The impact of natural selection on the genome: Emerging patterns in Drosophila and Arabidopsis

Species in the Drosophila and Arabidopsis species groups share the feature of having relatively small, streamlined genomes. In Drosophila, evidence for pervasive negative and positive selection is overturning long-held views about the functional significance of noncoding DNA, the frequency of positive selection, and the extent to which coding and noncoding polymorphism and divergence between species is neutral. However, despite sharing some similarities with Drosophila, Arabidopsis shows quite distinct patterns of selective constraint and positive selection. Two conspicuous differences between these species groups are their effective population sizes and population structure, which may explain lower levels of selective constraint in coding and noncoding DNA of Arabidopsis, more evidence for balancing selection and less evidence for canonical signature of positive selection than in Drosophila species. As more comparative genomic data accumulate in the Arabidopsis group, the combination of polymorphism and divergence data allow these initial contrasts to be quantified on a genomic scale.