DIVERGENT SELECTION, NOT LIFE-HISTORY PLASTICITY VIA FOOD LIMITATION, DRIVES MORPHOLOGICAL DIVERGENCE BETWEEN PREDATOR REGIMES INGAMBUSIA HUBBSI

Abstract

We recently tested the hypothesis of ecological speciation in a post-Pleistocene radiation of Bahamas mosquitofish (Gambusia hubbsi) inhabiting inland blue holes (vertical, water-filled caves) on Andros Island, the Bahamas (Langerhans et al. 2007). Combining morphological, molecular, and behavioral data, our results suggested that enhanced premating isolation evolved as a byproduct of differences in body shape between predator regimes, due to assortative mating for body shape. Our results strongly supported the hypothesis of ecological speciation, and parallel results were observed across species within the genus, suggesting a past history of such ecological speciation within the Gambusia lineage. Downhower et al. (in press) question the morphological results described for G. hubbsi in our paper, contending that prior work on life-history variation in G. hubbsi (primarily Downhower et al. 2000, 2002) invalidates our conclusions. Here we show that this contention is based on a mischaracterization of our study, inaccurate quotation of our paper, and misinterpretation of the relevance of their life-history data for morphological results observed in our study. Downhower et al. (in press) suggest—without providing any direct evidence—that differences in body shape between predator regimes in G. hubbsi reflect correlative effects of phenotypic plasticity in life-history traits in response to variation in food availability. To make such a conclusion, we would have to set aside the following facts: (1) morphological differences were observed in adult males, who do not exhibit the life-history traits in question (e.g., number and size of offspring), (2) results matched well-supported a priori predictions of body shape variation based on divergent selection on locomotor performance, (3) results matched empirical results observed in other fish presumably experiencing similar forms of divergent selection between predator regimes, (4) the observed morphological differences are known to influence swimming performance in Gambusia fish and consequently influence endurance and survival with predators, (5) consistent results were observed for males both within and between Gambusia species, (6) any differences in food availability among blue holes are unknown, and (7) available evidence to date indicates that differences in body shape partly reflect genetic divergence, and not solely phenotypic plasticity. The criticisms in Downhower et al. (in press) can be summarized as three arguments: (1) incorrect attribution of evidence for divergent selection between predator regimes in G. hubbsi to several previous