Evolution Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! === Natural selection === {{main|Natural selection}} {{See also|Dollo's law of irreversibility}} 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:<ref name="Lewontin70" /> * 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.<ref name="Hurst">{{cite journal |last=Hurst |first=Laurence D. |author-link=Laurence Hurst |title=Fundamental concepts in genetics: genetics and the understanding of selection |date=February 2009 |journal=Nature Reviews Genetics |volume=10 |issue=2 |pages=83–93 |doi=10.1038/nrg2506 |pmid=19119264 |s2cid=1670587 }}</ref> This [[teleonomy]] is the quality whereby the process of natural selection creates and preserves traits that are [[teleology in biology|seemingly fitted]] for the [[function (biology)|functional]] roles they perform.<ref>{{harvnb|Darwin|1859|loc=[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=477 Chapter XIV]}}</ref> Consequences of selection include [[Assortative mating|nonrandom mating]]<ref>{{Cite journal |last1=Otto |first1=Sarah P. |author-link1=Sarah Otto |last2=Servedio |first2=Maria R. |author-link2=Maria Servedio|last3=Nuismer |first3=Scott L. |title=Frequency-Dependent Selection and the Evolution of Assortative Mating |journal=Genetics |date=August 2008 |volume=179 |issue=4 |pages=2091–2112 |doi=10.1534/genetics.107.084418 |pmc=2516082 |pmid=18660541}}</ref> and [[genetic hitchhiking]]. The central concept of natural selection is the [[fitness (biology)|evolutionary fitness]] of an organism.<ref name="Orr">{{cite journal |last=Orr |first=H. Allen |author-link=H. Allen Orr |date=August 2009 |title=Fitness and its role in evolutionary genetics |journal=Nature Reviews Genetics |volume=10 |issue=8 |pages=531–539 |doi=10.1038/nrg2603 |pmc=2753274 |pmid=19546856 |issn=1471-0056}}</ref> Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.<ref name="Orr" /> 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.<ref name="Haldane">{{cite journal |last=Haldane |first=J. B. S. |s2cid=4185793 |author-link=J. B. S. Haldane |date=14 March 1959 |title=The Theory of Natural Selection To-Day |url=https://archive.org/details/sim_nature-uk_1959-03-14_183_4663/page/710 |journal=Nature |volume=183 |issue=4663 |pages=710–713 |bibcode=1959Natur.183..710H |doi=10.1038/183710a0 |pmid=13644170}}</ref> 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.<ref name="Orr"/> 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''."<ref name="Lande">{{cite journal |last1=Lande |first1=Russell |author-link1=Russell Lande |last2=Arnold |first2=Stevan J. |date=November 1983 |title=The Measurement of Selection on Correlated Characters |journal=Evolution |volume=37 |issue=6 |pages=1210–1226 |doi=10.1111/j.1558-5646.1983.tb00236.x |pmid=28556011 |issn=0014-3820 |jstor=2408842|s2cid=36544045 |doi-access= }}</ref> 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.<ref name="Futuyma_2005" /> 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.<ref>{{cite journal |last1=Goldberg |first1=Emma E. |last2=Igić |first2=Boris |date=November 2008 |title=On phylogenetic tests of irreversible evolution |journal=Evolution |volume=62 |issue=11 |pages=2727–2741 |doi=10.1111/j.1558-5646.2008.00505.x |issn=0014-3820 |pmid=18764918|s2cid=30703407 }}</ref><ref>{{cite journal |last1=Collin |first1=Rachel |last2=Miglietta |first2=Maria Pia |date=November 2008 |title=Reversing opinions on Dollo's Law |journal=[[Trends (journals)|Trends in Ecology & Evolution]] |volume=23 |issue=11 |pages=602–609 |doi=10.1016/j.tree.2008.06.013 |pmid=18814933}}</ref> 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 [[atavism]]s.<ref>{{cite journal |last1=Tomić |first1=Nenad |last2=Meyer-Rochow |first2=Victor Benno |s2cid=40851098 |year=2011 |title=Atavisms: Medical, Genetic, and Evolutionary Implications |url=https://archive.org/details/sim_perspectives-in-biology-and-medicine_summer-2011_54_3/page/332 |journal=[[Perspectives in Biology and Medicine]] |volume=54 |issue=3 |pages=332–353 |doi=10.1353/pbm.2011.0034 |pmid=21857125}}</ref> [[File:Genetic Distribution.svg|thumb|left|upright=1.45|These charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of [[phenotypic trait]] and the y-axis variable is the number of organisms.{{imagefact|date=December 2022}} Group A is the original population and Group B is the population after selection.<br /> '''·''' Graph 1 shows [[directional selection]], in which a single extreme [[phenotype]] is favoured.