Evolution

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This article is about evolution in biology. Evolution_sentence_0

For related articles, see Outline of evolution. Evolution_sentence_1

For other uses, see Evolution (disambiguation). Evolution_sentence_2

For a more accessible and less technical introduction to this topic, see Introduction to evolution. Evolution_sentence_3

Evolution is change in the heritable characteristics of biological populations over successive generations. Evolution_sentence_4

These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Evolution_sentence_5

Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution_sentence_6

Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population. Evolution_sentence_7

It is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. Evolution_sentence_8

The scientific theory of evolution by natural selection was conceived independently by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. Evolution_sentence_9

Evolution by natural selection was first demonstrated by the observation that more offspring are often produced than can possibly survive. Evolution_sentence_10

This is followed by three observable facts about living organisms: (1) traits vary among individuals with respect to their morphology, physiology and behaviour (phenotypic variation), (2) different traits confer different rates of survival and reproduction (differential fitness) and (3) traits can be passed from generation to generation (heritability of fitness). Evolution_sentence_11

Thus, in successive generations members of a population are more likely to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments. Evolution_sentence_12

In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation. Evolution_sentence_13

All life on Earth shares a last universal common ancestor (LUCA) that lived approximately 3.5–3.8 billion years ago. Evolution_sentence_14

The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms. Evolution_sentence_15

Existing patterns of biodiversity have been shaped by repeated formations of new species (speciation), changes within species (anagenesis) and loss of species (extinction) throughout the evolutionary history of life on Earth. Evolution_sentence_16

Morphological and biochemical traits are more similar among species that share a more recent common ancestor, and can be used to reconstruct phylogenetic trees. Evolution_sentence_17

Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology. Evolution_sentence_18

Their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture, medicine and computer science. Evolution_sentence_19

History of evolutionary thought Evolution_section_0

Main article: History of evolutionary thought Evolution_sentence_20

Further information: History of speciation Evolution_sentence_21

Classical times Evolution_section_1

The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles. Evolution_sentence_22

Such proposals survived into Roman times. Evolution_sentence_23

The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura (On the Nature of Things). Evolution_sentence_24

Medieval Evolution_section_2

In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. Evolution_sentence_25

This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Evolution_sentence_26

Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be. Evolution_sentence_27

Pre-Darwinian Evolution_section_3

In the 17th century, the new method of modern science rejected the Aristotelian approach. Evolution_sentence_28

It sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. Evolution_sentence_29

However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types. Evolution_sentence_30

John Ray applied one of the previously more general terms for fixed natural types, "species", to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. Evolution_sentence_31

The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan. Evolution_sentence_32

Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. Evolution_sentence_33

In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Evolution_sentence_34

Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, and Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism (or "filament"). Evolution_sentence_35

The first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents. Evolution_sentence_36

(The latter process was later called Lamarckism.) Evolution_sentence_37

These ideas were condemned by established naturalists as speculation lacking empirical support. Evolution_sentence_38

In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. Evolution_sentence_39

In the meantime, Ray's ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin. Evolution_sentence_40

Darwinian revolution Evolution_section_4

The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by Charles Darwin in terms of variable populations. Evolution_sentence_41

Darwin used the expression "descent with modification" rather than "evolution". Evolution_sentence_42

Partly influenced by An Essay on the Principle of Population (1798) by Thomas Robert Malthus, Darwin noted that population growth would lead to a "struggle for existence" in which favourable variations prevailed as others perished. Evolution_sentence_43

In each generation, many offspring fail to survive to an age of reproduction because of limited resources. Evolution_sentence_44

This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism. Evolution_sentence_45

Darwin developed his theory of "natural selection" from 1838 onwards and was writing up his "big book" on the subject when Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Evolution_sentence_46

Their separate papers were presented together at an 1858 meeting of the Linnean Society of London. Evolution_sentence_47

At the end of 1859, Darwin's publication of his "abstract" as On the Origin of Species explained natural selection in detail and in a way that led to an increasingly wide acceptance of Darwin's concepts of evolution at the expense of alternative theories. Evolution_sentence_48

Thomas Henry Huxley applied Darwin's ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Evolution_sentence_49

Some were disturbed by this since it implied that humans did not have a special place in the universe. Evolution_sentence_50

