User:Imbehind/sandbox
Evolucija je naziv za proces uslijed kojeg dolazi do promjena nasljednih osobina bioloških populacija tijekom velikog broja uzastopnih generacija.[1][2] Evolucijski procesi dovode do bioraznolikosti na svakom nivou biološke organizacije, uključujući nivoe vrsta, individualnih organizama i molekula.[3]
Neprestano formiranje novih vrsta (specijacija), promjene unutar vrsta (anageneza) i nestanak vrsta (izumiranje) tijekom cijele evolucijske povijesti života na Zemlji obilježeno je pojavom brojnih zajedničkih morfoloških i biokemijskih osobina organizama, uključujući i međusobno identične sekvence DNK.[4] Ove zajedničke osobine su sličnije među vrstama koje dijele nedavnog zajedničkog pretka i mogu se koristiti kako bi rekonstruirali biološko "stablo života" zasnovano na evolucijskim poveznicama (filogenetika) koristeći kako postojeće žive vrste tako i fosile. Fosilni zapisi u sebi uključuju prikaz prograsije u razvoju živoga svijeta od ranog biogenog grafita,[5] preko fosila mikrobnih naslaga ,[6][7][8] do fosiliziranih višestaničnih organizama. Svi danas postojeći oblici bioraznolikosti su oblikovani procesima specijacije, ali i izumiranja.[9]
Sredinom 19. stoljeća Charles Darwin je postavio znanstvenu Teoriju evolucije putem prirodnog odabira koju je objavio u svojoj knjizi Postanak vrsta (1859) te ga zbog toga danas smatramo ocem kako Teorije o evoluciji tako i moderne biologije. Iako Teorija evolucije ne objašnjava kako je život nastao, u potpunosti objašnjava svu raznolikost života koju danas susrećemo i smatra se najvažnijom teorijom iz područja znanosti o životu na kojoj se temelje sva suvremena istraživanja i znanstvene spoznaje u biologiji. Iz temeljnih postulata teorije evolucije nužno proizlazi i teorija o zajedničkom porijeklu odnosno pretpostavlja postojanje posljednjeg univerzalnog zajedničkog pretka, životnog oblika koji je živio prije 3.5 - 3.8 milijardi godina i koji predak svim danas poznatim živim vrstama.[10][11][12][5][6][8] Teoriju o univerzalnom zajedničkom porijeklu je također prvi formulirao Darwin.
Evolucija putem prirodnog odabira je proces koji je temeljen na opažanju da živi svijet u pravilu proizvodi ukupno više potomstva nego što to potomstvo ima mogućnost preživljavanja te na činjenicama koje znamo o populaciji: 1) osobine variriaju među jedinkama u smislu morfologije, fiziologije i ponašanja (fenotipska varijacija), 2) različite varijacije uzrokuju različite stope preživljavanja i reprodukcije (diferencijalna prilagođenost) i 3) osobine se mogu prenositi iz generacije u generaciju (nasljeđivanje sposobnosti preživljavanja).[13] Zbog toga su jedinke u svakoj novoj generaciji neke populacije zamijenjene potomstvom onih roditelja koji su bolje prilagođeni i imaju veće šanse za preživljavanje i razmnožavanje u okolišu u kojem se prirodan odabir odvija.
Spomenuti način djelovanja evolucije stvara privid svrhovitosti ili usmjerenosti ka cilju samog procesa prirodnog odabira koji naizgled ciljano stvara i prilagođava nužne osobine organizma funkcionalnoj ulozi koju obavljaju (telenomija).[14] Procesi po kojima se promjene odvijaju iz generacije u generaciju nazivamo evolucijskim procesima i mehanizmima.[15] Četiri najčešće spominjana mehanizma su su prirodan odabir (uključujući i seksualnu selekciju), genetski otklon, mutacije i genetski tok uslijed genetskih primjesa. Mutacije i genetski tok uzrokuju varijacije, a prirodna selekcija i genetski otklon ih probiru.
