Evolution

This article is about evolution in biology. For other articles with similar names, see Evolution (disambiguation).
For a non-technical introduction to the topic, please see Introduction to evolution.
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Evolution
Mechanisms and processes

Adaptation
Genetic drift
Gene flow
Mutation
Selection
Speciation

Research and history

Evidence
History
Modern synthesis
Social effect

Evolutionary biology fields

Ecological genetics
Evolutionary development
Human evolution
Molecular evolution
Phylogenetics
Population genetics
Evo-devo

Biology Portal ·

In biology, evolution is the process in which some of a population's inherited traits become more common, at the expense of others, from generation to generation. This is usually measured in terms of the variant genes, known as alleles, that encode the competing traits. As differences in and between populations accumulate over time, speciation, the development of new species from existing ones, can occur. All organisms, including extinct species, are related by common descent through numerous speciation events starting from a single ancestor.[1][2]

Mutation of the genes, migration between populations, and the reshuffling of genes during sexual reproduction creates variation in organisms. While a certain random component, known as genetic drift, is involved, the variation is also acted on by natural selection, in which organisms which happen to have combinations of traits that help them to survive and reproduce more than others in the population will, on average, have more offspring, passing more copies of these beneficial traits on to the next generation. This leads to advantageous traits becoming more common in each generation, while disadvantageous traits become rarer. [1][3][4] Given enough time, this passive process can result in varied adaptations to changing environmental conditions.[5]

The theory of evolution by natural selection was first put forth in detail in Charles Darwin's 1859 book On the Origin of Species. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis.[5] With its enormous explanatory and predictive power, this theory has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.[6][7][8]

Contents

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Basic processes

Evolution consists of two basic types of processes: those that introduce new genetic variation into a population, and those that affect the frequencies of existing genes. Paleontologist Stephen Jay Gould once summarized this as "variation proposes and selection disposes".[9] There is a certain amount of variation in apparent traits, or phenotypes, in populations. This phenotypic variation is the result of genotypes, the specific genetic makeup encoded on DNA molecules. Variants in gene sequences in the individuals of a population and the interaction of a genotype with the environment are involved in phenotypic plasticity. There may be one or more functional variants of a gene or locus, and these variants are called alleles. Most sites in the genome (i.e., complete DNA sequence) of a species are identical in all individuals in the population; sites with more than one allele are called polymorphic, or segregating, sites.

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Variation

Genetic variation is often the result of a new mutation in a single individual; in subsequent generations the frequency of that variant may fluctuate in the population, becoming more or less prevalent relative to other alleles at the site. This change in allele frequency is the commonly accepted definition of evolution, and all evolutionary forces act by driving allele frequency in one direction or another. Variation disappears when it reaches the point of fixation—when it either reaches a frequency of zero and disappears from the population, or reaches a frequency of one and replaces the ancestral allele entirely.

Variation is also produced during the production of gametes and union at fertilization to produce a zygote (there is genetic recombination that produce variation). In some organisms (like bacteria and plants) the lateral transfer of genetic material or Horizontal gene transfer plays a significant role, and the mixing of genetic material by hybridization (mixing species) produces significant variation (like in plants and birds).

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Heredity

A section of a model of a DNA molecule.
A section of a model of a DNA molecule.[10]

Gregor Mendel's work provided the first firm basis to the idea that heredity occurred in discrete units. He noticed several traits in peas that occur in only one of two forms (e.g., the peas were either "round" or "wrinkled"), and was able to show that the traits were: heritable (passed from parent to offspring); discrete (i.e., if one parent had round peas and the other wrinkled, the progeny were not intermediate, but either round or wrinkled); and were distributed to progeny in a well-defined and predictable manner (Mendelian inheritance). His research laid the foundation for the concept of discrete heritable traits, known today as genes. After Mendel's work was "rediscovered" in 1900, it was found that the concepts could have wide applicability, and that most complex traits were polygenetic and not controlled by single unit characters.

Later research gave a physical basis to the notion of genes, and eventually identified DNA as the genetic material, and identified genes as discrete elements within DNA. DNA is not perfectly copied, and rare mistakes (mutations) in genes can affect traits that the genes control (e.g., pea shape).

A gene can have modifications such as DNA methylation, which do not change the nucleotide sequence of a gene, but do result in the epigenetic inheritance of a change in the expression of that gene in a trait. Another epigenetic mechanism is via micro RNA's, and RNA interference which serve regulatory roles in gene transcription and translation.

Non-DNA based forms of heritable variation exist, such as transmission of the secondary structures of prions or structural inheritance of patterns in the rows of cilia in protozoans such as Paramecium[11] and Tetrahymena. Investigations continue into whether these mechanisms allow for the production of specific beneficial heritable variation in response to environmental signals. If this were shown to be the case, then some instances of evolution would lie outside of the typical Darwinian framework, which avoids any connection between environmental signals and the production of heritable variation. However, the processes that produce these variations are rather rare, often reversible, and leave the genetic information intact.

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Mutation

Mutation can occur because of "copy errors" during DNA replication.
Mutation can occur because of "copy errors" during DNA replication.

