Asymmetry in Reproductive Character Displacement

A commonly held view in studies of character displacement is that character states of both speciesare shifted in areas of sympatry. This view has been confirmed in an overwhelming number of cases for ecologicalcharacter displacement. Excluding species pairs in which one of the two interacting species is found onlywithin the distribution of the other species and species displaying gynogenesis, the pattern of reproductivecharacter displacement is asymmetrical in that the shift in character states between areas of symaptry andallopatry occurs in only one of the two interacting species. Hypotheses for the reasons behind this asymmetryin reproductive character displacement include (1) homogenization by gene flow, (2) other mechanisms of reproductiveisolation, and (3) sufficient reproductive isolation being provided by one of the interacting species exhibitinga pattern of reproductive character displacement. Because reproductive isolation can be achieved by divergenceat any point in a sequence of premating reproductive behaviors and postmating developments, it is necessaryto understand the mechanisms of reproductive isolation of two interacting taxa in areas of sympatry and allopatryand to analyze the relative contributions of potential factors to reproductive isolation to disentangle hypothesesfor the patterns of asymmetry.