Početkom 20. stoljeća moderna evolucijska sinteza je integrirala klasičnu genetiku Gregora Mendela sa Darwinovom Teorijom evolucije putom prirodnog odabira kroz disciplinu populacijske genetike, a ključna važnost prirodnog odabira kao uzroka procesa evolucije je prihvaćena u svim granama biologije. Štoviše, prethodni stavovi i razmišljanja o evoluciji, kao što su ortogeneza, evolucionizam te hipoteze o "prirodnom" smijeru evolucijskog procesa u smislu stalnog trenda uvećanja kompleksnosti živih organizama, postaju suvišni i više se ne smatraju znanstvenima.[16] Znanstvenici nastavljaju proučavati različite aspekte evolucijske biologije postavljajući i testirajući nove hipoteze, stvarajući nove matematičke modele teorijske biologije, kao i biološke teorije, koristeći empirijske podatke, te i dalje provode opsežne eksperimente na terenu i u laboratoriju.
U smislu praktične primjene, naše razumijevanje evolucije je bilo ključno u razvoju brojnih novih znanstvenih disciplina kao i industrijskih polja primjene stečenih spoznaja o evoluciji, uključujući posebno poljoprivredu, ljudsku i veterinarsku medicinu, kao i znanost o životu uopće. [17][18][19] Otkrića u evolucijskoj biologiji značajano su utjecala ne samo na tradicionalne znanstvene discipline na polju biologije nego i na druge akademske discipline, uključujući biološku antropologiju, evolucijsku psihologiju pa čak i na informacijske znanosti, a posebno na razvoj umjetne inteligencije i primjenu genetskih algoritama nastalih na principima Darwinove teorije evolucije na rješavanje problema optimizacije u računalnim znanostima.[20][21]
History of evolutionary thought
Classical times
The proposal that one type of
Medieval
In contrast to these
Pre-Darwinian
In the 17th century, the new
Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species.[31] Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, and Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism (or "filament").[32] The first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809,[33] which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.[34][35] (The latter process was later called Lamarckism.)[34][36][37][38] These ideas were condemned by established naturalists as speculation lacking empirical support. In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, Ray's ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.[39][40][41]
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 Charles Darwin in terms of variable populations. Partly influenced 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.[48] 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.[49] 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 cells 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.[50] 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.[35][51][52] 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.[53]
The 'modern synthesis'
In the 1920s and 1930s the so-called
Further syntheses
Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the biological hierarchy, from genes to species. One extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations (evolution) acts on patterns of change within individual organisms (development).[59][60][61] Since the beginning of the 21st century and in light of discoveries made in recent decades, some biologists have argued for an extended evolutionary synthesis, which would account for the effects of non-genetic inheritance modes, such as epigenetics, parental effects, ecological and cultural inheritance, and evolvability.[62][63]
Heredity
Evolution in organisms occurs through changes in heritable traits—the inherited characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents.[64] Inherited traits are controlled by genes and the complete set of genes within an organism's genome (genetic material) is called its genotype.[65]
The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.[66] As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn.[67]
Heritable traits are passed from one generation to the next via DNA, a molecule that encodes genetic information.[65] DNA is a long biopolymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.[68] However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by quantitative trait loci (multiple interacting genes).[69][70]
Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA. These phenomena are classed as epigenetic inheritance systems.[71] 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.[72][73] 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.[74] 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. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.[75] Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.[76][77]
Variation
An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation.[70] 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.[78]
Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The Hardy–Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.[79]
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 identical in all individuals of that species.[80] However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.[81]
Mutation
Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect. Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.[82]
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.[83] Extra copies of genes are a major source of the raw material needed for new genes to evolve.[84] This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors.[85] 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.[86]
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.[87][88] Other types of mutations can even generate entirely new genes from previously noncoding DNA.[89][90]
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.