Pangenesis and heredity Evolution_section_5

The mechanisms of reproductive heritability and the origin of new traits remained a mystery. Evolution_sentence_51

Towards this end, Darwin developed his provisional theory of pangenesis. Evolution_sentence_52

In 1865, Gregor Mendel reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Evolution_sentence_53

Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory. Evolution_sentence_54

August Weismann made the important distinction between germ cells that give rise to gametes (such as sperm and egg cells) and the somatic cells of the body, demonstrating that heredity passes through the germ line only. Evolution_sentence_55

Hugo de Vries connected Darwin's pangenesis theory to Weismann's germ/soma cell distinction and proposed that Darwin's pangenes were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cell's structure. Evolution_sentence_56

De Vries was also one of the researchers who made Mendel's work well known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline. Evolution_sentence_57

To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. Evolution_sentence_58

In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. Evolution_sentence_59 B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. Evolution_sentence_60

The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled. Evolution_sentence_61

The 'modern synthesis' Evolution_section_6

Main article: Modern synthesis (20th century) Evolution_sentence_62

In the 1920s and 1930s the so-called modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that applied generally to any branch of biology. Evolution_sentence_63

The modern synthesis explained patterns observed across species in populations, through fossil transitions in palaeontology, and complex cellular mechanisms in developmental biology. Evolution_sentence_64

The publication of the structure of DNA by James Watson and Francis Crick with contribution of Rosalind Franklin in 1953 demonstrated a physical mechanism for inheritance. Evolution_sentence_65

Molecular biology improved understanding of the relationship between genotype and phenotype. Evolution_sentence_66

Advancements were also made in phylogenetic systematics, mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees. Evolution_sentence_67

In 1973, evolutionary biologist Theodosius Dobzhansky penned that "nothing in biology makes sense except in the light of evolution," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent explanatory body of knowledge that describes and predicts many observable facts about life on this planet. Evolution_sentence_68

Further syntheses Evolution_section_7

Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the biological hierarchy, from genes to species. Evolution_sentence_69

One extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations (evolution) acts on patterns of change within individual organisms (development). Evolution_sentence_70

Since the beginning of the 21st century and in light of discoveries made in recent decades, some biologists have argued for an extended evolutionary synthesis, which would account for the effects of non-genetic inheritance modes, such as epigenetics, parental effects, ecological inheritance and cultural inheritance, and evolvability. Evolution_sentence_71

Heredity Evolution_section_8

Further information: Introduction to genetics, Genetics, Heredity, and Reaction norm Evolution_sentence_72

Evolution in organisms occurs through changes in heritable traits—the inherited characteristics of an organism. Evolution_sentence_73

In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents. Evolution_sentence_74

Inherited traits are controlled by genes and the complete set of genes within an organism's genome (genetic material) is called its genotype. Evolution_sentence_75

The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. Evolution_sentence_76

These traits come from the interaction of its genotype with the environment. Evolution_sentence_77

As a result, many aspects of an organism's phenotype are not inherited. Evolution_sentence_78

For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. Evolution_sentence_79

However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn. Evolution_sentence_80

Heritable traits are passed from one generation to the next via DNA, a molecule that encodes genetic information. Evolution_sentence_81

DNA is a long biopolymer composed of four types of bases. Evolution_sentence_82

The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Evolution_sentence_83

Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Evolution_sentence_84

Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Evolution_sentence_85

Within cells, the long strands of DNA form condensed structures called chromosomes. Evolution_sentence_86

The specific location of a DNA sequence within a chromosome is known as a locus. Evolution_sentence_87

If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. Evolution_sentence_88

DNA sequences can change through mutations, producing new alleles. Evolution_sentence_89

If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. Evolution_sentence_90

However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by quantitative trait loci (multiple interacting genes). Evolution_sentence_91

Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA. Evolution_sentence_92

These phenomena are classed as epigenetic inheritance systems. Evolution_sentence_93

DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference and the three-dimensional conformation of proteins (such as prions) are areas where epigenetic inheritance systems have been discovered at the organismic level. Evolution_sentence_94

Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalisation. Evolution_sentence_95

Heritability may also occur at even larger scales. Evolution_sentence_96

For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. Evolution_sentence_97

This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Evolution_sentence_98

Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors. Evolution_sentence_99

Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis. Evolution_sentence_100

Sources of variation Evolution_section_9

Main article: Genetic variation Evolution_sentence_101

Further information: Genetic diversity and Population genetics Evolution_sentence_102

