Editing Natural selection (section)

==Mechanism==

===Heritable variation, differential reproduction===
[[File:Lichte en zwarte versie berkenspanner crop.jpg|thumb|upright=1.2|During the [[Industrial Revolution]], pollution killed many [[lichen]]s, leaving tree trunks dark. A [[industrial melanism|dark (melanic)]] morph of the [[peppered moth]] largely replaced the formerly usual light morph (both shown here). Since the moths are subject to [[predation]] by birds hunting by sight, the colour change offers better [[camouflage]] against the changed background, suggesting natural selection at work.]]
{{Main|Genetic variation}}

Natural variation occurs among the individuals of any population of organisms. Some differences may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring. If the traits that give these individuals a reproductive advantage are also [[heritable]], that is, passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way the [[natural environment]] of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described.<ref name=Michigan>{{cite web |title=Evolution and Natural Selection |url=http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html |publisher=University of Michigan |access-date=9 November 2016 |date=10 October 2010}}</ref> This gives the appearance of purpose, but in natural selection there is no intentional choice.{{efn|In [[sexual selection]], a female animal making a choice of mate may be argued to be intending to get the best mate; there is no suggestion that she has any intention to improve the bloodline in the manner of an animal breeder.}} Artificial selection is [[Teleology|purposive]] where natural selection is not, though [[teleology in biology|biologists often use teleological language]] to describe it.<ref name=Stanford>{{cite web |title=Teleological Notions in Biology |url=http://plato.stanford.edu/entries/teleology-biology/ |website=Stanford Encyclopedia of Philosophy |access-date=28 July 2016 |date=18 May 2003}}</ref>

The [[peppered moth]] exists in both light and dark colours in Great Britain, but during the [[Industrial Revolution]], many of the trees on which the moths rested became blackened by [[soot]], giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial [[Manchester]] were dark. The balance was reversed by the effect of the [[Clean Air Act 1956]], and the dark moths became rare again, demonstrating the influence of natural selection on [[peppered moth evolution]].<!--<ref name="Peppered Moth">{{cite web |url=http://www.millerandlevine.com/km/evol/Moths/moths.html |title=The Peppered Moth – An Update |last=Miller |first=Kenneth R. |author-link=Kenneth R. Miller |date=August 1999 |website=millerandlevine.com |publisher=Miller And Levine Biology |access-date=9 November 2016}}</ref>--><ref>{{cite journal|last1=van't Hof |first1=Arjen E. |last2=Campagne |first2=Pascal |last3=Rigden |first3=Daniel J |display-authors=etal |title=The industrial melanism mutation in British peppered moths is a transposable element |journal=Nature |date=June 2016 |volume=534 |issue=7605 |pages=102–105 |doi=10.1038/nature17951 |pmid=27251284|bibcode=2016Natur.534..102H |s2cid=3989607 }}</ref> A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies the [[camouflage]] of moths to [[predation]] risk.<ref name=Walton2018>{{cite journal |last1=Walton |first1=Olivia |last2=Stevens |first2=Martin |title=Avian vision models and field experiments determine the survival value of peppered moth camouflage |journal=Communications Biology |date=2018 |volume=1 |page=118 |doi=10.1038/s42003-018-0126-3 |pmid = 30271998|pmc=6123793 }}</ref>

