Repeated Trait Evolution Driven by Divergent Natural Selection at Early and Late Stages of Speciation

Repeated Trait Evolution Driven by Divergent Natural Selection at Early and Late Stages of Speciation Spencer J. Ingley Department of Biology, BYU Doctor of Philosophy Speciation – the process by which new species arise – is of fundamental importance in the biological sciences. The means by which new species arise, and the relationship among living species, has been a topic that has captivated both lay and scientific observers for centuries. In recent years, the study of speciation has enjoyed increased attention, resulting in significant advances in our understanding of how species form. Although our understanding of the processes that contribute to speciation has increased dramatically in recent years, our knowledge of how reproductive barriers accumulate as speciation proceeds is still limited. Thus, studies that evaluate trait divergence and its consequences at early verses late stages of divergence can provide valuable insight into the speciation process. Chapter 1 of my dissertation focuses on the role of animal personality in the speciation process. Animal personality – defined as consistent individual differences in behavioral tendencies – has been identified as a key player in several ecological and evolutionary processes, yet the role of personality in speciation remains unexplored. In this chapter I discuss the ways by which personality can contribute to a suite of reproductive barriers and drive the speciation process. Chapters 2 through 5 provide a case study evaluating how selection acts on traits at early and late stages of speciation, using the Neotropical Livebearing fish genus Brachyrhaphis as a model system. Brachyrhaphis is ideally suited for this research because several species pairs and population pairs within species occur in similarly divergent selective regimes. I first present results from a field demographic study that shows that the strength of divergent selection acting on life-history traits in populations from divergent predation environments diminishes as speciation proceeds. I then show that population pairs at different stages of divergence are evolving similar morphological patterns along parallel trajectories. At both early and late stages of divergence, populations from environments with dense predator populations have a body shape that appears to be optimized for burst-speed swimming, and important component of predator escape. In contrast, populations from environments lacking predators have a body shape optimized for endurance swimming ability, which is important in environments where competition for foods and mates is high. Next, I show that populations from divergent predation environments do indeed differ in their swimming abilities according to our predictions, reflecting a population level trade-off between burst and endurance swimming ability. Although population level trade-offs were strong, I found no evidence of within population level trade-offs, suggesting that populations have arrived at novel solutions to between population trade-offs that were not present within ancestral populations. Finally, I show that these specialized swimming modes are locally adaptive, and that divergent ecology selects against immigrants, effectively reducing gene flow between populations from divergent environments. Together, these studies provide a valuable glimpse into the repeatability and predictability of trait divergence at different stages of speciation.