Evolution can occur if there is enough genetic variation within a population. Evolution_sentence_103

Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Evolution_sentence_104

Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species. Evolution_sentence_105

However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes. Evolution_sentence_106

An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. Evolution_sentence_107

A substantial part of the phenotypic variation in a population is caused by genotypic variation. Evolution_sentence_108

The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. Evolution_sentence_109

The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Evolution_sentence_110

Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely. Evolution_sentence_111

Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. Evolution_sentence_112

But with blending inheritance, genetic variation would be rapidly lost, making evolution by natural selection implausible. Evolution_sentence_113

The Hardy–Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. Evolution_sentence_114

The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift. Evolution_sentence_115

Mutation Evolution_section_10

Main article: Mutation Evolution_sentence_116

Mutations are changes in the DNA sequence of a cell's genome. Evolution_sentence_117

When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect. Evolution_sentence_118

Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial. Evolution_sentence_119

Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome. Evolution_sentence_120

Extra copies of genes are a major source of the raw material needed for new genes to evolve. Evolution_sentence_121

This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. Evolution_sentence_122

For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene. Evolution_sentence_123

New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. Evolution_sentence_124

This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Evolution_sentence_125

Other types of mutations can even generate entirely new genes from previously noncoding DNA. Evolution_sentence_126

The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions. Evolution_sentence_127

When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions. Evolution_sentence_128

For example, polyketide synthases are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line. Evolution_sentence_129

Sex and recombination Evolution_section_11

Further information: Sexual reproduction, Genetic recombination, and Evolution of sexual reproduction Evolution_sentence_130

In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. Evolution_sentence_131

In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. Evolution_sentence_132

In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes. Evolution_sentence_133

Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles. Evolution_sentence_134

Sex usually increases genetic variation and may increase the rate of evolution. Evolution_sentence_135

The two-fold cost of sex was first described by John Maynard Smith. Evolution_sentence_136

The first cost is that in sexually dimorphic species only one of the two sexes can bear young. Evolution_sentence_137

This cost does not apply to hermaphroditic species, like most plants and many invertebrates. Evolution_sentence_138

The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes. Evolution_sentence_139

Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. Evolution_sentence_140

The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Evolution_sentence_141

Gene flow Evolution_section_12

Further information: Gene flow Evolution_sentence_142

Gene flow is the exchange of genes between populations and between species. Evolution_sentence_143

It can therefore be a source of variation that is new to a population or to a species. Evolution_sentence_144

Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses. Evolution_sentence_145

Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Evolution_sentence_146

Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. Evolution_sentence_147

In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Evolution_sentence_148

Horizontal transfer of genes from bacteria to eukaryotes such as the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis has occurred. Evolution_sentence_149

An example of larger-scale transfers are the eukaryotic bdelloid rotifers, which have received a range of genes from bacteria, fungi and plants. Evolution_sentence_150

Viruses can also carry DNA between organisms, allowing transfer of genes even across biological domains. Evolution_sentence_151

Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria. Evolution_sentence_152

It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea. Evolution_sentence_153

Mechanisms Evolution_section_13

From a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Evolution_sentence_154

Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, gene flow and mutation bias. Evolution_sentence_155

Natural selection Evolution_section_14

Main article: Natural selection Evolution_sentence_156

Evolution by means of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. Evolution_sentence_157

It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts: Evolution_sentence_158

Evolution_unordered_list_0

  • Variation exists within populations of organisms with respect to morphology, physiology, and behaviour (phenotypic variation).Evolution_item_0_0
  • Different traits confer different rates of survival and reproduction (differential fitness).Evolution_item_0_1
  • These traits can be passed from generation to generation (heritability of fitness).Evolution_item_0_2

More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Evolution_sentence_159

Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage. Evolution_sentence_160

This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Evolution_sentence_161

Consequences of selection include nonrandom mating and genetic hitchhiking. Evolution_sentence_162

The central concept of natural selection is the evolutionary fitness of an organism. Evolution_sentence_163

Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. Evolution_sentence_164

However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes. Evolution_sentence_165

For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness. Evolution_sentence_166

If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. Evolution_sentence_167

These traits are said to be "selected for." Evolution_sentence_168

Examples of traits that can increase fitness are enhanced survival and increased fecundity. Evolution_sentence_169

Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer—they are "selected against." Evolution_sentence_170

Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. Evolution_sentence_171

However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see Dollo's law). Evolution_sentence_172

However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. "Throwbacks" such as these are known as atavisms. Evolution_sentence_173

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. Evolution_sentence_174

The first is directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Evolution_sentence_175

Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. Evolution_sentence_176

This would be when either short or tall organisms had an advantage, but not those of medium height. Evolution_sentence_177

Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. Evolution_sentence_178

This would, for example, cause organisms to eventually have a similar height. Evolution_sentence_179

Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. Evolution_sentence_180

"Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Evolution_sentence_181

Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and nonliving parts) within the system...." Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system. Evolution_sentence_182

These relationships involve the life history of the organism, its position in the food chain and its geographic range. Evolution_sentence_183

This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. Evolution_sentence_184

Natural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Evolution_sentence_185

Selection can act at multiple levels simultaneously. Evolution_sentence_186

An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Evolution_sentence_187

Selection at a level above the individual, such as group selection, may allow the evolution of cooperation. Evolution_sentence_188

Genetic hitchhiking Evolution_section_15

Further information: Genetic hitchhiking, Hill–Robertson effect, and Selective sweep Evolution_sentence_189

Recombination allows alleles on the same strand of DNA to become separated. Evolution_sentence_190

However, the rate of recombination is low (approximately two events per chromosome per generation). Evolution_sentence_191

As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as linkage. Evolution_sentence_192

This tendency is measured by finding how often two alleles occur together on a single chromosome compared to expectations, which is called their linkage disequilibrium. Evolution_sentence_193

A set of alleles that is usually inherited in a group is called a haplotype. Evolution_sentence_194

This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Evolution_sentence_195

Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size. Evolution_sentence_196

Sexual selection Evolution_section_16

Further information: Sexual selection Evolution_sentence_197

A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates. Evolution_sentence_198

Traits that evolved through sexual selection are particularly prominent among males of several animal species. Evolution_sentence_199

Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises the survival of individual males. Evolution_sentence_200

This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake, sexually selected traits. Evolution_sentence_201

Genetic drift Evolution_section_17

Further information: Genetic drift and Effective population size Evolution_sentence_202

Genetic drift is the random fluctuations of allele frequencies within a population from one generation to the next. Evolution_sentence_203

When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward at each successive generation because the alleles are subject to sampling error. Evolution_sentence_204

This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Evolution_sentence_205

Genetic drift may therefore eliminate some alleles from a population due to chance alone. Evolution_sentence_206

Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles. Evolution_sentence_207

The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift. Evolution_sentence_208

Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift. Evolution_sentence_209

This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature. Evolution_sentence_210

However, a more recent and better-supported version of this model is the nearly neutral theory, where a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population. Evolution_sentence_211

Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft. Evolution_sentence_212

The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations. Evolution_sentence_213

The number of individuals in a population is not critical, but instead a measure known as the effective population size. Evolution_sentence_214

The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest. Evolution_sentence_215

The effective population size may not be the same for every gene in the same population. Evolution_sentence_216

It is usually difficult to measure the relative importance of selection and neutral processes, including drift. Evolution_sentence_217

The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research. Evolution_sentence_218

Gene flow Evolution_section_18

Further information: Gene flow, Hybrid (biology), and Horizontal gene transfer Evolution_sentence_219

Gene flow involves the exchange of genes between populations and between species. Evolution_sentence_220

The presence or absence of gene flow fundamentally changes the course of evolution. Evolution_sentence_221

Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the Bateson-Dobzhansky-Muller model, even if both populations remain essentially identical in terms of their adaptation to the environment. Evolution_sentence_222

If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Evolution_sentence_223

Thus, exchange of genetic information between individuals is fundamentally important for the development of the Biological Species Concept (BSC). Evolution_sentence_224

During the development of the modern synthesis, Sewall Wright developed his shifting balance theory, which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution. Evolution_sentence_225

However, recently there has been substantial criticism of the importance of the shifting balance theory. Evolution_sentence_226

Mutation bias Evolution_section_19

Mutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. Evolution_sentence_227

This is related to the idea of developmental bias. Evolution_sentence_228

Haldane and Fisher argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. Evolution_sentence_229