Genetic variation arises due to random mutations that occur at a certain rate in the genomes of all organisms. Mutations are permanent, transmissible changes to the genetic material (usually DNA or RNA) of a cell, and can be caused by: "copying errors" in the genetic material during cell division; by exposure to radiation, chemicals, or viruses. In multicellular organisms, mutations can be subdivided into germline mutations that occur in the gametes and thus can be passed on to progeny, and somatic mutations that can lead to the malfunction or death of a cell and can cause cancer.

Mutations that are not affected by natural selection are called neutral mutations. Their frequency in the population is governed by mutation rate, genetic drift and selective pressure on linked alleles. It is understood that most of a species' genome, in the absence of selection, undergoes a steady accumulation of neutral mutations.

Individual genes can be affected by point mutations, also known as SNPs, in which a single base pair is altered. The substitution of a single base pair may or may not affect the function of the gene (see mutation) while deletions and insertions of a single or several base pairs usually results in a non-functional gene.[12]

Mobile elements, transposons, make up a major fraction of the genomes of plants and animals and appear to have played a significant role in the evolution of genomes. These mobile insertional elements can jump within a genome and alter existing genes and gene networks to produce evolutionary change and diversity. [13]

On the other hand, gene duplications, which may occur via a number of mechanisms, are believed to be one major source of raw material for evolving new genes as tens to hundreds of genes are duplicated in animal genomes every million years.[14] Most genes belong to larger "families" of genes derived from a common ancestral gene (two genes from a species that are in the same family are dubbed "paralogs"). Another mechanism causing gene duplication is intergenic recombination, particularly 'exon shuffling', i.e., an aberrant recombination that joins the 'upstream' part of one gene with the 'downstream' part of another. Genome duplications and chromosome duplications also appear to have served a significant role in evolution. Genome duplication has been the driving force in the Teleostei genome evolution, where up to four genome duplications are thought to have happened, resulting in species with more than 250 chromosomes.

Large chromosomal rearrangements do not necessarily change gene function, but do generally result in reproductive isolation, and, by definition, speciation (species (in sexual organisms) are usually defined by the ability to interbreed). An example of this mechanism is the fusion of two chromosomes in the homo genus that produced human chromosome 2; this fusion did not occur in the chimp lineage, resulting in two separate chromosomes in extant chimps.

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Horizontal gene transfer

A phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and eukaryotes. Originally proposed by Carl Woese.
A phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and eukaryotes. Originally proposed by Carl Woese.

Horizontal gene transfer (HGT) (or Lateral gene transfer) is any process in which an organism transfers genetic material (i.e. DNA) to another organism that is not its offspring. This mechanism allows for the transfer of genetic material between unrelated organisms of the same species or of different species and is a form of gene flow.

Many mechanisms for horizontal gene transfer have been observed, such as antigenic shift, reassortment, and hybridization. Viruses can transfer genes between species via transduction.[15] Bacteria can incorporate genes from other dead bacteria or plasmids via transformation, exchange genes with living bacteria via conjugation, and can have plasmids "set up residence separate from the host's genome".[16] Hybridization is highly significant in plant speciation [17] , and one out of ten species of birds are known to hybridize.[18] There are also examples of hybridization in mammals and insects [19] , however it most often produces sterile offspring.

HGT has been shown to result in the spread of antibiotic resistance across bacterial populations.[20] Furthermore, findings indicate that HGT has been a major mechanism for prokaryotic and eukaryotic evolution.[21][22]

HGT complicates the inference of the phylogeny of life, as the original metaphor of a tree of life no longer fits. Rather, since genetic information is passed to other organisms and other species in addition to being passed from parent to offspring, "biologists [should] use the metaphor of a mosaic to describe the different histories combined in individual genomes and use [the] metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes."[23]

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Mechanisms of evolution

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Selection and adaptation

A peacock's tail is the canonical example of sexual selection.
A peacock's tail is the canonical example of sexual selection.

Natural selection comes from differences in survival and reproduction. Differential mortality is the survival rate of individuals to their reproductive age. Differential fertility is the total genetic contribution to the next generation. Note that, whereas mutations and genetic drift are random, natural selection is not, as it preferentially selects for different mutations based on differential fitnesses. For example, rolling dice is random, but always picking the higher number on two rolled dice is not random. The central role of natural selection in evolutionary theory has given rise to a strong connection between that field and the study of ecology.

Natural selection can be subdivided into two categories: ecological selection occurs when organisms that survive and reproduce increase the frequency of their genes in the gene pool over those that do not survive; and sexual selection occurs when organisms which are more attractive to the opposite sex because of their features reproduce more and thus increase the frequency of those features in the gene pool.

Natural selection operates on mutations in a number of different ways. Arguably the most common form of selection is stabilizing selection, which decreases the frequency of harmful mutations; "living fossils" may be a result of this. Other forms of natural selection include directional selection, which increases the frequency of a beneficial mutation, and artificial selection, the purposeful breeding of a species.

Through the process of natural selection, organisms become better adapted to their environments. Adaptation is any evolutionary process that increases the fitness of the individual, or sometimes the trait that confers increased fitness, e.g., a stronger prehensile tail or greater visual acuity. Note that adaptation is context-sensitive; a trait that increases fitness in one environment may decrease it in another.

Most biologists believe that adaptation occurs through the accumulation of many mutations of small effect. However, macromutation is an alternative process for adaptation that involves a single, very large-scale mutation.

