Meeting Abstract
The net opportunity for sexual selection depends on the joint contributions of precopulatory selection, which arises from variance in mating success, and postcopulatory selection, which arises from variance in fertilization success. These two components of sexual selection can act in concert or in opposition to one another, but it is generally unknown whether the opportunities for pre- and postcopulatory selection tend to covary in predictable fashion. To test for a general relationship between pre- and postcopulatory sexual selection, we collected data from 148 species of squamate reptiles (116 lizards, 32 snakes). In squamates, competition for territories and mates often favors large body size in males, such that the degree of male-biased sexual size dimorphism (SSD) can be treated as a proxy for the intensity of precopulatory selection. In species experiencing strong postcopulatory selection, males typically invest heavily in testicular tissue to increase sperm production, such that testis size (relative to body size) can be treated as a proxy for the intensity of postcopulatory selection. Using both conventional and phylogenetically based analyses, we found a significant negative relationship between relative testis size and male-biased SSD across squamates. Lizards and snakes thus appear to be arrayed along an axis ranging from those that experience relatively strong pre- and weak postcopulatory selection, to those that experience relatively strong post- and weak precopulatory selection. This suggests that precopulatory selection may constrain the opportunity for postcopulatory selection, but that when precopulatory selection is relaxed, the opportunity for postcopulatory selection may increase. This inverse relationship between these two components of sexual selection suggests that their impacts on trait evolution may be non-independent.