This opposing-pressures argument was long used to dismiss the possibility of internal tendencies in evolution, until the molecular era prompted renewed interest in neutral evolution. Evolution_sentence_230

Noboru Sueoka and Ernst Freese proposed that systematic biases in mutation might be responsible for systematic differences in genomic GC composition between species. Evolution_sentence_231

The identification of a GC-biased E. coli mutator strain in 1967, along with the proposal of the neutral theory, established the plausibility of mutational explanations for molecular patterns, which are now common in the molecular evolution literature. Evolution_sentence_232

For instance, mutation biases are frequently invoked in models of codon usage. Evolution_sentence_233

Such models also include effects of selection, following the mutation-selection-drift model, which allows both for mutation biases and differential selection based on effects on translation. Evolution_sentence_234

Hypotheses of mutation bias have played an important role in the development of thinking about the evolution of genome composition, including isochores. Evolution_sentence_235

Different insertion vs. deletion biases in different taxa can lead to the evolution of different genome sizes. Evolution_sentence_236

The hypothesis of Lynch regarding genome size relies on mutational biases toward increase or decrease in genome size. Evolution_sentence_237

However, mutational hypotheses for the evolution of composition suffered a reduction in scope when it was discovered that (1) GC-biased gene conversion makes an important contribution to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation. Evolution_sentence_238

Contemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. Evolution_sentence_239

More recent work showed that the original "pressures" theory assumes that evolution is based on standing variation: when evolution depends on the introduction of new alleles, mutational and developmental biases in introduction can impose biases on evolution without requiring neutral evolution or high mutation rates. Evolution_sentence_240

Several recent studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation. Evolution_sentence_241

Outcomes Evolution_section_20

Evolution influences every aspect of the form and behaviour of organisms. Evolution_sentence_242

Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. Evolution_sentence_243

These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Evolution_sentence_244

Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. Evolution_sentence_245

In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. Evolution_sentence_246

These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Evolution_sentence_247

Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in allele frequency and adaptation. Evolution_sentence_248

In general, macroevolution is regarded as the outcome of long periods of microevolution. Evolution_sentence_249

Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. Evolution_sentence_250

However, in macroevolution, the traits of the entire species may be important. Evolution_sentence_251

For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. Evolution_sentence_252

In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction. Evolution_sentence_253

A common misconception is that evolution has goals, long-term plans, or an innate tendency for "progress", as expressed in beliefs such as orthogenesis and evolutionism; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity. Evolution_sentence_254

Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the biosphere. Evolution_sentence_255

For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size, and constitute the vast majority of Earth's biodiversity. Evolution_sentence_256

Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable. Evolution_sentence_257

Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time. Evolution_sentence_258

Adaptation Evolution_section_21

Further information: Adaptation Evolution_sentence_259

Adaptation is the process that makes organisms better suited to their habitat. Evolution_sentence_260

Also, the term adaptation may refer to a trait that is important for an organism's survival. Evolution_sentence_261

For example, the adaptation of horses' teeth to the grinding of grass. Evolution_sentence_262

By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Evolution_sentence_263

Adaptations are produced by natural selection. Evolution_sentence_264

The following definitions are due to Theodosius Dobzhansky: Evolution_sentence_265

Evolution_ordered_list_1

  1. Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.Evolution_item_1_3
  2. Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.Evolution_item_1_4
  3. An adaptive trait is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.Evolution_item_1_5

Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. Evolution_sentence_266

An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Evolution_sentence_267

Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. Evolution_sentence_268

An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability). Evolution_sentence_269

Adaptation occurs through the gradual modification of existing structures. Evolution_sentence_270

Consequently, structures with similar internal organisation may have different functions in related organisms. Evolution_sentence_271

This is the result of a single ancestral structure being adapted to function in different ways. Evolution_sentence_272

The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor. Evolution_sentence_273

However, since all living organisms are related to some extent, even organs that appear to have little or no structural similarity, such as arthropod, squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called deep homology. Evolution_sentence_274

During evolution, some structures may lose their original function and become vestigial structures. Evolution_sentence_275

Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Evolution_sentence_276

Examples include pseudogenes, the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds, the presence of hip bones in whales and snakes, and sexual traits in organisms that reproduce via asexual reproduction. Evolution_sentence_277