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Recombination

In asexual organisms, variants in genes on the same chromosome will always be inherited together—they are linked, by virtue of being on the same DNA molecule. However, sexual organisms, in the production of gametes, shuffle linked alleles on homologous chromosomes inherited from the parents via meiotic recombination. This shuffling allows independent assortment of alleles (mutations) in genes to be propagated in the population independently. This allows bad mutations to be purged and beneficial mutations to be retained more efficiently than in asexual populations.

However, the meitoic recombination rate is not very high - on the order of one crossover (recombination event between homologous chromosomes) per chromosome arm per generation. Therefore, linked alleles are not perfectly shuffled away from each other, but tend to be inherited together. This tendency may be measured by comparing the co-occurrence of two alleles, usually quantified as linkage disequilibrium (LD). A set of alleles that are often co-propagated is called a haplotype. Strong haplotype blocks can be a product of strong positive selection.

Recombination is mildly mutagenic, which is one of the proposed reasons why it occurs with limited frequency. Recombination also breaks up gene combinations that have been successful in previous generations, and hence should be opposed by selection. However, recombination could be favoured by negative frequency-dependent selection (this is when rare variants increase in frequency) because it leads to more individuals with new and rare gene combinations being produced.

When alleles cannot be separated by recombination (for example in mammalian Y chromosomes), there is an observable reduction in effective population size, known as the Hill-Robertson effect, and the successive establishment of bad mutations, known as Muller's ratchet.

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Genetic drift

Genetic drift is the change in allele frequency from one generation to the next due to sampling variance. The frequency of an allele in the offspring generation will vary according to a probability distribution of the frequency of the allele in the parent generation. Thus, over time even in the absence of selection upon the alleles, allele frequencies tend to "drift" upward or downward, eventually becoming "fixed" - that is, going to 0% or 100% frequency. Thus, fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population due to chance alone. Two separate populations that begin with the same allele frequencies therefore might drift apart by random fluctuation into two divergent populations with different allele sets (for example, alleles present in one population could be absent in the other, or vice versa).

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Gene flow and population structure

Map of the world showing distribution of camelids. Solid black lines indicate possible migration routes.
Map of the world showing distribution of camelids. Solid black lines indicate possible migration routes.

Gene flow, also called migration, is the exchange of genetic variation between populations, when geography and culture are not obstacles. Ernst Mayr thought that gene flow is likely to be homogenising, and therefore counteracting selective adaptation. Obstacles to gene flow result in reproductive isolation, a necessary condition for speciation.

The free movement of alleles through a population may also be impeded by population structure, the size and geographical distribution of a population. For example, most real-world populations are not actually fully interbreeding; geographic proximity has a strong influence on the movement of alleles within the population. Population structure has profound effects on possible mechanisms of evolution.

The effect of genetic drift depends strongly on the size of the population: drift is important in small mating populations, where chance fluctuations from generation to generation can be large. The relative importance of natural selection and genetic drift in determining the fate of new mutations also depends on the population size and the strength of selection. Natural selection is predominant in large populations, while genetic drift is in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size—smaller populations require a shorter time for fixation.

An example of the effect of population structure is the founder effect, in which a population temporarily has very few individuals as a result of a migration or population bottleneck, and therefore loses much genetic variation. In this case, a single, rare allele may suddenly increase very rapidly in frequency within a specific population if it happened to be prevalent in a small number of "founder" individuals. The frequency of the allele in the resulting population can be much higher than otherwise expected, especially for deleterious, disease-causing alleles. Since population size has a profound effect on the relative strengths of genetic drift and natural selection, changes in population size can alter the dynamics of these processes considerably.

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Speciation and extinction

An Allosaurus skeleton.
An Allosaurus skeleton.

Speciation is the process by which new biological species arise. This may take place by various mechanisms. Allopatric speciation occurs in populations that become isolated geographically, such as by habitat fragmentation or migration.[24] Sympatric speciation occurs when new species emerge in the same geographic area.[25][26] Ernst Mayr's peripatric speciation is a type of speciation that exists in between the extremes of allopatry and sympatry. Peripatric speciation is a critical underpinning of the theory of punctuated equilibrium. An example of rapid sympatric speciation can be clearly observed in the triangle of U, where new species of Brassica sp. have been made by the fusing of separate genomes from related plants.

Extinction is the disappearance of species (i.e., gene pools). The moment of extinction is generally defined as occurring at the death of the last individual of that species. Extinction is not an unusual event on a geological time scale—species regularly appear through speciation, and disappear through extinction. The Permian-Triassic extinction event was the Earth's most severe extinction event, rendering extinct 90% of all marine species and 70% of all terrestrial vertebrate species. In the Cretaceous-Tertiary extinction event, many forms of life perished (including approximately 50% of all genera), the most commonly mentioned among them being the non-avian dinosaurs. The Holocene extinction event is a current mass extinction, involving the rapid extinction of tens or hundreds of thousands of species each year. Scientists consider human activities to be the primary cause of the ongoing extinction event, as well as the related influence of climate change.[27]

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Cooperation

Generally mathematical models incorporating mutation and natural selection have been used to model adaptation and evolution. Recent trends now incorporate "game theory" as more applicable to generating reliable models.[28] This work and others studies have focused attention on cooperation as a fundamental property needed for evolution to construct new levels of organization. Selfish replicators sacrificing their own reproductive potential to cooperate seems paradoxical in a competitive world. However a number of mechanisms have demonstrated the capacity to generate cooperation, and even altruism, such as kin selection, direct reciprocity, indirect reciprocity, network reciprocity, and group selection. The ubiquity of cooperation in the natural world and studies from the last twenty years reveal cooperation as a significant principle in constructive evolution.[29][30]

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Evidence of evolution

Tiktaalik in context: one of many species that track the evolutionary development of fish fins into tetrapod limbs.
Tiktaalik in context: one of many species that track the evolutionary development of fish fins into tetrapod limbs.