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.[95] 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.[96] Sex usually increases genetic variation and may increase the rate of evolution.[97][98]
The two-fold cost of sex was first described by John Maynard Smith.[99] 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.[100] 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.[100][101][102][103]
Gene flow
Gene flow is the exchange of genes between populations and between species.[104] 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 eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria. It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea.[111]
Evolucijski mehanizmi
Iz Neo-Darwinističke perspektive, evolucija nastupa kada postoje promjene u frekvencijama alela unutar populacije organizama u kojoj dolazi do razmjene nasljednog materijala između jediniki. [79] Primjerice, kada alele koje diktiraju crnu boju u populaciji moljaca postanu uobičajene. Mehanizmi koji dovode do promjene u frekvencijama alela uključuju prirodni odabir, genski otklon, gensko stopiranje, mutacije i genski tok.
Prirodna selekcija
Evolucija uslijed prirodne selekcije je proces kojim osobine koje pospješuju preživljavanje i reprodukciju postaju uobičajene sa svakom novom generacijom neke populacije. Često se naziva "samorazumljivim" mehanizmom zato što nužno slijedi iz tri jednostavne i lako provjerljive činjenice:[13]
- U populacijama organizama postoje varijacije u smislu morfologije, fiziologije i ponašanja (fenotipske varijacije).
- Različite osobine organizama dovode do razlika u njihovoj sposobnosti preživljavanja i ramnožavanja (diferencijal prilagođenosti).
- Te osobine mogu biti proslijeđne iz jedne generacije u narednu (nasljeđivanje prilagođenosti).
Više potomstva se proizvodi nego što ga može preživjeti i takvi uvjeti među jedinkama u nekoj populaciji uzrokuju natjecanje za preživljavanje i mogućnost reprodukcije. Zbog toga jedinke sa osobinama koje im donose prednost nad konkurencijom imaju veću vjerojatnost da proslijede te svoje osobine u narednu generaciju od jedinki koje nemaju osobine koje im to omogućavaju.[112]
Centralni koncept teorije evolucije je evolucijska prilagođenost ili sposobnost preživljavanja nekog organizma. [113] Prilagođenost se mjeri sposobnošću organizma da preživi i da se uspješno razmnoži što određuje značaj genskog doprinosa organizma sljedećoj generaciji. [113] Međutim, prilagođenost ne mora biti određena ukupnim brojem potomaka - umjesto toga prilagođenost određujemo udjelom potomaka koji nose gene organizma u narednim generacijama.[114] Kada bi primjerice organizam imao sposobnost uspješnog preživljavanja i brzog razmnožavanja, ali takvu da su njegovi potomci premali i preslabi da bi preživjeli, takav organizam bi imao jako mali genetski doprinos budućim generacijama i imao bi jako nisku ukupnu evolucijsku prilagođenost.[113]
If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected for." Examples of traits that can increase fitness are enhanced survival and increased
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
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.[121] 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.[122][123] This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake, sexually selected traits.[124]
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.[126][127][128] Selection can act at multiple levels simultaneously.[129] An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome.[130] Selection at a level above the individual, such as group selection, may allow the evolution of cooperation, as discussed below.[131]
Usmjerena mutacija
Osim što je jedan od glavnih uzročnika varijacija, mutacija također može funkcionirati kao evolucijski mehanizam u slučajevima kada na molekularnom nivou postoje različite vjerojatnosti da se točno određena mutacija dogodi. Taj proces nazivamo usmjerenom mutacijom.[132] Primjerice, ako uzmemo dva genotipa, jedan sa nukleotidom G, a drugi sa nukleotidom A na istoj poziciji u DNK sekvenci, ali se mutacija iz G u A događa češće nego mutacija iz A u G, onda će proces evolucije teći na način da će naizgled preferirati genotipove koji sadrže nukleotid A.[133] Sklonost ili pristranost ka umetanju umjesto brisanju nukleotida, ili obrnuto, u različitim taksonima može dovesti do evolucije genotipa različitih veličina.[134][135] Razvojne ili mutacijske sklonosti, odnosno usmjerenja, također su zamijećene u morfološkoj evoluciji.[136][137] Primjerice, sukladno teorijama evolucije koje uzimaju u obzir utjecaj fenotipa na proces evolucije, mutacije mogu uzrokovati gensku asimilaciju osobina koje su uvjetovane od strane okoliša.[138][139][140]
Efekti usmjerene mutacije su superponirani na druge evolucijske procese. Ako prirodni odabir ne favorizira ni jednu od dvije mutacije posebno, ali nema dodatne prednosti da se zadrže i jedna i druga, onda će mutacija koja se događa češće najvjeroatnije biti ona koja će na kraju biti zadržana u populaciji.[141][142] Mutacije koje dovode do gubitka funkcionalnosti gena su znatno uobičajenije od mutacija koje stvaraju nove, potpuno funkcionalne gene. Prirodnim odabirom takve mutacije se obično jako brzo uklanjaju iz genotipa. Međutim, ako je selekcijski pritisak slab, takva mutacija usmjerena prema gubitku genske funkcionalnosti može utjecati na evolucijski proces,[143] Primjerice, geni koji utječu na stvaranje pigmenta u koži organizma koji žive u špiljama ili duboko u oceanima gdje nema svjetla nemaju nikakvu funkciju i vrlo brzo bivaju eliminirani iz genotipa populacije.[144] Ova vrsta gubitka neke funkcije organizma se može dogoditi kako uslijed usmjerene mutacije, tako i/ili zbog toga što je ta funkcija imala i negativne posljedice ili je zahtijevala dodatan utrošak energije organizma. Jednom kada je benefit očuvanja funkcije nestao, prirodna selekcija vodi u gubitak funkcije. Kada prirodni odabir ne favorizira gubitak funkcije, brzina kojom procesom evolucije dolazi do gubitka funkcije ovisi više o učestalosti mutacije nego o efektivnoj veličini populacije,[145] što nam govori da na takve evolucijske promjene više utječe usmjerena mutacija nego genetski otklon.
