Ecological Speciation is the development of reproductive isolation as a result of divergent adaptation to different environments. As populations diverge, post-zygotic isolation effects such as low hybrid fitness and zygotic inviability are expected to become increasingly dominant. However, for genetically similar incipient species, post-zygotic effects may not be sufficient to enforce a reduction in gene-flow. Models of allopatric speciation predict that pre-mating isolation may play an important role in reinforcing barriers between species, regardless of genetic incompatibilities. However, evidence for these models is mixed, and remains controversial. In this paper, we examine the extent to which pre-mating isolation resulting from divergent sexually-selected traits is sufficient to generate incipient species. We evolved populations of sexually-reproducing digital organisms that use sexual selection to choose their mates. These populations are then divided, and each half allowed to adapt to divergent environmental conditions (allopatry). We then reunited these populations for a single round of mating and measured the rate of hybridization. We found that sexual selection significantly reduces the number of hybrid matings between populations. Further, we found that post-zygotic effects were only minimally present, despite adaptation to distinct environments, and that there was little difference in both pre-mating and post-zygotic effects between distinct sets of environments. We conclude that sexual selection is a strong force for generating incipient species, even while post-zygotic effects have minimal impact.

Genetic spaces are often described in terms of fitness landscapes or genotype-to- phenotype maps, where each potential genetic sequence is associated with a set of properties and connected to other genotypes that are a single mutation away. The positions close to a genotype make up its mutational landscape and, in aggregate, determine the short-term evolutionary potential of a population. Populations with wider ranges of phenotypes in their mutational neighborhood tend to be more evolvable. Likewise, those with fewer phenotypic changes available in their local neighborhoods are more mutationally robust. As such, forces that alter the distribution of phenotypes available by mutation can have a profound effect on subsequent evolutionary dynamics. We demonstrate that cyclically-changing environments can push populations toward more evolvable mutational landscapes where a wide range of alternate phenotypes are available, though purely deleterious mutations remain suppressed. We further show that populations in environments with drastic changes shift phenotypes more readily than those in environments with more benign changes. We trace this effect to repeated population bottlenecks in the harsh environments, which result in shorter coalescence times and keep populations in regions of the mutational landscape where the phenotypic shifts in question are more likely to occur.