Does sexual selection cause speciation? This long-standing but controversial question is receiving a lot of attention currently, partly because of the special role that mate choice can play in determining gene flow. Sexual selection is thought to cause reproductive isolation when male mating signals and female preferences diversify because that can lead to sexual isolation among populations. After many years of relative obscurity this question is now in the limelight, and evidence is beginning to accumulate in its support. Yet for the most part, this evidence is limited to inferring a role for sexual selection when mating traits differ between closely related species that avoid mating with each other. We remain remarkably ignorant of how sexual selection causes reproductive isolation and when it is likely to do so. We lack answers to fundamental questions such as: Is sexual selection a primary driver of speciation, or is it limited to certain taxa or circumstances? How important is it relative to natural selection or drift? If sexual selection is involved, is it arbitrary with respect to environment or is ultimately the product of ecologically-based divergent selection? Which kinds of sexual selection play a role -- sexual selection by sensory drive, good genes, or sexual conflict? What is the genetic basis of traits that confer sexual isolation?
Work in my lab tackles these questions directly. We investigate behavioral and ecological causes of divergence in mating traits, the genetic basis of traits involved in sexual isolation, and are using a comparative approach to evaluate the generality of early results from model systems. To address these questions we use a combination of field observations and experiments, laboratory experiments, quantitative and molecular genetics, and comparative methods. We work at the intersection of several fields, incorporating conceptual underpinnings and methodology from evolutionary genetics, evolutionary ecology, behavioral ecology, and sensory biology. This highly integrative and multilevel approach has proven powerful for uncovering the processes guiding the evolution of behavior and the processes of speciation.
My lab uses an ideal system to study these questions -- species pairs of stickleback fish (Gasterosteus spp.) found in the postglacial lakes of British Columbia. These are extremely young species -- less than 13,000 years old -- providing a window on the speciation process. Evolutionary replication (7 species pairs of sympatric limnetic and benthic sticklebacks) allows direct experiments to test the evolutionary mechanisms involved. We also capitalize on genomic tools available for the threespine stickleback, which has been developed into a premier system for studying the genetics of adaptation.
This work is designed to understand the genetics of species differences in traits that confer sexual isolation in sticklebacks. This work is especially exciting as a counterpoint to work on the genetics of speciation in model taxa (e.g., Drosophila) where species are millions of years old and there is extensive intrinsic genetic isolation. The very recent origin of the stickleback species allows us to investigate the genetic changes that occur during the process of speciation. In addition, much work on the genetics of speciation ignores the evolutionary causes of genetic change. Our work starts by identifying those causes and their effect on phenotypes, and then explores the way this shapes the genome. We are characterizing the genetic architecture of traits known to isolate the species, such as male nuptial color, body size and shape. We use a combination of quantitative genetics, QTL mapping, and admixture mapping to achieve these goals. This work is relevant to testing several models of speciation, and will lay the foundation for further work investigating sexual selection and speciation. Our eventual aim is twofold: to find the genomic regions responsible for color differences, and explore how selection has acted on those regions to structure the genome.
We have a rare opportunity to gain insight into the speciation process by watching it go in the reverse direction. Ecological speciation is a powerful mechanism of diversification. But species that diverge because of differences in ecology are vulnerable to ecological change. We are witnessing a natural experiment - ecological disturbance has triggered the breakdown of a species pair through hybridization. To try to find the silver lining in a sad event (the limnetic-benthic species pairs are listed as endangered in Canada), my lab is using this reverse speciation to understand the forward process of diversification. We're investigating the loss of both premating and postmating isolation – characterizing what HAS been lost and identifying the causes. This work has important implications both for basic science (speciation processes) and conservation (the effects of invasive species on diversity). To this end, we hope to facilitate recovery of this pair, and to help avoid similar problems for the other pairs.
We are collaborating with an electrical engineer (Xiaobo Tan) and computer scientist (Phil McKinley) at MSU to advance our understanding of both behavioral evolution and robotics. We plan to do this by creating realistic two-way interactions between live and robotic fish, designed within an evolutionary computational framework. We focus on predator inspection, an adaptive antipredator behavior, to study sophisticated non-cooperative and cooperative behaviors in fish, and to synthesize control and learning strategies for autonomous robotic systems operating in noisy and unstructured environments. This work is a real meeting-of-the-minds, and requires concerted, interrelated research effort in biology, robotics, and computing. Evolutionary computing will enable the design of realistic robotic fish, and the exploration of important evolutionary questions. The insight from biology will be exploited to enable adaptive behavior for autonomous robotic systems, and the use of robots will give us unprecedented flexibility to test behavioral hypotheses.
Sexual signal evolution
My lab is collaborating with Tom Getty and Chris Klausmeier at the Kellogg Biological Stationa set of projects investigating the dynamics of rapid evolutionary change in sexual signals - traits like frog calls and bird plumage that are typically used by males to attract and secure mates. Our work highlights the loss of mating signals rather than their elaboration, and we use both empirical studies in stickleback fish and field crickets, and mathematical modeling to understand the evolutionary loss of these important traits. To date, most research on sexual selection has emphasized the exaggeration of male signals and very strong female preferences for those signals. Both of these should reduce diversity in sexual signaling systems resulting in one biggest, best male signal. Yet, mating signals are extremely diverse and recent studies in organisms ranging from insects to vertebrates suggest that male sexual traits are frequently lost, despite the contention that female preferences should maintain them. We hypothesize that the dynamics of both trait loss and elaboration contribute to the diversity of sexual systems we see in nature. But, why and how are male signals lost?
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