Genetic drift
Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles are subject to sampling error.[146] As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or 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 began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[147]
It is usually difficult to measure the relative importance of selection and neutral processes, including drift.[148] The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.[149]
The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[150] Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.[151] This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.[152][153] However, a more recent and better-supported version of this model is the nearly neutral theory, where a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.[112] Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft.[146][154][155]
The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[156] The number of individuals in a population is not critical, but instead a measure known as the effective population size.[157] 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.[157] The effective population size may not be the same for every gene in the same population.[158]
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.[159] 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.[160] 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.[154]
Gene flow
Gene flow involves the exchange of genes between populations and between species.
If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Thus, exchange of genetic information between individuals is fundamentally important for the development of the
During the development of the modern synthesis, Sewall Wright developed his shifting balance theory, which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution.[161] However, recently there has been substantial criticism of the importance of the shifting balance theory.[162]
Outcomes
Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical
These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in gene frequency and adaptation.[164] In general, macroevolution is regarded as the outcome of long periods of microevolution.[165] Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved.[166] However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.[167][168][169]
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.[170][171][172] 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.[173] For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size,[174] and constitute the vast majority of Earth's biodiversity.[175] 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.[176] Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[177][178]
Adaptation
Adaptation is the process that makes organisms better suited to their habitat.[179][180] 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.[181] 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.[182]
- 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.[183]
- 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.[184]
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.[185] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[186] Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[187][188] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[189][190] 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).[191][192][193][194][195]
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.[197] However, since all living organisms are related to some extent,[198] 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.[199][200]
During evolution, some structures may lose their original function and become
However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.[212] One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation.[212] Within cells, molecular machines such as the bacterial flagella[213] and protein sorting machinery[214] evolved by the recruitment of several pre-existing proteins that previously had different functions.[164] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.[215][216]
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
Cooperation
Not all co-evolved interactions between species involve conflict.[224] 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.[225] 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.[226]
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.[227]
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.[228] 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.[229] Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.[230]
Speciation
Speciation is the process where a species diverges into two or more descendant species.[231]
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.[240] 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.[241][242] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.[243]
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.[244]
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.[231] 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.[245] 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.[246]
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.[247] Generally, sympatric speciation in animals requires the evolution of both genetic differences and non-random mating, to allow reproductive isolation to evolve.[248]
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.[249] 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.[250] 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.[251] This happened about 20,000 years ago,[252] and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.[253] 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.[254]
