How Can We Resolve Lewontin's Paradox?

Abstract

We discuss the genetic, demographic, and selective forces that are likely to be at play in restricting observed levels of DNA sequence variation in natural populations to a much smaller range of values than would be expected from the distribution of census population sizes alone-Lewontin's Paradox. While several processes that have previously been strongly emphasized must be involved, including the effects of direct selection and genetic hitchhiking, it seems unlikely that they are sufficient to explain this observation without contributions from other factors. We highlight a potentially important role for the less-appreciated contribution of population size change; specifically, the likelihood that many species and populations may be quite far from reaching the relatively high equilibrium diversity values that would be expected given their current census sizes.

Keywords: coalescent time; diversity; effective population size; genetic drift; hitchhiking; mutation.

Fig. 1.

(A) Displays the nucleotide site diversity (π) as a function of the scaled mutation rate (θ = 4N_eu) for the Jukes–Cantor mutational model, with four alleles at a site and equal frequencies of mutation between each allele. (B) Shows the equilibrium GC content under neutrality and the corresponding infinite population equilibrium value of π, under a mutational model in which all mutation rates between possible basepairs are equal, except for an elevated rate of GC > AT transitions.