===Fitness===

{{Main|Fitness (biology)}}

The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "[[survival of the fittest]]", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular [[genotype]] corresponds to the average effect on all individuals with that genotype.<ref name=Orr2009>{{cite journal |last1=Orr |first1=H. Allen |title=Fitness and its role in evolutionary genetics |journal=Nat Rev Genet |date=August 2009 |volume=10 |issue=8 |pages=531–539 |doi=10.1038/nrg2603 |pmc=2753274 |pmid=19546856 }}</ref>
A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection".<ref>{{cite journal |title=The Cost of Natural Selection |last=Haldane |first=J. B. S. |author-link=J. B. S. Haldane |journal=Current Science |volume=63 |issue=9/10 |date=November 1992 |pages=612–625}}</ref> Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman.<ref>{{cite journal | last1=Kleinman | first1=A. | year=2014 | title=The basic science and mathematics of random mutation and natural selection | journal=Statistics in Medicine | volume=33 | issue=29 | pages=5074–5080 | doi=10.1002/sim.6307 | pmid=25244620 | doi-access=free }}</ref> An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment.<ref>{{cite journal | pmc=5534434 | pmid=27609891 | doi=10.1126/science.aag0822 | volume=353 | issue=6304 | title=Spatiotemporal microbial evolution on antibiotic landscapes | year=2016 | journal=Science | pages=1147–51 | last1=Baym | first1=M. | last2=Lieberman | first2=T. D. | last3=Kelsic | first3=E. D. | last4=Chait | first4=R. | last5=Gross | first5=R. | last6=Yelin | first6=I. | last7=Kishony | first7=R. | bibcode=2016Sci...353.1147B}}</ref> In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. [[Richard Lenski]]'s classic [[E. coli long-term evolution experiment|''E. coli'' long-term evolution experiment]] is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest").<ref name="pmid18524956">{{cite journal |last2=Borland |first2=Christina Z. |last3=Lenski |first3=Richard E. |year=2008 |title=Historical contingency and the evolution of a key innovation in an experimental population of ''Escherichia coli'' |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=23 |pages=7899–906 |bibcode=2008PNAS..105.7899B |doi=10.1073/pnas.0803151105 |jstor=25462703 |pmc=2430337 |pmid=18524956 |last1=Blount |first1=Zachary D.|doi-access=free }}</ref> The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.<ref>{{Cite journal |title=Distribution of fixed beneficial mutations and the rate of adaptation in asexual populations |first1=B. H. |last1=Good |first2=I. M. |last2=Rouzine |first3=D. J. |last3=Balick |first4=O. |last4=Hallatschek |first5=M. M. |last5=Desai |date=27 February 2012 |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=13 |pages=4950–4955 |doi=10.1073/pnas.1119910109|pmid=22371564 |pmc=3323973 |doi-access=free }}</ref>

===Competition===
{{Main|Competition (biology)}}

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on a [[Limiting factor|limited]] supply of a resource such as food, water, or [[Territory (animal)|territory]].<ref>{{harvnb|Begon|Townsend|Harper|1996}}</ref> Competition may be [[intraspecific competition|within]] or [[interspecific competition|between species]], and may be direct or indirect.<ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal |last1=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |author-link2=Michael Benton |last3=Ferry |first3=Paul A. |date=23 August 2010 |title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land |journal=[[Biology Letters]]|volume=6 |issue=4 |pages=544–547 |doi=10.1098/rsbl.2009.1024 |pmc=2936204 |pmid=20106856}}</ref> Species less suited to compete should [[competitive exclusion principle|in theory either adapt or die out]], since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among larger [[clade]]s.<ref name="SahneyBentonFerry2010LinksDiversityVertebrates"/><ref name="Jardine2012">{{cite journal |last1=Jardine |first1=Phillip E. |last2=Janis |first2=Christine M. |last3=Sahney |first3=Sarda |last4=Benton |first4=Michael J. |date=1 December 2012 |title=Grit not grass: Concordant patterns of early origin of hypsodonty in Great Plains ungulates and Glires |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=365–366 |pages=1–10 |doi=10.1016/j.palaeo.2012.09.001|bibcode=2012PPP...365....1J }}</ref>

Competition is modelled by [[r/K selection theory|''r/K'' selection theory]], which is based on [[Robert MacArthur]] and [[E. O. Wilson]]'s work on [[Insular biogeography|island biogeography]].<ref>{{harvnb|MacArthur|Wilson|2001}}</ref> In this theory, selective pressures drive evolution in one of two stereotyped directions: ''r''- or ''K''-selection.<ref>{{cite journal |last=Pianka |first=Eric R. |author-link=Eric Pianka |date=November–December 1970 |title=On ''r''- and ''K''-Selection |journal=[[The American Naturalist]] |volume=104 |number=940 |pages=592–597 |doi=10.1086/282697 |jstor=2459020|bibcode=1970ANat..104..592P |s2cid=83933177 }}</ref> These terms, ''r'' and ''K'', can be illustrated in a [[Logistic function#In ecology: modeling population growth|logistic model]] of [[population dynamics]]:<ref name=Verhulst>{{cite journal |last=Verhulst |first=Pierre François |author-link=Pierre François Verhulst |year=1838 |title=''Notice sur la loi que la population suit dans son accroissement'' |url=https://archive.org/details/correspondancem02belggoog |language=fr |journal=Correspondance Mathématique et Physique |location=Brussels, Belgium |volume=10 |pages=113–121 |oclc=490225808}}</ref>

<math display="block">\frac{dN}{dt}=rN\left(1 - \frac{N}{K}\right) \qquad \!</math>
where ''r'' is the [[Population growth#Population growth rate|growth rate]] of the population (''N''), and ''K'' is the [[carrying capacity]] of its local environmental setting. Typically, ''r''-selected species exploit empty [[Ecological niche|niches]], and produce many offspring, each with a relatively low [[probability]] of surviving to adulthood. In contrast, ''K''-selected species are strong competitors in crowded niches, and [[Parental investment|invest]] more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood.<ref name=Verhulst/>