A fundamental question in evolutionary biology is how biodiversity evolves and is maintained. In many species, males vary markedly in anatomical, physiological, and behavioral traits that constitute discrete alternative strategies for obtaining mates. A proposed mechanism for the maintenance of such variation is frequency dependence, which states that if a given male strategy becomes rare its success in reproductive competition will increase relative to the other strategies therefore preventing its extinction. In the fish Xiphophorus nigrensis, there are two strategies that are correlated with male body size, small 'sneaker' males that pre-empt female choice by forcing copulations and larger 'courting' males that produce behavioral displays females find attractive. To test the frequency dependence hypothesis, sneaking and courting male X. nigrensis will be distributed at different frequencies in semi-natural enclosures and their reproductive success measured using genetic markers. The success of the strategies in nature will also be measured by comparing strategy frequency in the adult population to strategy frequency in a sample of offspring collected from pregnant females. In addition, this study will examine the effects of frequency dependence on the genetic diversity within individual broods produced by females. Sneaker male X. nigrensis produce ejaculates that contain both a higher proportion of living sperm and sperm that is longer lived, and as a result it is expected that variation in the quality of sperm between strategies will influence the rates of multiple paternity. This research will broaden our understanding of how frequency dependence affects patterns of genetic variation both at the population level and among offspring within broods. The broader impacts of this project involve training of a graduate student, mentorship of undergraduates and outreach to elementary school children.
