Similar rates of protein adaptation in Drosophila miranda and D. melanogaster, two species with different current effective population sizes.
BACKGROUND
Adaptive protein evolution is common in several Drosophila species investigated. Some studies point to very weak selection operating on amino-acid mutations, with average selection intensities on the order of Nes approximately in D. melanogaster and D. simulans. Species with lower effective population sizes should undergo less adaptation since they generate fewer mutations and selection is ineffective on a greater proportion of beneficial mutations.


RESULTS
Here I study patterns of polymorphism and divergence at 91 X-linked loci in D. miranda, a species with a roughly 5-fold smaller effective population size than D. melanogaster. Surprisingly, I find a similar fraction of amino-acid mutations being driven to fixation by positive selection in D. miranda and D. melanogaster. Genes with higher rates of amino-acid evolution show lower levels of neutral diversity, a pattern predicted by recurrent adaptive protein evolution. I fit a hitchhiking model to patterns of polymorphism in D. miranda and D. melanogaster and estimate an order of magnitude higher selection coefficients for beneficial mutations in D. miranda.


CONCLUSIONS
This analysis suggests that effective population size may not be a major determinant in rates of protein adaptation. Instead, adaptation may not be mutation-limited, or the distribution of fitness effects for beneficial mutations might differ vastly between different species or populations. Alternative explanation such as biases in estimating the fraction of beneficial mutations or slightly deleterious mutation models are also discussed.
Bachtrog
Doris
D
Department of Integrative Biology, University of California Berkeley, Berkeley, CA 94720, USA. dbachtrog@berkeley.edu
D019943:Amino Acid Substitution; D000818:Animals; D004330:Drosophila / Q000145:classification / Q000235:genetics*; D029721:Drosophila Proteins / Q000235:genetics*; D019143:Evolution, Molecular; D009154:Mutation; D011110:Polymorphism, Genetic; D011156:Population Density; D012641:Selection, Genetic
D029721:Drosophila Proteins