Examples of vestigial structures in humans include wisdom teeth, the coccyx, the vermiform appendix, and other behavioural vestiges such as goose bumps and primitive reflexes. Evolution_sentence_278

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. Evolution_sentence_279

One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. Evolution_sentence_280

However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Evolution_sentence_281

Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously had different functions. Evolution_sentence_282

Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes. Evolution_sentence_283

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. Evolution_sentence_284

This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. Evolution_sentence_285

These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. Evolution_sentence_286

It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. Evolution_sentence_287

It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes. Evolution_sentence_288

Coevolution Evolution_section_22

Further information: Coevolution Evolution_sentence_289

Interactions between organisms can produce both conflict and cooperation. Evolution_sentence_290

When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Evolution_sentence_291

Here, the evolution of one species causes adaptations in a second species. Evolution_sentence_292

These changes in the second species then, in turn, cause new adaptations in the first species. Evolution_sentence_293

This cycle of selection and response is called coevolution. Evolution_sentence_294

An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. Evolution_sentence_295

In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake. Evolution_sentence_296

Cooperation Evolution_section_23

Further information: Co-operation (evolution) Evolution_sentence_297

Not all co-evolved interactions between species involve conflict. Evolution_sentence_298

Many cases of mutually beneficial interactions have evolved. Evolution_sentence_299

For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil. Evolution_sentence_300

This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis. Evolution_sentence_301

Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system. Evolution_sentence_302

Coalitions between organisms of the same species have also evolved. Evolution_sentence_303

An extreme case is the eusociality found in social insects, such as bees, termites and ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. Evolution_sentence_304

On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Evolution_sentence_305

Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. Evolution_sentence_306

If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer. Evolution_sentence_307

Such cooperation within species may have evolved through the process of kin selection, which is where one organism acts to help raise a relative's offspring. Evolution_sentence_308

This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on. Evolution_sentence_309

Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms. Evolution_sentence_310

Speciation Evolution_section_24

Main article: Speciation Evolution_sentence_311

Further information: Assortative mating and Panmixia Evolution_sentence_312

Speciation is the process where a species diverges into two or more descendant species. Evolution_sentence_313

There are multiple ways to define the concept of "species." Evolution_sentence_314

The choice of definition is dependent on the particularities of the species concerned. Evolution_sentence_315

For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Evolution_sentence_316

Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic. Evolution_sentence_317

The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Evolution_sentence_318

Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." Evolution_sentence_319

Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes, and this is called the species problem. Evolution_sentence_320

Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species. Evolution_sentence_321

Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Evolution_sentence_322

Gene flow may slow this process by spreading the new genetic variants also to the other populations. Evolution_sentence_323

Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Evolution_sentence_324

Such hybrids are generally infertile. Evolution_sentence_325

In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. Evolution_sentence_326

However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. Evolution_sentence_327

The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example. Evolution_sentence_328

Speciation has been observed multiple times under both controlled laboratory conditions (see laboratory experiments of speciation) and in nature. Evolution_sentence_329

In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. Evolution_sentence_330

There are four primary geographic modes of speciation. Evolution_sentence_331

The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Evolution_sentence_332

Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. Evolution_sentence_333

As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed. Evolution_sentence_334

The second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. Evolution_sentence_335

This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Evolution_sentence_336

Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change. Evolution_sentence_337

The third mode is parapatric speciation. Evolution_sentence_338

This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Evolution_sentence_339

Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Evolution_sentence_340

Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. Evolution_sentence_341

One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Evolution_sentence_342

Here, plants evolve that have resistance to high levels of metals in the soil. Evolution_sentence_343

Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Evolution_sentence_344

Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance. Evolution_sentence_345

Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat. Evolution_sentence_346

This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Evolution_sentence_347

Generally, sympatric speciation in animals requires the evolution of both genetic differences and nonrandom mating, to allow reproductive isolation to evolve. Evolution_sentence_348

One type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. Evolution_sentence_349

This is not common in animals as animal hybrids are usually sterile. Evolution_sentence_350

This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. Evolution_sentence_351

However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. Evolution_sentence_352

This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already. Evolution_sentence_353

An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa crossbred to give the new species Arabidopsis suecica. Evolution_sentence_354

This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Evolution_sentence_355

Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms. Evolution_sentence_356

Speciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged. Evolution_sentence_357

In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. Evolution_sentence_358

As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils. Evolution_sentence_359

