Evolution
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Evolution is the change in the
The
In the early 20th century,
All life on Earth—including
Heredity
Evolution in organisms occurs through changes in heritable characteristics—the inherited characteristics of an organism. In humans, for example,
The complete set of observable traits that make up the structure and behaviour of an organism is called its
Heritable characteristics are passed from one generation to the next via
Sources of variation
Evolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is very similar among all individuals of that species.[28] However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in genotype can lead to dramatic differences in phenotype both within and between species.
An individual organism's phenotype results from both its genotype and the influence of the environment it has lived in.[27] The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.[29]
Mutation
Mutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms.[30] When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect.
About half of the mutations in the coding regions of protein-coding genes are deleterious — the other half are neutral. A small percentage of the total mutations in this region confer a fitness benefit.[31] Some of the mutations in other parts of the genome are deleterious but the vast majority are neutral. A few are beneficial.
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.[32] Extra copies of genes are a major source of the raw material needed for new genes to evolve.[33] This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors.[34] 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.[35]
New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. 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.[36][37] Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed de novo gene birth.[38][39]
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 (exon shuffling).[40][41] 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.[42] For example, polyketide synthases are large enzymes that make antibiotics; they contain up to 100 independent domains that each catalyse one step in the overall process, like a step in an assembly line.[43]
One example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor (MC1R) disrupt the pattern. The majority of pig breeds carry MC1R mutations disrupting wild-type colour and different mutations causing dominant black colouring.[44]
Sex and recombination
In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes.[45] 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.[46] Sex usually increases genetic variation and may increase the rate of evolution.[47][48]
The two-fold cost of sex was first described by John Maynard Smith.[49] The first cost is that in sexually dimorphic species only one of the two sexes can bear young. This cost does not apply to hermaphroditic species, like most plants and many invertebrates. 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.[50] Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. 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.[50][51][52][53] Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.[54][55]
Gene flow
Gene flow is the exchange of genes between populations and between species.[56] It can therefore be a source of variation that is new to a population or to a species. 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.
Gene transfer between species includes the formation of
Large-scale gene transfer has also occurred between the ancestors of
Epigenetics
Some heritable changes cannot be explained by changes to the sequence of nucleotides in the DNA. These phenomena are classed as epigenetic inheritance systems.[64] 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.[65] 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.[66] Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations.[67] Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.[68][69]
Evolutionary forces
From a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms,[70] for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias.
Natural selection
Evolution by natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:[7]
- Variation exists within populations of organisms with respect to morphology, physiology and behaviour (phenotypic variation).
- Different traits confer different rates of survival and reproduction (differential fitness).
- These traits can be passed from generation to generation (heritability of fitness).
More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. 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.[71] 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.[72] Consequences of selection include nonrandom mating[73] and genetic hitchhiking.
The central concept of natural selection is the evolutionary fitness of an organism.[74] Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.[74] 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.[75] 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.[74]
If an allele increases fitness more than the other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be "selected for." Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele likely becoming rarer—they are "selected against."[76]
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.[25] 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.[77][78] 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.[79]
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. The first is
Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an
Natural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species.[83][84][85] Selection can act at multiple levels simultaneously.[86] An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome.[87] Selection at a level above the individual, such as group selection, may allow the evolution of cooperation.[88]
Genetic drift
Genetic drift is the random fluctuation of allele frequencies within a population from one generation to the next.[89] When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward[clarification needed] in each successive generation because the alleles are subject to sampling error.[90] This drift halts when an allele eventually becomes fixed, either by disappearing from the population or by replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that begin with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[91]
According to the neutral theory of molecular evolution most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[92] In this model, most genetic changes in a population are thus the result of constant mutation pressure and genetic drift.[93] This form of the neutral theory has been debated since it does not seem to fit some genetic variation seen in nature.[94][95] A better-supported version of this model is the nearly neutral theory, according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.[71] Other theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft.[90][96][97] Another concept is constructive neutral evolution (CNE), which explains that complex systems can emerge and spread into a population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting,[98][99][100] and it has been applied in areas ranging from the origins of the spliceosome to the complex interdependence of microbial communities.[101][102][103]
The time it takes a neutral allele to become fixed by genetic drift depends on population size; fixation is more rapid in smaller populations.[104] The number of individuals in a population is not critical, but instead a measure known as the effective population size.[105] 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.[105] The effective population size may not be the same for every gene in the same population.[106]
It is usually difficult to measure the relative importance of selection and neutral processes, including drift.[107] The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.[108]
Mutation bias
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. This is related to the idea of developmental bias. Haldane[109] and Fisher[110] 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. This opposing-pressures argument was long used to dismiss the possibility of internal tendencies in evolution,[111] until the molecular era prompted renewed interest in neutral evolution.