Evolution has left numerous signs of the histories of different species. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species.

The development of molecular genetics, and particularly of DNA sequencing, has allowed biologists to study the record of evolution left in organisms' genetic structures. The degrees of similarity and difference in the DNA sequences of modern species allows geneticists to reconstruct their lineages. It is from DNA sequence comparisons that figures such as the 95% genotypic similarity between humans and chimpanzees are obtained.[31][32]

Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance, monotremes and most marsupials are found only in Australia, showing that their common ancestor with placental mammals lived before the submerging of the ancient land bridge between Australia and Asia.

Scientists correlate all of the above evidence, drawn from paleontology, anatomy, genetics, and geography, with other information about the history of Earth. For instance, paleoclimatology attests to periodic ice ages during which the world's climate was much cooler, and these are often found to match up with the spread of species which are better-equipped to deal with the cold, such as the woolly mammoth.

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Morphological evidence

Letter c in the picture indicates the undeveloped hind legs of a baleen whale, vestigial remnants of its terrestrial ancestors.
Letter c in the picture indicates the undeveloped hind legs of a baleen whale, vestigial remnants of its terrestrial ancestors.

Fossils are critical evidence for estimating when various lineages originated. Since fossilization of an organism is an uncommon occurrence, usually requiring hard parts (like teeth, bone, or pollen), the fossil record provides only sparse and intermittent information about ancestral lineages.[33]

The fossil record provides several types of data important to the study of evolution. First, the fossil record contains the earliest known examples of life itself, as well as the earliest occurrences of individual lineages. For example, the first complex animals date from the early Cambrian period, approximately 520 million years ago. Second, the records of individual species yield information regarding the patterns and rates of evolution, showing whether, for example, speciation occurs gradually and incrementally, or in relatively brief intervals of geologic time. Thirdly, the fossil record is a document of large-scale patterns and events in the history of life. For example, mass extinctions frequently resulted in the loss of entire groups of species, while leaving others relatively unscathed. Recently, molecular biologists have used the time since divergence of related lineages to calibrate the rate at which mutations accumulate, and at which the genomes of different lineages evolve.

Phylogenetics, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. The appendages on bat wings, for example, are very structurally similar to human hands, and may constitute a vestigial structure. Vestigial structures are idiosyncratic anatomical features such as the panda's "thumb", which indicate how an organism's evolutionary lineage constrains its adaptive development. Other examples of vestigial structures include the degenerate eyes of blind cave-dwelling fish, and the presence of hip bones in whales and snakes. Such structures may exist with little or no function in a more current organism, yet have a clear function in an ancestral species. Examples of vestigial structures in humans include wisdom teeth, the coccyx and the vermiform appendix.

These anatomical similarities in extant and fossil organisms can give evidence of the relationships between different groups of organisms. Important fossil evidence includes the connection of distinct classes of organisms by so-called "transitional" species, such as the Archaeopteryx, which provided early evidence for intermediate species between dinosaurs and birds,[34] and the recently-discovered Tiktaalik, which clarifies the development from fish to animals with four limbs.[35]

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Molecular evidence

By comparing the DNA sequences of species, we can find out their evolutionary relationships. The resultant phylogenetic trees are typically congruent with traditional taxonomy, and are often used to either strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons.[36] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes.[37][38] The sequence of the 16S rRNA gene, a vital gene encoding a part of the ribosome, was used to find the broad phylogenetic relationships between all extant life. This analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern bacteria, and the subsequent split led to modern archaea and eukaryotes.

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or by detecting their physical manifestations. As an example, the appearance of oxygen in the earth's atmosphere is linked to the evolution of photosynthesis.

The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and RNA polymerase, are found in everything from the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunits, largely affecting the regulation and protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNA, RNA, amino acids, and the lipid bilayer, give support to the theory of common descent. The chirality of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right- or left-handed molecular chirality, the simplest hypothesis is that the choice was made randomly by early organisms and passed on to all extant life through common descent. Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes, "dead" genes which steadily accumulate mutations.[39]

There is also a large body of molecular evidence for a number of different mechanisms for large evolutionary changes, among them: genome and gene duplication, which facilitates rapid evolution by providing substantial quantities of genetic material under weak or no selective constraints; horizontal gene transfer, the process of transferring genetic material to another cell that is not an organism's offspring, allowing for species to acquire beneficial genes from each other; recombination, capable of reassorting large numbers of different alleles and of establishing reproductive isolation; and endosymbiosis, the incorporation of genetic material and biochemical composition of a separate species, a process observed in organisms such as the protist hatena and used to explain the origin of organelles such as mitochondria and plastids as the absorption of ancient prokaryotic cells into ancient eukaryotic ones.[40][41]

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Ancestry of organisms

Morphologic similarities in the Hominidae family are evidence of common descent.
Morphologic similarities in the Hominidae family are evidence of common descent.

The theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool. Evidence for common descent is inferred from traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds, even those which do not fly, have wings. Today, there is strong evidence from genetics that all organisms have a common ancestor. For example, every living cell makes use of nucleic acids as its genetic material, and uses the same 20 amino acids as the building blocks for proteins. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary.[42]

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History of life

Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the journal Nature arguing that formations such as this possess 3.5 billion year old fossilized algae microbes. If true, they would be the earliest known life on earth.
Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the journal Nature arguing that formations such as this possess 3.5 billion year old fossilized algae microbes. If true, they would be the earliest known life on earth.

The origin of life from self-catalytic chemical reactions is not a part of biological evolution, but rather of pre-evolutionary abiogenesis. However, disputes over what defines life make the point at which such increasingly complex sets of reactions became true organisms unclear. Not much is yet known about the earliest developments in life. There is no scientific consensus regarding the relationship of the three domains of organisms (Archaea, Bacteria, and Eukaryota) or regarding the precise reactions involved in abiogenesis. Attempts to shed light on the origin of life generally focus on the behavior of macromolecules—particularly RNA—and the behavior of complex systems.

Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the 4.57 billion year history of Earth. Oxygenic photosynthesis emerged around 3 billion years ago, and the subsequent emergence of an oxygen-rich atmosphere made the development of aerobic cellular respiration possible around 2 billion years ago. In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion, a geologically brief period of remarkable biological diversity, originated all the major body plans, or phyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes.

About 500 million years ago (mya), plants and fungi colonized the land, and were soon followed by arthropods and other animals. Amphibians first appeared around 300 mya, followed by reptiles, then mammals around 200 mya and birds around 100 mya. The human genus arose around 2 mya, while the earliest modern humans lived 200 thousand years ago.

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Study of evolution

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History of modern evolutionary thought

Gregor Mendel's work on the inheritance of traits in pea plants laid the foundation for genetics.
Gregor Mendel's work on the inheritance of traits in pea plants laid the foundation for genetics.

Although the idea of evolution has existed since classical antiquity, being first discussed by Greek philosophers such as Anaximander, the first convincing exposition of a mechanism by which evolutionary change could occur was not proposed until Charles Darwin and Alfred Russel Wallace jointly presented the theory of evolution by natural selection to the Linnean Society of London in separate papers in 1858. Shortly after, the publication of Darwin's On the Origin of Species by Charles Darwin popularized and provided detailed support for the theory.

However, Darwin had no working mechanism for inheritance. This was provided by Gregor Mendel, whose research revealed that distinct traits were inherited in a well-defined and predictable manner.[43]

In the 1930s, Darwinian natural selection and Mendelian inheritance were combined to form the modern evolutionary synthesis. In the 1940s, the identification of DNA as the genetic material by Oswald Avery and colleagues, and the articulation of the double-helical structure of DNA by James Watson and Francis Crick, provided a physical basis for the notion that genes were encoded in DNA. Since then, the role of genetics in evolutionary biology has become increasingly central.[44]

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Academic disciplines

Scholars in a number of academic disciplines continue to document examples of evolution, contributing to a deeper understanding of its underlying mechanisms. Every subdiscipline within biology both informs and is informed by knowledge of the details of evolution, such as in ecological genetics, human evolution, molecular evolution, and phylogenetics. Areas of mathematics (such as bioinformatics), physics, chemistry, and other fields all make important contributions to current understanding of evolutionary mechanisms. Even disciplines as far removed as geology and sociology play a part, since the process of biological evolution has coincided in time and space with the development of both the Earth and human civilization.

Evolutionary biology is a subdiscipline of biology concerned with the origin and descent of species, as well as their changes over time. It was originally an interdisciplinary field including scientists from many traditional taxonomically-oriented disciplines. For example, it generally includes scientists who may have a specialist training in particular organisms, such as mammalogy, ornithology, or herpetology, but who use those organisms to answer general questions in evolution. Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles.

Evolutionary developmental biology (informally, evo-devo) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. The discovery of genes regulating development in model organisms allowed for comparisons to be made with genes and genetic networks of related organisms.

Physical anthropology emerged in the late 19th century as the study of human osteology, and the fossilized skeletal remains of other hominids. At that time, anthropologists debated whether their evidence supported Darwin's claims, because skeletal remains revealed temporal and spatial variation among hominids, but Darwin had not offered an explanation of the specific mechanisms that produce variation. With the recognition of Mendelian genetics and the rise of the modern synthesis, however, evolution became both the fundamental conceptual framework for, and the object of study of, physical anthropologists. In addition to studying skeletal remains, they began to study genetic variation among human populations (population genetics); thus, some physical anthropologists began calling themselves biological anthropologists.