Extinction Evolution_section_25

Further information: Extinction Evolution_sentence_360

Extinction is the disappearance of an entire species. Evolution_sentence_361

Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction. Evolution_sentence_362

Nearly all animal and plant species that have lived on Earth are now extinct, and extinction appears to be the ultimate fate of all species. Evolution_sentence_363

These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events. Evolution_sentence_364

The Cretaceous–Paleogene extinction event, during which the non-avian dinosaurs became extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe, with approximately 96% of all marine species driven to extinction. Evolution_sentence_365

The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Evolution_sentence_366

Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century. Evolution_sentence_367

Human activities are now the primary cause of the ongoing extinction event; global warming may further accelerate it in the future. Evolution_sentence_368

Despite the estimated extinction of more than 99 percent of all species that ever lived on Earth, about 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described. Evolution_sentence_369

The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered. Evolution_sentence_370

The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (the competitive exclusion principle). Evolution_sentence_371

If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction. Evolution_sentence_372

The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors. Evolution_sentence_373

Evolutionary history of life Evolution_section_26

Main article: Evolutionary history of life Evolution_sentence_374

See also: Timeline of evolutionary history of life Evolution_sentence_375

Origin of life Evolution_section_27

Further information: Abiogenesis, Earliest known life forms, Panspermia, and RNA world hypothesis Evolution_sentence_376

The Earth is about 4.54 billion years old. Evolution_sentence_377

The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago, during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. Evolution_sentence_378

Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia. Evolution_sentence_379

Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. Evolution_sentence_380

Commenting on the Australian findings, Stephen Blair Hedges wrote, "If life arose relatively quickly on Earth, then it could be common in the universe." Evolution_sentence_381

In July 2016, scientists reported identifying a set of 355 genes from the last universal common ancestor (LUCA) of all organisms living on Earth. Evolution_sentence_382

More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Evolution_sentence_383

Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.9 million are estimated to have been named and 1.6 million documented in a central database to date, leaving at least 80 percent not yet described. Evolution_sentence_384

Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed. Evolution_sentence_385

The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions. Evolution_sentence_386

The beginning of life may have included self-replicating molecules such as RNA and the assembly of simple cells. Evolution_sentence_387

Common descent Evolution_section_28

Further information: Common descent and Evidence of common descent Evolution_sentence_388

All organisms on Earth are descended from a common ancestor or ancestral gene pool. Evolution_sentence_389

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events. Evolution_sentence_390

The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Evolution_sentence_391

Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Evolution_sentence_392

Third, vestigial traits with no clear purpose resemble functional ancestral traits. Evolution_sentence_393

Fourth, organisms can be classified using these similarities into a hierarchy of nested groups, similar to a family tree. Evolution_sentence_394

Modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species. Evolution_sentence_395

To solve this problem and others, some authors prefer to use the "Coral of life" as a metaphor or a mathematical model to illustrate the evolution of life. Evolution_sentence_396

This view dates back to an idea briefly mentioned by Darwin but later abandoned. Evolution_sentence_397

Past species have also left records of their evolutionary history. Evolution_sentence_398

Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. Evolution_sentence_399

By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. Evolution_sentence_400

However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Evolution_sentence_401

Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry. Evolution_sentence_402

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. Evolution_sentence_403

For example, all living cells use the same basic set of nucleotides and amino acids. Evolution_sentence_404

The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations. Evolution_sentence_405

For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed. Evolution_sentence_406

Evolution of life Evolution_section_29

Main articles: Evolutionary history of life and Timeline of evolutionary history of life Evolution_sentence_407

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago. Evolution_sentence_408

No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years. Evolution_sentence_409

The eukaryotic cells emerged between 1.6–2.7 billion years ago. Evolution_sentence_410

The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis. Evolution_sentence_411

The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes. Evolution_sentence_412

Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants. Evolution_sentence_413

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period. Evolution_sentence_414

The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria. Evolution_sentence_415

In January 2016, scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells. Evolution_sentence_416

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Evolution_sentence_417

Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct. Evolution_sentence_418

Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis. Evolution_sentence_419

About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals. Evolution_sentence_420

Insects were particularly successful and even today make up the majority of animal species. Evolution_sentence_421

Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago. Evolution_sentence_422

However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes. Evolution_sentence_423