Noboru Sueoka[112] and Ernst Freese[113] proposed that systematic biases in mutation might be responsible for systematic differences in genomic GC composition between species. The identification of a GC-biased E. coli mutator strain in 1967,[114] 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.
For instance, mutation biases are frequently invoked in models of codon usage.[115] Such models also include effects of selection, following the mutation-selection-drift model,[116] which allows both for mutation biases and differential selection based on effects on translation. Hypotheses of mutation bias have played an important role in the development of thinking about the evolution of genome composition, including isochores.[117] Different insertion vs. deletion biases in different taxa can lead to the evolution of different genome sizes.[118][119] The hypothesis of Lynch regarding genome size relies on mutational biases toward increase or decrease in genome size.
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[120] and (2) bacterial genomes frequently have AT-biased mutation.[121]
Contemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work[111] showed that the original "pressures" theory assumes that evolution is based on standing variation: when evolution depends on events of mutation that introduce new alleles, mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.[111][122] Several studies report that the mutations implicated in adaptation reflect common mutation biases[123][124][125] though others dispute this interpretation.[126]
Genetic hitchhiking
Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). 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.[127] 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. A set of alleles that is usually inherited in a group is called a haplotype. 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.[128] 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.[96]
Sexual selection
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.[130] Traits that evolved through sexual selection are particularly prominent among males of several animal species. 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.[131][132] This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake, sexually selected traits.[133]
Natural outcomes
Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding
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.[141][142][143] 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.[144] For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size,[145] and constitute the vast majority of Earth's biodiversity.[146] 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.[147] Indeed, the evolution of microorganisms is particularly important to evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[148][149]
Adaptation
Adaptation is the process that makes organisms better suited to their habitat.[150][151] Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. 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. Adaptations are produced by natural selection.[152] The following definitions are due to Theodosius Dobzhansky:
- Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[153]
- 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.[154]
- 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.[155]
Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. 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.[156] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[157] Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[158][159] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[160][161] 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).[162][163][164][165]
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. 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.[167] However, since all living organisms are related to some extent,[168] 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.[169][170]
During evolution, some structures may lose their original function and become vestigial structures.
However, many traits that appear to be simple adaptations are in fact
An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.
Coevolution
Interactions between organisms can produce both conflict and cooperation. 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. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution.[192] 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. 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.[193]
Cooperation
Not all co-evolved interactions between species involve conflict.[194] Many cases of mutually beneficial interactions have evolved. 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.[195] This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis. 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.[196]
Coalitions between organisms of the same species have also evolved. 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. 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. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.[197]
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.[198] 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.[199] Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.[200]
Speciation
Speciation is the process where a species diverges into two or more descendant species.[201]
There are multiple ways to define the concept of "species". The choice of definition is dependent on the particularities of the species concerned.