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.[255] 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.[168]
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.[259] 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).[59] 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.[127] 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.[263]
Evolutionary history of life
million years ago) |
Origin of life
The
More than 99 percent of all species, amounting to over five billion species,[271] that ever lived on Earth are estimated to be extinct.[272][273] Estimates on the number of Earth's current species range from 10 million to 14 million,[274][275] of which about 1.9 million are estimated to have been named[276] and 1.6 million documented in a central database to date,[277] leaving at least 80 percent 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
Common descent
All organisms on Earth are descended from a common ancestor or ancestral gene pool.[198][282] Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[283] The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups—similar to a family tree.[284] However, modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.[285][286]
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.[287] By comparing the anatomies of both modern and extinct species, paleontologists 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.[288] 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.[289] 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.[290]
Evolution of life
Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.[292][293] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[294] The eukaryotic cells emerged between 1.6–2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[295][296] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[297] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[298]
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.[302] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[303]
About 500 million years ago, plants and fungi colonised the land and were soon followed by
Applications
Concepts and models used in evolutionary biology, such as natural selection, have many applications.[309]
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.[310] 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.[311]
Understanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders.[312] For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves.[313] This helped identify genes required for vision and pigmentation.[314]
Many human diseases are not static phenomena, but capable of evolution. Viruses, bacteria, fungi and
In computer science, simulations of evolution using evolutionary algorithms and artificial life started in the 1960s and were extended with simulation of artificial selection.[323] Artificial evolution became a widely recognised optimisation method as a result of the work of Ingo Rechenberg in the 1960s. He used evolution strategies to solve complex engineering problems.[324] Genetic algorithms in particular became popular through the writing of John Henry Holland.[325] Practical applications also include automatic evolution of computer programmes.[326] Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers and also to optimise the design of systems.[327]
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.[59] However, evolution remains a contentious concept for some theists.[329]
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.[164][330][331] 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.[332] 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.[333] While other scientific fields such as cosmology[334] and Earth science[335] 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
- Argument from poor design
- Biocultural evolution
- Biological classification
- Evidence of common descent
- Evolutionary anthropology
- Evolutionary ecology
- Evolutionary epistemology
- Evolutionary neuroscience
- Evolution of biological complexity
- Evolution of plants
- Timeline of the evolutionary history of life
- Unintelligent design
- Universal Darwinism
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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, NY: 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, MA: 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.
- Gould, Stephen Jay (2002). The Structure of Evolutionary Theory. Cambridge, MA: Belknap Press of Harvard University Press. OCLC 47869352.
- Hall, Brian K.; Olson, Wendy, eds. (2003). Keywords and Concepts in Evolutionary Developmental Biology. Cambridge, MA: Harvard University Press. OCLC 50761342.
- ISBN 978-0-19-507951-7.
- 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". New Scientist. Retrieved 2011-05-30.
- "Evolution Resources from the National Academies". Washington, D.C.: National Academy of Sciences. Retrieved 2011-05-30.
- "Understanding Evolution: your one-stop resource for information on evolution". University of California, Berkeley. Retrieved 2011-05-30.
- "Evolution of Evolution – 150 Years of Darwin's 'On the Origin of Species'". Arlington County, VA: National Science Foundation. Retrieved 2011-05-30.
- Human Timeline (Interactive) – Smithsonian, National Museum of Natural History (August 2016).
- Experiments concerning the process of biological evolution
- Lenski, Richard E. "Experimental Evolution". Michigan State University. Retrieved 2013-07-31.
- Chastain, Erick; Livnat, Adi; PMID 24979793.
- Online lectures
- Carroll, Sean B. "The Making of the Fittest". Archived from the original on 2011-07-18. Retrieved 2011-05-30.
- Stearns, Stephen C. "Principles of Evolution, Ecology and Behavior". Archived from the original on 2011-03-23. Retrieved 2011-08-30.