Meeting Abstract

4.4  Sunday, Jan. 4  Disruptive selection and the evolution of variation within species MARTIN, R.A.*; PFENNIG, D.W.; Univ. of North Carolina, Chapel Hill; Univ. of North Carolina, Chapel Hill martinra@email.unc.edu

What evolutionary and ecological factors account for the amazing diversity seen within most natural populations? A long-standing hypothesis is that disruptive selection promotes the evolution of such diversity by favoring extreme phenotypes. Two distinct mechanisms can cause disruptive selection. First, disruptive selection can occur when extreme phenotypes specialize on separate resources, for which intermediate phenotypes are competitively inferior. Second, disruptive selection can occur when competition is more intense between phenotypically similar individuals, because of shared resource use. This results in the most common phenotypes experiencing intense competition and rare phenotypes experience reduced competition. By the first mechanism, fitness is frequency-independent, meaning that fitness is not expected to vary with the proportion of intermediate and extreme phenotypes in a population. By the second mechanism, however, fitness is frequency-dependent. Although recent theory has emphasized the role of frequency-dependent interactions as the primary cause of disruptive selection, frequency-dependent and frequency-independent competitive interactions are not mutually exclusive. We show that disruptive selection is acting on tadpoles of the spadefoot toad tadpole, Spea multiplicata. We also show that both frequency-dependent and frequency-independent effects drive this disruptive selection. In particular, we show that intermediate phenotypes are less well adapted for handling the available resources than are extreme phenotypes. We also show that competition is more intense between phenotypically similar individuals. In general, disruptive selection may be common in nature, and both frequency-dependent and frequency-independent effects may drive such selection.