Any gradual directional change; now most commonly used to refer to the cumulative changes in the characteristics of populations of organisms from generation to generation. Evolution occurs by the fixation of changes (mutations) in the structure of the genetic material, and the passing on of these changes from ancestor to descendant. It is well demonstrated over geological time by the sequence of organisms preserved in the fossil record. There are two opposing schools of thought regarding the pattern and tempo of evolution. The gradualist school is based on a model of evolution in which species change gradually through time by slow directional change within a lineage, producing a long graded series of differing forms. The punctuated equilibria school is based on a model in which species are relatively stable and long-lived in geological time, and that new species appear during outbursts of rapid speciation, followed by the differential success of certain of the newly formed species.
This article is about evolution in biology. For other uses, see Evolution (disambiguation).In biology, evolution is change in the heritable traits of a population over successive generations, as determined by shifts in the allele frequencies of genes. Over time, this process can result in speciation, the development of new species from existing ones. Evolution is thus the source of the vast diversity of life on Earth, including the many extinct species attested to in the fossil record.
The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift; these two mechanisms act on the genetic variation created by mutation, genetic recombination, and gene flow. Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce.
The modern understanding of evolution is based on the theory of natural selection, which was first set out in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace and popularized in Darwin's 1859 book The Origin of Species. In the 1930s, Darwinian natural selection was combined with the theory of Mendelian heredity to form the modern evolutionary synthesis, also known as "Neo-Darwinism". The modern synthesis describes evolution as a change in the frequency of alleles within a population from one generation to the next.
Although there is overwhelming scientific consensus supporting the validity of evolution, it has been at the center of many social and religious controversies since its inception because of its potential implications for the origins of humankind.
Study of evolution
History of evolutionary thought
The idea of biological evolution has existed since ancient times, notably among Greek philosophers such as Anaximander and Epicurus and Indian philosophers such as Patañjali. However, scientific theories of evolution were not proposed until the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin.
The transmutation of species was accepted by many scientists before 1859, but Charles Darwin's On The Origin of Species by Means of Natural Selection provided the first convincing exposition of a mechanism by which evolutionary change could occur: natural selection. After many years of working in private on his theory, Darwin was motivated to publish his work on evolution when he received a letter from Alfred Russel Wallace in which Wallace revealed his own, independent discovery of natural selection.
The publication of Darwin's book sparked a great deal of scientific and social debate. Although the occurrence of biological evolution of some sort came to be widely accepted by scientists, Darwin's specific ideas about evolution—that it occurred gradually, through natural selection—were actively attacked and contested. Additionally, while Darwin was able to observe variation, and to infer natural selection and thereby adaptation, he was unable to explain how variation might arise or be altered over generations.
Work on plant hybridity by a contemporary of Darwin's, Gregor Mendel, revealed that certain traits in peas occurred in discrete forms (that is, they were either one distinct trait or another, such as "round" or "wrinkled") and were inherited in a well-defined and predictable manner. When Mendel's work was "rediscovered" in 1901, it was initially interpreted as supporting an anti-Darwinian "jumping", saltationist form of evolution, and contradicting the biometricians' gradualism.
However, the simple version of the theory of early Mendelians soon gave way to the classical genetics of Thomas Hunt Morgan and his school, which thoroughly grounded and articulated the applications of Mendelian laws to biology. Following this, the work of population geneticists and zoologists in the 1930s and 1940s synthesized Darwinian evolution with genetics, creating the modern evolutionary synthesis. Genes were then still theoretical entities, and many paleontologists and embryologists were inclined to dismiss them as being of no, or minor, importance, but subsequent advancements have made genetics a key aspect of evolutionary biology.
The most significant recent developments in evolutionary biology have been the improved understanding of and advances in genetics. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process (see molecular evolution): the mutation of segments of DNA. Williams' 1966 Adaptation and natural selection: A Critique of some Current Evolutionary Thought and Richard Dawkins' The Selfish Gene marked a departure from the idea of groups or organisms as units of selection toward the modern gene-centered view of evolution. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a mechanism of evolution.
Debates over various aspects of how evolution occurs have continued. One prominent debate was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of gradual transitions between species in the fossil record, as well as the absence of change or stasis that is observed over significant intervals of time.
