Uniform Selection as a Diversifying Force in Evolution: Evidence from Drosophila

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Abstract
We have examined whether uniform directional selection enhances genetic divergence among conspecific populations and between sibling species. Selection for increased knockdown resistance to ethanol was imposed on two pairs of sibling species Drosophila: five populations of D. melanogaster and one population of sibling D. simulans; and five populations of D. pseudoobscura and one population of D. persimilis. After selection, lines were measured for the selected character and for other characters potentially associated with the selected phenotype. Selection enhanced divergence among D. melanogaster populations for the selected character and for other characters potentially associated with the selected phenotype. Selection enhanced divergence among D. melanogaster populations for the selected character and several other characters showing a correlated response to selection, but divergence among D. pseudoobscura populations was not affected by selection. Selection increased the divergence between D. simulans and D. melanogaster for the selected character more than it increased the divergence among D. melanogaster populations. However, D. melanogaster and D. simulans showed similar correlated responses, and differences between these species for the correlated characters were not increased by selection. Divergence between D. persimilis and D. pseudoobscura for the selected character and correlated characters was not altered by selection. These results suggest that conspecific populations do not necessarily become genetically or phenotypically similar as a consequence of selection and also that the potential for genetic divergence under uniform selection does not necessarily show a quantum increase as a result of speciation. The results also imply that genetic-covariance matrices may not be constant during evolution.