Editing Human variability (section)

====Genetic drift====
{{Main|Genetic drift}}
[[Genetic drift]] is one method by which variability occurs in populations.<ref>{{Cite web|url=http://www.nature.com/scitable/definition/random-genetic-drift-genetic-drift-201|title=random genetic drift / genetic drift {{!}} Learn Science at Scitable|website=www.nature.com|access-date=2016-11-16}}</ref> Unlike [[natural selection]], genetic drift occurs when [[allele]]s decrease randomly over time and not as a result of selection bias.<ref>{{Cite web|url=http://evolution.berkeley.edu/evolibrary/article/evo_24|title=Genetic drift|website=evolution.berkeley.edu|access-date=2016-11-16}}</ref> Over a long history, this can cause significant shifts in the underlying genetic distribution of a population. We can model [[genetic drift]] with the Wright-Fisher model. In a population of N with 2N genes, there are two alleles with frequencies p and q. If the previous generation had an allele with frequency p, then the probability that the next generation has k of that allele is:<ref name="Hartl_p112">{{harvnb|Hartl|Clark|2007|p=112}}</ref><ref>{{harvnb|Tian|2008|p=11}}</ref>

<math>{2N \choose k}  p^k q^{2N-k} </math>

Over time, one allele will be fixed when the frequency of that allele reaches 1 and the frequency of the other allele reaches 0. The probability that any allele is fixed is proportional to the frequency of that allele. For two alleles with frequencies p and q, the probability that p will be fixed is p. The expected number of generations for an allele with frequency p to be fixed is:<ref>{{harvnb|Hedrick|2005|p=315}}</ref>

<math>
\bar{T}_\text{fixed} = \frac{-4N_e(1-p) \ln (1-p)}{p}
</math>

Where ''N''<sub>''e''</sub> is the effective population size.<ref name="Charlesworth09">{{cite journal|last=Charlesworth|first=Brian|date=March 2009|title=Fundamental concepts in genetics: Effective population size and patterns of molecular evolution and variation|journal=[[Nature Reviews Genetics]]|volume=10|issue=3|pages=195–205|doi=10.1038/nrg2526|issn=1471-0056|pmid=19204717|s2cid=205484393|author-link=Brian Charlesworth}}</ref>