Academic disciplines
Scholars in a number of academic disciplines continue to document examples of the theory 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 foundational contributions to the theory of evolution. 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. 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 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. 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);
Evidence of evolution
Evolution has left numerous records which reveal the history of different species. 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, and the recently-discovered Tiktaalik, which clarifies the development from fish to animals with four limbs.
The development of molecular genetics, and particularly of DNA sequencing, has allowed biologists to study the record of evolution left in the organisms' genetic structures.
Additional evidence of ancestry includes idiosyncratic structures present in certain organisms, such as the panda's "thumb", which indicate how an organism's evolutionary lineage constrains its adaptive development.
Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species.
Scientists correlate all of the above evidence, drawn from paleontology, anatomy, genetics, and geography, with other information about the history of Earth.
Morphological evidence
Fossils are critical evidence for estimating when various lineages originated.
The fossil record provides several types of data important to the study of evolution. Second, the records of individual species yield information regarding the patterns and rates of evolution, showing for example if species evolve into new species (speciation) gradually and incrementally, or in relatively brief intervals of geologic time.
Phylogenetics, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions.
Molecular evidence
Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogenetic trees are typically congruent with traditional taxonomy, and are often used to strengthen or correct taxonomic classifications. 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. The analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life.
The proteomic evidence also supports the universal ancestry of life.
Molecular evidence also offers a mechanism for large evolutionary changes. Horizontal gene transfer, the process in which an organism transfers genetic material (i.e. DNA) to another cell that is not its offspring, allows for large sudden evolutionary leaps in a species by incorporating beneficial genes evolved in another species. Rather than evolving eukaryotic organelles slowly, this theory offers a mechanism for a large evolutionary changes by incorporating the genetic material and biochemical composition of a separate species. Hatena, a protist, is an extant organism that is undergoing endosymbiotic evolution.
Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes, i.e., 'dead' genes, which steadily accumulate mutations.
Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. As an example, the appearance of oxygen in the earth's atmosphere is linked to the evolution of photosynthesis.
Theoretical evidence
Mathematical models of evolution, pioneered by the likes of Sewall Wright, Ronald Fisher and J. For example, the "Out of Africa" theory of human origins, which states that modern humans developed in Africa and a small sub-population migrated out (undergoing a population bottleneck), implies that modern populations should show the signatures of this migration pattern.
Ancestry of organisms
See also: Common descentIn biology, 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.
Information about the early development of life includes input from the fields of geology and planetary science. However, a great deal of information about the early Earth has been destroyed by
geological processes over the course of time.
History of life
The chemical evolution (or abiogenesis) from self-catalytic chemical reactions to life (see Origin of life) is not a part of biological evolution, but it is unclear at which point such increasingly complex sets of reactions became what we would consider, today, to be living organisms.
Not much is known about the earliest developments in life. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, Eukaryota) or the origin of life.
The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides.
In the last billion years, simple multicellular plants and animals began to appear in the oceans.
The evolutionary process may be exceedingly slow. Studies on guppies by David Reznick at the University of California, Riverside, however, have shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record.
The ancestry of living organisms has traditionally been reconstructed from morphology, but is increasingly supplemented with phylogenetic — the reconstruction of phylogenies by the comparison of genetic (usually DNA) sequence.
Modern synthesis
Charles Darwin was able to observe variation, infer natural selection and thereby adaptation, but didn't know the basis of heritability.
The blending problem was solved when the population geneticists R.A. Haldane, married Darwinian evolutionary theory to population genetic theory, which was based on Mendelian genetics (genes as discrete units of heredity).
The problem of what the mechanisms might be was solved in principle with 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.
Heredity
Gregor Mendel's work provided the first firm basis to the idea that heredity occurred in discrete units.
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.
Non-DNA based forms of heritable variation exist, such transmission of the secondary structures of prions, and structural inheritance of patterns in the rows of cilia in protozoans such as Paramecium and Tetrahymena. 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.
Variation
Evolutionary changes are the product of evolutionary forces acting on genetic variation. This phenotypic variation is the result of variants in gene sequences among the individuals of a population.
All genetic variation begins as a new mutation in a single individual; 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.
Mechanisms of evolution
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 variation. Gould once phrased this succinctly as "variation proposes and selection disposes."
Mutation
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;
Mutations that are not affected by natural selection are called 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.
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.
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. 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 abberant 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).
