Nick Barton*
Institute of Science and Technology (IST) Austria, Klosterneuburg, Austria
Citation: Barton N (2010) Understanding Adaptation in Large Populations. PLoS Genet 6(6): e1000987. doi:10.1371/journal.pgen.1000987
Editor: Harmit S. Malik, Fred Hutchinson Cancer Research Center, United States of America
Published: June 17, 2010
Copyright: © 2010 Nick Barton. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The author received no specific funding for this article.
Competing interests: The author has declared that no competing interests exist.
* E-mail: nick.barton@ist.ac.at
For the past half-century, population genetics has been dominated by studies of molecular evolution, interpreted under the neutral theory. This predicts that the rate of substitution equals the rate of neutral mutation and that the genetic diversity within populations depends on the product of population size and neutral mutation rate, 4Nμ. Yet, diversity clearly does not increase in direct proportion to population size [1], [2]. Bacterial populations are typically more diverse than insects, which in turn are more diverse than mammals, but these differences span only an order of magnitude, even though actual population sizes vary far more. We can see that the standard neutral theory makes no sense for very abundant species; it predicts that genes share common ancestry 2N generations back, which may often be older than the species, and, for microbes, older than the planet itself.
In abundant species, conventional random drift must be negligible; diversity is instead limited by occasional drastic bottlenecks and by recurrent selective sweeps [3]. Often, the net rate of such sporadic events is described by defining an “effective size,” Ne, which is much smaller than the actual census size. However, this effective size is only a description of the level of neutral diversity and does not tell us how random drift influences the adaptive alleles that actually matter to the organism. It is crucial to distinguish, here, between short-term factors such as sex ratio or variance in offspring number that increase the rate of random drift, and more drastic events such as bottlenecks or selective sweeps that affect the whole population. The former may reduce the short-term effective population size by as much as an order of magnitude below the census number [4], but nevertheless, random drift will be negligible if the census number is sufficiently high. In contrast, selective sweeps and severe bottlenecks are essentially independent of the typical population number and limit neutral diversity in the long term.
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