On April 2, 2007, A.Th.E.N.S. Presents

pervasive positive selection among nearly neutral amino acid replacements

by Professor Daniel Hartl

A comparison of levels of polymorphism and divergence between synonymous sites and amino acid replacement sites in a gene is potentially informative about the magnitude of selective forces associated with amino acid replacements. We have analyzed 91 genes in which polymorphism data from D. melanogaster are compared with divergence from a reference strain of D. simulans. The framework of the analysis is Bayesian and assumes that the distribution of selective effects (Malthusian fitnesses) is Gaussian with a mean that differs for each gene. The 91 genes include some with biased expression in testes, others with biased expression in ovaries, and still others with unbiased expression. The estimated parameters inferred from the levels of polymorphism and divergence of these genes are examined in light of selection and drift in the divergence of amino acid sequences.