Selection and adaptation
Natural selection comes from differences in survival and reproduction . Note that, whereas mutations and genetic drift are random, natural selection is not, as it preferentially selects for different mutations based on differential fitnesses. 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. 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 also operates on mutations in several different ways:
Positive or directional selection increases the frequency of a beneficial mutation, or pushes the mean in either direction. Purifying or stabilizing selection maintains a common trait in the population by decreasing the frequency of harmful mutations and weeding them out of the population. It is argued that stabilizing selection is the most common form of natural selection. Balancing selection maintains variation within a population through a number of mechanisms, including: Heterozygote advantage or overdominance, where the heterozygote is more fit than either of the homozygous forms (exemplified by human sickle cell anemia conferring resistance to malaria) Frequency-dependent selection, where rare variants either have increased fitness or decreased fitness, because of their rarity. Disruptive selection favors both extremes, and results in a bimodal distribution of gene frequency. It comes in two forms: Background selection occurs when a deleterious mutation is selected against, and linked mutations are eliminated along with the deleterious variant, resulting in lower genetic polymorphism in the surrounding region. Genetic hitchhiking occurs when a beneficial allele is selected for, and linked alleles, which can be neutral or beneficial, are pushed towards fixation along with the beneficial allele.Through the process of natural selection, organisms become better adapted to their environments.
Evolution does not act in a linear direction towards a pre-defined "goal" — it only responds to various types of adaptationary changes. The belief in a teleological evolution of this sort is known as orthogenesis, and is not supported by the scientific understanding of evolution.
Most biologists believe that adaptation occurs through the accumulation of many mutations of small effect.
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. This shuffling allows independent assortment of alleles (mutations) in genes to be propagated in the population independently.
However, the meitoic recombination rate is not very high - on the order of one crossover (recombination event between homomolgous chromosomes) per chromosome arm per generation.
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), we see a reduction in effective population size, known as the Hill-Robertson effect, and the successive establishment of bad mutations, known as Muller's ratchet.
Gene flow and Population structure
Gene flow (also called gene admixture or simply migration) is the exchange of genetic variation between populations, when geography and culture are not obstacles.
The free movement of alleles through a population may also be impeded by population structure.
An example of the effect of population structure is the so-called founder effect, resulting from a migration or population bottleneck, in which a population temporarily has very few individuals, and therefore loses a lot of genetic variation. 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.
Drift
Genetic drift describes changes in allele frequency from one generation to the next due to sampling variance. Thus, over time even in the absence of selection upon the alleles, allele frequencies will tend to "drift" upward or downward, eventually becoming "fixed" - that is, going to 0% or 100% frequency. 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).
The consequence of genetic drift depends strongly on the size of the population (generally abbreviated as N): drift is important in small mating populations (see Founder effect and Population bottleneck), 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: when N times s (population size times strength of selection) is small, genetic drift predominates. Thus, natural selection is predominant in large populations, or equivalently, genetic drift is stronger 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, with smaller populations requiring a shorter time to fixation.
Horizontal gene transfer
Horizontal gene transfer (HGT) (or Lateral gene transfers) is any process in which an organism transfers genetic material (i.e. This mechanism allows for the transfer of genetic material between unrelated organisms of the same species or of different species.
Many mechanisms for horizontal gene transfer have been observed, such as antigenic shift, reassortment, and hybridization. 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".
HGT has been shown to result in the spread of antibiotic resistance across bacterial populations. Furthermore, findings indicate that HGT has been a major mechanism for prokaryotic and eukaryotic evolution.
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."
Speciation and extinction
Speciation is the process by which new biological species arise.
Extinction is the disappearance of species (i.e. gene pools). In the Cretaceous-Tertiary extinction event many forms of life perished (including approximately 50% of all genera), the
most often mentioned among them being the extinction of the non-avian dinosaurs.
Misunderstandings about modern evolutionary biology
Although the modern synthesis is a central theory in science, many misunderstandings about ideas of modern synthesis prevail among some within the general population.
Distinctions between theory and fact
Further information: Scientific Theory See also: Theory vs. FactStephen Jay Gould explained that "evolution is a theory.
The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. A theory is an attempt to identify and describe relationships between phenomena or things, and generates falsifiable predictions which can be tested through controlled experiments and empirical observation.
