Understanding Adaptation in Large Populations
Perspective Understanding Adaptation in Large Populations Nick Barton* Institute of Science and Technology (IST) Austria, Klosterneuburg, Austria For the past half-century, population independent of whatever long-term factors a surrounding genome whose diversity genetics has been dominated by studies determine neutral diversity. Resistance to depends only on the number of favour- of molecular evolution, interpreted under organophosphate insecticides is due to able mutations that enter in every gener- the neutral theory. This predicts that the specific amino-acid changes in the active ation, 2Nm, and whose length depends on rate of substitution equals the rate of site of the target enzyme acetyl-cholines- the inverse of the time to fixation, ,S/ neutral mutation and that the genetic terase, with the most resistant alleles log(S/m) (Figure 1). diversity within populations depends on having three changes. Karasov et al. show In very large populations, the distinc- the product of population size and that although the same amino acids tion between adaptation from new muta- neutral mutation rate, 4Nm.Yet,diver- (indeed, because of constraints from the tions—invoked here by Karasov et al. sity clearly does not increase in direct genetic code, the same nucleotide changes) [5]—and adaptation from standing varia- proportion to population size [1,2]. are always involved, these have arisen tion becomes blurred. If there is selection s Bacterial populations are typically more independently many times on different against resistance alleles before insecticide diverse than insects, which in turn are local haplotypes. Most striking is that is applied, then resistance alleles will be more diverse than mammals, but these complex resistance alleles have arisen present at a frequency of ,m/s, and each differences span only an order of magni- through successive mutations, with no allele will have originated on average tude, even though actual population sizes need for recombination, and all within ,1/s generations back; in this example, vary far more.
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