Speciation has been observed multiple times under both controlled laboratory conditions and in nature.[210] In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.[211][212] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.[213]
The second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.[214]
The third mode is parapatric speciation. 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. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.[201] Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines.[215] Here, plants evolve that have resistance to high levels of metals in the soil. 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. 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.[216]
Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population.[217] Generally, sympatric speciation in animals requires the evolution of both genetic differences and nonrandom mating, to allow reproductive isolation to evolve.[218]
One type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids.[219] 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.[220] 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.[221] This happened about 20,000 years ago,[222] and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.[223] 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.[224]
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.[225] 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. 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.[139]
Extinction
Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.
The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.[229] 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).[237] 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.[84] 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.[238]
Applications
Concepts and models used in evolutionary biology, such as natural selection, have many applications.[239]
Artificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the domestication of plants and animals.[240] 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. 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.[241]
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
Evolutionary theory has many
In
Evolutionary history of life
million years ago) |
Origin of life
The Earth is about
More than 99% of all species, amounting to over five billion species,[266] that ever lived on Earth are estimated to be extinct.[234][235] Estimates on the number of Earth's current species range from 10 million to 14 million,[267][268] of which about 1.9 million are estimated to have been named[269] and 1.6 million documented in a central database to date,[270] leaving at least 80% not yet described.
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.[10] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.[271][272] The beginning of life may have included self-replicating molecules such as RNA[273] and the assembly of simple cells.[274]
Common descent
All organisms on Earth are descended from a common ancestor or ancestral
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.[278][279] 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. This view dates back to an idea briefly mentioned by Darwin but later abandoned.[280]
Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[281] By comparing the anatomies of both modern and extinct species, palaeontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.
More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and amino acids.[282] 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.[283] 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.[284]
Evolution of life
Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.[286][287] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[288] The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[289][290] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[291] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[292]
The history of life was that of the
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. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[296] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[297]
About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals.
History of evolutionary thought
Classical antiquity
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.[304] Such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura (lit. 'On the Nature of Things').[305][306]
Middle Ages
In contrast to these materialistic views, Aristotelianism had considered all natural things as actualisations of fixed natural possibilities, known as forms.[307][308] This became part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. 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.[309]
A number of Arab Muslim scholars wrote about evolution, most notably Ibn Khaldun, who wrote the book Muqaddimah in 1377 AD, in which he asserted that humans developed from "the world of the monkeys", in a process by which "species become more numerous".[310]
Pre-Darwinian
The
Other
Darwinian revolution
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
Pangenesis and heredity
The mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of pangenesis.[328] 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). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.[329] 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. 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. 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.[330] 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.[331][332] In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.[333]
The 'modern synthesis'
In the 1920s and 1930s, the modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that included random genetic drift, mutation, and gene flow. This new version of evolutionary theory focused on changes in allele frequencies in population. It explained patterns observed across species in populations, through fossil transitions in palaeontology.[333]
Further syntheses
Since then, further syntheses have extended evolution's explanatory power in the light of numerous discoveries, to cover biological phenomena across the whole of the biological hierarchy from genes to populations.[334]
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.[335] Molecular biology improved understanding of the relationship between genotype and phenotype. Advances 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.[336] 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.[337]
One extension, known as evolutionary developmental biology and informally called "evo-devo", emphasises how changes between generations (evolution) act on patterns of change within individual organisms (development).[237][338] Since the beginning of the 21st century, 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.[339][340]
Social and cultural responses
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. Today, the modern evolutionary synthesis is accepted by a vast majority of scientists.[237] However, evolution remains a contentious concept for some theists.[342]
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.[135][343][344] 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.[345] 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.[346] While other scientific fields such as cosmology[347] and Earth science[348] also conflict with literal interpretations of many religious texts, evolutionary biology experiences significantly more opposition from religious literalists.
The teaching of evolution in American secondary school biology classes was uncommon in most of the first half of the 20th century. The
See also
- Devolution (biology) – Notion that species can revert to primitive forms
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- Piatigorsky, Joram; Kantorow, Marc; Gopal-Srivastava, Rashmi; Tomarev, Stanislav I. (1994). "Recruitment of enzymes and stress proteins as lens crystallins". In Jansson, Bengt; Jörnvall, Hans; Rydberg, Ulf; et al. (eds.). Toward a Molecular Basis of Alcohol Use and Abuse. Experientia. Vol. 71. Basel; Boston: PMID 8032155.