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Misunderstandings

There are a number of common misunderstandings about evolution, some of which have hindered its general acceptance.[45][46][47] Critics of evolution frequently assert that evolution is "just a theory", a misunderstanding of the meaning of theory in a scientific context: whereas in colloquial speech a theory is a conjecture or guess, in science a theory is "a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make" [48] Critics also state that evolution is not a fact, although from a scientific viewpoint evolution is considered both a theory and a fact.[49][50][51] A related, more extreme claim is that evolution is a "theory in crisis", generally based on misrepresenting the scientific support and evidence for evolutionary theory.[52]

Another common misunderstanding is the idea that one species, such as humans, can be more "highly evolved" or "advanced" than another. It is often assumed that evolution must lead to greater complexity, or that devolution ("backwards" evolution) can occur. Scientists consider evolution a non-directional process that does not proceed toward any ultimate goal; advancements are only situational, and organisms' complexity can either increase, decrease, or stay the same, depending on which is advantageous, and thus selected for.[53]

Evolution is also frequently misinterpreted as stating that humans evolved from monkeys; based on this, some critics argue that monkeys should no longer exist. This misunderstands speciation, which frequently involves a subset of a population cladogenetically splitting off before speciating, rather than an entire species simply turning into a new one. Additionally, biologists have never claimed that humans evolved from monkeys—only that humans and monkeys share a common ancestor, as do all organisms.[54]

It is also frequently claimed that speciation has only been inferred, never directly observed. In reality, the evolution of numerous new species has been observed.[55] A similar claim is that only microevolution, not macroevolution, has been observed; however, macroevolution has been observed as well, and modern evolutionary synthesis draws little distinction between the two, considering macroevolution to simply be microevolution on a larger scale.[56]

Other widespread misunderstandings of evolution include the idea that evolution violates the second law of thermodynamics, which applies to isolated systems, not open systems like the earth, which absorbs light from the sun an radiates heat to space; and that evolution cannot create new physical information, although this regularly occurs whenever a novel mutation or gene duplication arises.[57]

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Social and religious controversies

This caricature of Charles Darwin as an ape reflects the cultural backlash against evolution and common descent.
This caricature of Charles Darwin as an ape reflects the cultural backlash against evolution and common descent.

Ever since the publication of The Origin of Species in 1859, evolution has been a source of nearly constant controversy. In general, controversy has centered on the philosophical, social, and religious implications of evolution, not on the science of evolution itself; the proposition that biological evolution occurs through the mechanism of natural selection is completely uncontested within the scientific community.[58]

As Darwin recognized early on, perhaps the most controversial aspect of evolutionary thought is its applicability to human beings. Specifically, many object to the idea that all diversity in life, including human beings, arose through natural processes without a need for supernatural intervention. Although many religions, such as Catholicism, have reconciled their beliefs with evolution through theistic evolution, creationists argue against evolution on the basis that it contradicts their theistic origin beliefs.[59] In some countries—notably the United States—these tensions between scientific and religious teachings have fueled the ongoing creation-evolution controversy, a social and religious conflict especially centering on public education. While many other fields of science, such as cosmology[60] and earth science[61] also conflict with a literal interpretation of many religious texts, evolutionary biology has borne the brunt of these debates. Some also argue that evolutionary common descent "degrades" human beings by placing them on the same level as other animals, in contrast with past views of a great chain of being in which humans are "above" animals.

Evolution has been used to support philosophical and ethical choices which most contemporary scientists consider were neither mandated by evolution nor supported by science.[62] For example, the eugenic ideas of Francis Galton were developed into arguments that the human gene pool should be improved by selective breeding policies, including incentives for reproduction for those of "good stock" and disincentives, such as compulsory sterilization, "euthanasia", and later, prenatal testing, birth control, and genetic engineering, for those of "bad stock". Another example of an extension of evolutionary theory that is now widely regarded as unwarranted is "Social Darwinism", a term given to the 19th century Whig Malthusian theory developed by Herbert Spencer into ideas about "survival of the fittest" in commerce and human societies as a whole, and by others into claims that social inequality, racism, and imperialism were justified.[63]

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See also

For a more comprehensive list of topics, see Category:Evolution and Category:Evolutionary biology
  • Abiogenesis
  • Behavioral ecology
  • Convergent evolution
  • Divergent evolution
  • Evolution of sex
  • Evolutionary algorithm
  • Evolutionary art
  • Evolution of multicellularity
  • Evolutionary psychology
  • Evolutionary tree
  • Experimental evolution
  • Gene-centered view of evolution
  • Genetics
  • Gradualism
  • Human behavioral ecology
  • Kin selection
  • List of publications on evolution and human behavior
  • Paleontology
  • Parallel evolution
  • Punctuated equilibrium
  • Sociobiology
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Footnotes