Applications Evolution_section_30

Main articles: Applications of evolution, Selective breeding, and Evolutionary computation Evolution_sentence_424

Concepts and models used in evolutionary biology, such as natural selection, have many applications. Evolution_sentence_425

Artificial selection is the intentional selection of traits in a population of organisms. Evolution_sentence_426

This has been used for thousands of years in the domestication of plants and animals. Evolution_sentence_427

More recently, such selection has become a vital part of genetic engineering, with selectable markers such as antibiotic resistance genes being used to manipulate DNA. Evolution_sentence_428

Proteins with valuable properties have evolved by repeated rounds of mutation and selection (for example modified enzymes and new antibodies) in a process called directed evolution. Evolution_sentence_429

Understanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders. Evolution_sentence_430

For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Evolution_sentence_431

Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. Evolution_sentence_432

This helped identify genes required for vision and pigmentation. Evolution_sentence_433

Evolutionary theory has many applications in medicine. Evolution_sentence_434

Many human diseases are not static phenomena, but capable of evolution. Evolution_sentence_435

Viruses, bacteria, fungi and cancers evolve to be resistant to host immune defences, as well as pharmaceutical drugs. Evolution_sentence_436

These same problems occur in agriculture with pesticide and herbicide resistance. Evolution_sentence_437

It is possible that we are facing the end of the effective life of most of available antibiotics and predicting the evolution and evolvability of our pathogens and devising strategies to slow or circumvent it is requiring deeper knowledge of the complex forces driving evolution at the molecular level. Evolution_sentence_438

In computer science, simulations of evolution using evolutionary algorithms and artificial life started in the 1960s and were extended with simulation of artificial selection. Evolution_sentence_439

Artificial evolution became a widely recognised optimisation method as a result of the work of Ingo Rechenberg in the 1960s. Evolution_sentence_440

He used evolution strategies to solve complex engineering problems. Evolution_sentence_441

Genetic algorithms in particular became popular through the writing of John Henry Holland. Evolution_sentence_442

Practical applications also include automatic evolution of computer programmes. Evolution_sentence_443

Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers and also to optimise the design of systems. Evolution_sentence_444

Social and cultural responses Evolution_section_31

Further information: Social effects of evolutionary theory, 1860 Oxford evolution debate, Rejection of evolution by religious groups, Objections to evolution, and Evolution in fiction Evolution_sentence_445

In the 19th century, particularly after the publication of On the Origin of Species in 1859, the idea that life had evolved was an active source of academic debate centred on the philosophical, social and religious implications of evolution. Evolution_sentence_446

Today, the modern evolutionary synthesis is accepted by a vast majority of scientists. Evolution_sentence_447

However, evolution remains a contentious concept for some theists. Evolution_sentence_448

While various religions and denominations have reconciled their beliefs with evolution through concepts such as theistic evolution, there are creationists who believe that evolution is contradicted by the creation myths found in their religions and who raise various objections to evolution. Evolution_sentence_449

As had been demonstrated by responses to the publication of Vestiges of the Natural History of Creation in 1844, the most controversial aspect of evolutionary biology is the implication of human evolution that humans share common ancestry with apes and that the mental and moral faculties of humanity have the same types of natural causes as other inherited traits in animals. Evolution_sentence_450

In some countries, notably the United States, these tensions between science and religion have fuelled the current creation–evolution controversy, a religious conflict focusing on politics and public education. Evolution_sentence_451

While other scientific fields such as cosmology and Earth science also conflict with literal interpretations of many religious texts, evolutionary biology experiences significantly more opposition from religious literalists. Evolution_sentence_452

The teaching of evolution in American secondary school biology classes was uncommon in most of the first half of the 20th century. Evolution_sentence_453

The Scopes Trial decision of 1925 caused the subject to become very rare in American secondary biology textbooks for a generation, but it was gradually re-introduced later and became legally protected with the 1968 Epperson v. Arkansas decision. Evolution_sentence_454

Since then, the competing religious belief of creationism was legally disallowed in secondary school curricula in various decisions in the 1970s and 1980s, but it returned in pseudoscientific form as intelligent design (ID), to be excluded once again in the 2005 Kitzmiller v. Dover Area School District case. Evolution_sentence_455

The debate over Darwin's ideas did not generate significant controversy in China. Evolution_sentence_456

See also Evolution_section_32

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Evolution.