Editing Natural selection (section)

===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>