In this scientific sense, "facts" are what theories attempt to explain. In the same way, heritable variation, natural selection, and response to selection (e.g. in domesticated plants and animals) are "facts", and the generalization or extrapolation beyond these phenomena, and the explanation for them, is the "theory of evolution".
Evolution and devolution
One of the most common misunderstandings of evolution is that one species can be "more highly evolved" than another, that evolution is necessarily progressive and/or leads to greater "complexity", or that its converse is "devolution". Evolution provides no assurance that later generations are more intelligent or complex than earlier generations. The claim that evolution results in progress is not part of modern evolutionary theory;
In many cases evolution does involve "progression" towards more complexity, since the earliest lifeforms were extremely simple compared to many of the species existing today, and there was nowhere to go but up. Evolution caused life to become more complex, since becoming simpler wasn't advantageous.
Speciation
It is sometimes claimed that speciation — the origin of new species — has never been directly observed, and thus evolution cannot be called sound science. This is a misunderstanding of both science and evolution. Moreover, since the publication of On the Origin of Species scientists have confirmed Darwin's hypothesis by data gathered from sources that did not exist in his day, such as DNA similarity among species and new fossil discoveries. (See the hawthorn fly example.) Further, there are a number of examples of speciation in plants, and differences in ectodysplasin alleles in stickleback fish speciation has developed as a supermodel for studying gene alterations and speciation.
A variation of this assertion, that microevolution has been directly observed and macroevolution has not, is subject to the same counterarguments.
Entropy and life
It is claimed that evolution, by increasing complexity without supernatural intervention, violates the second law of thermodynamics. Simple calculations show that the Sun-Earth-space system does not violate the second law because the enormous increase in entropy due to the Sun and Earth radiating into space dwarfs the small decrease in entropy caused by the evolution of life.
In statistical thermodynamics entropy has been envisioned as a measure of the statistical "disorder" at a microstate level, leading to the mistaken idea that entropy implies increasing chaos.
The flow of matter and energy allows self-organization, enabling an increase in complexity without guidance or management.
Self assembly is ubiquitous in biological systems and for nanostructures under equilibrium and some in non-equilibrium conditions.
Information
Some assert that evolution cannot create information, or that information can only be created by an intelligence. Physical information exists regardless of the presence of an intelligence, and evolution allows for new information whenever a novel mutation or gene duplication occurs and is kept.
Japanese researchers demonstrated that nylon degrading ability can be obtained de novo in laboratory cultures of Pseudomonas aeruginosa strain POA, which initially had no enzymes capable of degrading nylon oligomers. Using the structural results, the authors propose "that the amino acid replacements in the catalytic cleft of a preexisting esterase with the beta-lactamase fold resulted in the evolution of the" nylon-digesting enzyme.
Social and religious controversies
Starting with the publication of The Origin of Species in 1859, the modern science of evolution has been a source of nearly constant controversy. In general, controversy has centered on the philosophical, cosmological, 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 has been almost completely uncontested within the scientific community for much of the 20th century.
As Darwin recognized early on, perhaps the most controversial aspect of evolutionary thought is its applicability to human beings. Many religious people are able to reconcile the science of evolution with their faith, or see no real conflict; The idea that faith and evolution are compatible has been called theistic evolution. Another group of religious people, generally referred to as creationists, consider evolutionary origin beliefs to be incompatible with their faith, their religious texts and their perception of design in nature, and so cannot accept what they call "unguided evolution".
One particularly contentious topic evoked by evolution is the biological status of humanity. Whereas the classical religious view can broadly be characterized as a belief in the great chain of being (in which people are "above" the animals but slightly "below" the angels), the science of evolution shows that humans are animals and share common ancestry with chimpanzees, gibbons, gorillas, and orangutans, which some people find offensive, for, in their opinion, it "degrades" humankind. A related conflict arises when critics combine the religious view of people's superior status with the mistaken notion that evolution is necessarily "progressive". Because animals that are inferior creatures do demonstrably exist, those criticising evolution sometimes incorrectly take this as supporting their claim that evolution is false.
In some countries — notably the United States — these and other tensions between religion and science have fueled what has been called the creation-evolution controversy, which, among other things, has generated struggles over teaching curricula.
Evolution has been used to support philosophical and ethical choices which most modern scientists argue are neither mandated by evolution nor supported by science. 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".
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