Are There Advantages of Having Sex in Novel Environments?

Abstract

The evolution of general-purpose genotypes (GPGs) is one of several hypotheses to explain the geographically widespread distribution of parthenogenetic organisms despite recognized disadvantages of asexual reproduction. Empirical tests of the GPG hypothesis, that have compared the performance of related parthenogenetic and sexual races in variable or stressful environments, have been equivocal (Bierzychudek 1989, Michaels and Bazzaz 1989, Weider 1993, Parker and Niklasson 1995, Gade and Parker 1997). Here we propose that employing chemical pollutants as environmental stressors can contribute relevant empirical evidence for testing the GPG hypothesis. In this context, the extent to which an environmental variable has been a selective force during the evolutionary history of the population under investigation, as well as the specific properties of the organism's biochemical/physiological stress-response machinery, may determine whether empirical observations support predictions of GPG or not. We shall consider these issues in particular with regard to chemical pollutants and will make specific predictions about the types of exposure scenarios likely to favor parthenogenesis versus mixis. We begin by briefly reviewing the issue of sex in an evolutionary context. We then discuss the extent to which novel environmental gradients may be relevant to testing predictions of the GPG hypothesis. Finally we make testable predictions about how the degree and scale of environmental heterogeneity are likely to favor different reproductive modes.