  1. 1.0 1.1 Futuyma, Douglas J. (2005). Evolution. Sunderland, Massachusetts: Sinauer Associates, Inc. ISBN 0-87893-187-2.
  2. Gould, Stephen J. (2002). The Structure of Evolutionary Theory. Belknap Press. ISBN 0-674-00613-5.
  3. Lande, R., Arnold, S.J. (1983). "The measurement of selection on correlated characters". Evolution 37: 1210–1226.
  4. Haldane, J.B.S. (1953). "The measurement of natural selection". Proceedings of the 9th International Congress of Genetics 1: 480–487.
  5. 5.0 5.1 Mechanisms: the processes of evolution. Understanding Evolution. University of California, Berkeley. Retrieved on 2006-07-14.
  6. Myers, PZ. "Ann Coulter: No evidence for evolution?", Pharyngula, scienceblogs.com, 2006-06-18. Retrieved on 2006-11-18.
  7. IAP Statement on the Teaching of Evolution Joint statement issued by the national science academies of 67 countries, including the United Kingdom's Royal Society (PDF file)
  8. From the American Association for the Advancement of Science, the world's largest general scientific society: 2006 Statement on the Teaching of Evolution (PDF file), AAAS Denounces Anti-Evolution Laws
  9. Stephen J. Gould (1997-06-12). Darwinian Fundamentalism. New York Review of Books. Retrieved on 2006-08-01.
  10. Created from PDB 1D65
  11. Beisson, J. & Sonneborn, T. M. (1965). Cytoplasmic inheritance of the organization of the cell cortex of Paramecium aurelia. Proc. natn. Acad Sci. U.S.A. 53, 275-282
  12. Snustad, P. and Simmons, A. 2000. Principles of Genetics, 2nd edition. John Wiley and Sons, Inc. New-York, p.20
  13. Aminetzach YT, Macpherson JM, Petrov DA. (1992). "Pesticide resistance via transposition-mediated adaptive gene truncation in Drosophila.". Science 309 (5735): 764-7..
  14. Carroll S.B,. Grenier J.K., Weatherbee S.D. (2005). From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Second Edition. Oxford: Blackwell Publishing. ISBN 1-4051-1950-0.
  15. enmicro.pdf
  16. Pennisi_2003.pdf
  17. Rieseberg LH, Raymond O, Rosenthal DM, Lai Z, Livingstone K, Nakazato T, Durphy JL, Schwarzbach AE, Donovan LA, Lexer C. (2003). "Major ecological transitions in wild sunflowers facilitated by hybridization.". Science 301 (5637): 1211-6..
  18. Grant, P.R., and Grant, B.R. (1992). "Hybridization in bird species". Science 256 (4061): 193-7..
  19. Gompert Z, Fordyce JA, Forister ML, Shapiro AM, Nice CC. (2006). "Homoploid hybrid speciation in an extreme habitat.". Science 314 (5807): 1923-5.
  20. Dzidic S, Bedekovic V. (2003). "Horizontal gene transfer-emerging multidrug resistance in hospital bacteria". Acta pharmacologica Sinica 24 (6): 519-526.
  21. Andersson JO (2005). "Lateral gene transfer in eukaryotes". Cellular and molecular life sciences 62 (11): 1182-1197.
  22. Katz LA (2002). "Lateral gene transfers and the evolution of eukaryotes: theories and data". International journal of systematic and evolutionary microbiology 52 (5): 1893-1900.
  23. Evolutionary Theory by Peter Gogarten, Ph.D.
  24. Hoskin et al (Oct 2005). "Reinforcement drives rapid allopatric speciation". Nature 437: 1353-1356.
  25. Savolainen et al (May 2006). "Sympatric speciation in palms on an oceanic island". Nature 441: 210-213.
  26. Barluenga et al (February 2006). "Sympatric speciation in Nicaraguan crater lake cichlid fish". Nature 439: 719-723.
  27. Leakey, Richard and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0-385-46809-1.
  28. Nowak et al. "Evolutionary dynamics of biological games"Science"'303'", 793-799 (2004), see also Nowak's book Evolutionary Dynamics
  29. Nowak et al. Five Rules for the Evolution of Cooperation Science 314, 1560 (2006)
  30. Sachs, J.L. Cooperation within and among species Journal of Evolutionary Biology 19, 1415 (2006)
  31. Chimpanzee Sequencing and Analysis Consortium (2005). "Initial sequence of the chimpanzee genome and comparison with the human genome". Nature 437: 69–87.
  32. Britten, R.J. (2002). "Divergence between samples of chimpanzee and human DNA sequences is 5%, counting indels". Proc Natl Acad Sci U S A 99: 13633–13635.
  33. Schweitzer M.H. et al (2005). "Soft-tissue vessels and cellular preservation in Tyrannosaurus rex". Science 307 (5717): 1952-1955.
  34. Feduccia, Alan (1996). The Origin and Evolution of Birds. New Haven: Yale University Press. ISBN 0-300-06460-8.
  35. Daeschler, Edward B., Shubin, Neil H., & Jenkins Jr, Farish A. (April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan". Nature 440: 757–763. DOI:10.1038/nature04639. Retrieved on 2006-07-14.
  36. Two sources: 'Genomic divergences between humans and other hominoids and the effective population size of the common ancestor of humans and chimpanzees'. and 'Quantitative Estimates of Sequence Divergence for Comparative Analysis of Mammalian Genomes' "[1] [2]"
  37. The picture labeled "Human Chromosome 2 and its analogs in the apes" in the article Comparison of the Human and Great Ape Chromosomes as Evidence for Common Ancestry shows how humans have a single chromosome which is two separate chromosomes in the nonhuman apes.
  38. The New York Times report Still Evolving, Human Genes Tell New Story, based on A Map of Recent Positive Selection in the Human Genome, states the International HapMap Project is "providing the strongest evidence yet that humans are still evolving" and details some of that evidence.
  39. Pseudogene evolution and natural selection for a compact genome. "[3]"
  40. Okamoto N, Inouye I. (2005). "A secondary symbiosis in progress". Science 310 (5746): 287.
  41. Okamoto N, Inouye I. (2006). "Hatena arenicola gen. et sp. nov., a Katablepharid Undergoing Probable Plastid Acquisition.". Protist Article in Print.