- from the original on 18 September 2015.
- OCLC 46660910.
- OCLC 45806501.
- OCLC 2126030.
- OCLC 9020616.
- Ridley, Mark (2004). Evolution. Oxford: Blackwell. ISBN 978-1-4051-0345-9.
- Stearns, Beverly Peterson; OCLC 803522914.
- OCLC 10458367.
- OCLC 939245154.
- OCLC 246124737.
Further reading
- Introductory reading
- Barrett, Paul H.; Weinshank, Donald J.; Gottleber, Timothy T., eds. (1981). A Concordance to Darwin's Origin of Species, First Edition. Ithaca, New York: OCLC 610057960.
- Carroll, Sean B. (2005). Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom. illustrations by Jamie W. Carroll, Josh P. Klaiss, Leanne M. Olds (1st ed.). New York: W.W. Norton & Company. OCLC 57316841.
- OCLC 51668497.
- Gould, Stephen Jay (1989). OCLC 18983518.
- OCLC 41420544.
- —— (2000). Darwin's Ghost: The Origin of Species Updated (1st ed.). New York: OCLC 42690131. American version.
- —— (2000). Darwin's Ghost: The Origin of Species Updated (1st ed.). New York:
- Mader, Sylvia S. (2007). Biology. Significant contributions by Murray P. Pendarvis (9th ed.). Boston, Massachusetts: OCLC 61748307.
- Maynard Smith, John (1993). OCLC 27676642.
- Pallen, Mark J. (2009). The Rough Guide to Evolution. Rough Guides Reference Guides. London; New York: OCLC 233547316.
- Advanced reading
- OCLC 86090399.
- Coyne, Jerry A.; OCLC 55078441.
- OCLC 729341924.
- Hall, Brian K.; Olson, Wendy, eds. (2003). Keywords and Concepts in Evolutionary Developmental Biology. Cambridge, Massachusetts: Harvard University Press. OCLC 50761342.
- OCLC 895048122.
- Maynard Smith, John; OCLC 30894392.
- Mayr, Ernst (2001). What Evolution Is. New York: Basic Books. OCLC 47443814.
- OCLC 233030259.
External links
- General information
- "Evolution" on In Our Time at the BBC
- "Evolution Resources from the National Academies". Washington, DC: National Academy of Sciences. Retrieved 30 May 2011.
- "Understanding Evolution: your one-stop resource for information on evolution". Berkeley, California: University of California, Berkeley. Retrieved 30 May 2011.
- "Evolution of Evolution – 150 Years of Darwin's 'On the Origin of Species'". Arlington County, Virginia: National Science Foundation. Archived from the original on 19 May 2011. Retrieved 30 May 2011.
- "Human Evolution Timeline Interactive". Smithsonian Institution, National Museum of Natural History. 28 January 2010. Retrieved 14 July 2018. Adobe Flash required.
- "History of Evolution in the United States". Salon. Retrieved 2021-08-24.
- Video (1980; Cosmos animation; 8:01): "Evolution" – Carl Sagan on YouTube
- Experiments
- Lenski, Richard E. "Experimental Evolution". East Lansing, Michigan: Michigan State University. Retrieved 31 July 2013.
- Chastain, Erick; Livnat, Adi; PMID 24979793.
- Online lectures
- "Evolution Matters Lecture Series". Harvard Online Learning Portal. Cambridge, Massachusetts: Harvard University. Archived from the original on 18 December 2017. Retrieved 15 July 2018.
- Stearns, Stephen C. "EEB 122: Principles of Evolution, Ecology and Behavior". Open Yale Courses. New Haven, Connecticut: Yale University. Archived from the original on 1 December 2017. Retrieved 14 July 2018.