<br /> '''·''' Graph 2 depicts [[stabilizing selection]], where the intermediate phenotype is favoured over the extreme traits.<br /> '''·''' Graph 3 shows [[disruptive selection]], in which the extreme phenotypes are favoured over the intermediate.]] 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 [[directional selection]], which is a shift in the average value of a trait over time—for example, organisms slowly getting taller.<ref>{{cite journal |last1=Hoekstra |first1=Hopi E. |last2=Hoekstra |first2=Jonathan M. |last3=Berrigan |first3=David |last4=Vignieri |first4=Sacha N. |last5=Hoang |first5=Amy |last6=Hill |first6=Caryl E. |last7=Beerli |first7=Peter |last8=Kingsolver |first8=Joel G. |date=31 July 2001 |title=Strength and tempo of directional selection in the wild |journal=PNAS |volume=98 |issue=16 |pages=9157–9160 |bibcode=2001PNAS...98.9157H |doi=10.1073/pnas.161281098 |pmc=55389 |pmid=11470913 |display-authors=3|doi-access=free }}</ref> Secondly, [[disruptive selection]] is selection for extreme trait values and often results in [[bimodal distribution|two different values]] becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in [[stabilising selection]] there is selection against extreme trait values on both ends, which causes a decrease in [[variance]] around the average value and less diversity.<ref name="Hurst" /><ref>{{cite journal |last=Felsenstein |first=Joseph |author-link=Joseph Felsenstein |date=November 1979 |title=Excursions along the Interface between Disruptive and Stabilizing Selection |journal=Genetics |volume=93 |issue=3 |pages=773–795 |doi=10.1093/genetics/93.3.773 |pmc=1214112 |pmid=17248980}}</ref> This would, for example, cause organisms to eventually have a similar height. 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 [[ecosystem]], that is, a system in which organisms interact with every other element, [[Abiotic component|physical]] as well as [[Biotic component|biological]], in their local environment. [[Eugene Odum]], a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and nonliving parts) within the system...."<ref name="Odum1971">{{harvnb|Odum|1971|p=8}}</ref> Each population within an ecosystem occupies a distinct [[Ecological niche|niche]], or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the [[food chain]] and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. Natural selection can act at [[unit of selection|different levels of organisation]], such as genes, cells, individual organisms, groups of organisms and species.<ref name="Okasha07">{{harvnb|Okasha|2006}}</ref><ref name="Gould">{{cite journal |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |date=28 February 1998 |title=Gulliver's further travels: the necessity and difficulty of a hierarchical theory of selection |journal=Philosophical Transactions of the Royal Society B |volume=353 |issue=1366 |pages=307–314 |doi=10.1098/rstb.1998.0211 |issn=0962-8436 |pmc=1692213 |pmid=9533127}}</ref><ref name="Mayr1997">{{cite journal |last=Mayr |first=Ernst |author-link=Ernst Mayr |date=18 March 1997 |title=The objects of selection |journal=PNAS |volume=94 |issue=6 |pages=2091–2094 |bibcode=1997PNAS...94.2091M |doi=10.1073/pnas.94.6.2091 |issn=0027-8424 |pmc=33654 |pmid=9122151|doi-access=free }}</ref> Selection can act at multiple levels simultaneously.<ref>{{harvnb|Maynard Smith|1998|pp=203–211; discussion 211–217}}</ref> An example of selection occurring below the level of the individual organism are genes called [[Transposable element|transposons]], which can replicate and spread throughout a genome.<ref>{{cite journal |last=Hickey |first=Donal A. |s2cid=6583945 |year=1992 |title=Evolutionary dynamics of transposable elements in prokaryotes and eukaryotes |journal=[[Genetica]] |volume=86 |issue=1–3 |pages=269–274 |doi=10.1007/BF00133725 |issn=0016-6707 |pmid=1334911}}</ref> Selection at a level above the individual, such as [[group selection]], may allow the evolution of cooperation.<ref>{{cite journal |last1=Gould |first1=Stephen Jay |last2=Lloyd |first2=Elisabeth A. |author-link2=Elisabeth Lloyd |date=12 October 1999 |title=Individuality and adaptation across levels of selection: how shall we name and generalise the unit of Darwinism? |journal=PNAS |volume=96 |issue=21 |pages=11904–11909 |bibcode=1999PNAS...9611904G |doi=10.1073/pnas.96.21.11904 |issn=0027-8424 |pmc=18385 |pmid=10518549 |doi-access=free }}</ref> Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. 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