  42. Oklahoma State - Horizontal Gene Transfer: "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic 'domains'. Thus determining the phylogenetic history of a species cannot be done conclusively by determining evolutionary trees for single genes."
  43. Bowler, Peter J. (1989). The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Baltimore: John Hopkins University Press.
  44. Rincon, Paul. "Evolution takes science honours", BBC News, 2005. Retrieved on 2006-07-16. According to the BBC, Colin Norman, news editor of Science, said "[S]cientists tend to take for granted that evolution underpins modern biology [...] Evolution is not just something that scientists study as an esoteric enterprise. It has very important implications for public health and for our understanding of who we are" and Dr. Mike Ritchie, of the school of biology at the University of St Andrews, UK said "The big recent development in evolutionary biology has obviously been the improved resolution in our understanding of genetics. Where people have found a gene they think is involved in speciation, I can now go and look how it has evolved in 12 different species of fly, because we've got the genomes of all these species available on the web."
  45. BBC Report on Biology Education in North America "In a study of 1,200 college freshmen, Professor Alters found that 45% of those who doubted the theory of evolution had specific misunderstandings about some of the science that has been used to support it."
  46. Constance Holden (1998). "SCIENCE EDUCATION: Academy Rallies Teachers on Evolution". Science 280 (5361): 194.
  47. Miller JD, Scott EC, Okamoto S. (2006). "Science communication. Public acceptance of evolution.". Science 313 (5788): 765-766.
  48. Stephen Hawking A Brief History of Time: From the Big Bang to Black Holes New York: Bantam Books 1988.
  49. Five Major Misconceptions about Evolution, Mark Isaak, Talkorigins, 2003
  50. Stephen Jay Gould, " Evolution as Fact and Theory"; Discover, Volume 2, Number 5, May 1981, p. 34-37, reprinted in Speak Out Against The New Right, Herbert F. Vetter (Editor), Beacon Press, 1982, ISBN 0807004863, Beacon Press, January 1982, ISBN 0807004871 and by Fenestra Books, October 31, 2004 ISBN 1587363577 and also in Hen's Teeth and Horse's Toes, Stephen Jay Gould, New York: W. W. Norton & Company, editions printed April 1983, November 28, 1984 and April 1994, pp. 253-262 ISBN 0393017168
  51. "Evolution: Fact and Theory", Richard E. Lenski, American Institute of Biological Sciences, 2000.
  52. Morton, G.R. (2002). The Imminent Demise of Evolution: The Longest Running Falsehood in Creationism
  53. Scientific American; Biology: Is the human race evolving or devolving?
  54. Index to Creationist Claims, Claim CC150 edited by Mark Isaak. The TalkOrigins Archive, 2005
  55. Boxhorn, Joseph. Observed Instances of Speciation. Talk Origins Archive.
  56. Theobald, Douglas L. '29+ Evidences for Macroevolution: The Scientific Case for Common Descent.' The Talk.Origins Archive. Vers. 2.83. 2004. 12 Jan, 2004 <http://www.talkorigins.org/faqs/comdesc/>
  57. Evolution and Information: The Nylon Bug. New Mexicans for Science and Reason.
  58. An overview of the philosophical, religious, and cosmological controversies by a philosopher who strongly supports evolution is: Daniel Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life (New York: Simon & Schuster, 1995). On the scientific and social reception of evolution in the 19th and early 20th centuries, see: Peter J. Bowler, Evolution: The History of an Idea, 3rd. rev. edn. (Berkeley: University of California Press, 2003).
  59. [4]
  60. Spergel, D. N., et al. (2003). "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters". The Astrophysical Journal Supplement Series 148: 175—194. DOI:10.1086/377226.
  61. Wilde, S. A., Valley J. W., Peck W. H. and Graham C. M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature 409: 175—178.
  62. Darwin strongly disagreed with attempts by Herbert Spencer and other to extrapolate evolutionary ideas to all possible subject matters; see Mary Midgley The Myths we Live By Routledge 2004 p62.
  63. On the history of eugenics and evolution, see Daniel Kevles, In the Name of Eugenics: Genetics and the Uses of Human Heredity (New York: Knopf, 1985).
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References

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External links

Evolution simulators
Basic topics in evolutionary biology
Evidence of evolution
Processes of evolution: adaptation - macroevolution - microevolution - speciation
Population genetic mechanisms: selection - genetic drift - gene flow - mutation
Evo-devo concepts: phenotypic plasticity - canalisation - modularity
Modes of evolution: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Other subfields: ecological genetics - human evolution - molecular evolution - phylogenetics - systematics
List of evolutionary biology topics | Timeline of evolution
Topics in population genetics
Key concepts: Hardy-Weinberg law | linkage disequilibrium | Fisher's fundamental theorem | neutral theory
Selection: natural | sexual | artificial | ecological
Genetic drift: small population size | population bottleneck | founder effect | coalescence
Founders: R.A. Fisher | J.B.S. Haldane | Sewall Wright
Related topics: evolution | microevolution | evolutionary game theory | fitness landscape | genetic genealogy
List of evolutionary biology topics
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