On Friday, Jenny Boughman, professor of zoology at Michigan State University and researcher for the BEACON Center for the Study of Evolution in Action, presented at the biology department’s winter 2011 Cramer Seminar Series.   Her presentation focused on polymorphism, sex dimorphism, and speciation in three-spine stickleback fish, specifically the effects of the environment on their evolution.

Boughman opened with remarks about how species differentiate, asking “what are the underlying forces that give rise to diversity, and what maintains this diversity?”  She also discussed the less predictable relationship between dimorphism-male characteristics v. female characteristics-and speciation, the development of two or more unique species.  She posed the question, “Does natural selection oppose sexual selection, or does it work in concert?”

Her lab has studied stickleback size and shape in 14 landmarks and lakes near Vancouver Island, British Columbia and found four distinct clusters of physiological characteristics: limnetic males, limnetic females, benthic males, and benthic females.  Both the small, slender limnetic species and the larger, more robust benthic species originate from the same marine ancestors. But, after disruptive selection acted on the population that migrated to this region, they evolved to show differences in mating traits, odor, color, and foraging patterns.

However, there was an anomaly in the study of Enos Lake on Vancouver Island.  As a result of fluctuating water levels, toxicity in the lake, and an ecological disturbance about 25 years ago, two stickleback species converged to have very similar traits. Yet, there still was a noticeable difference between the shape and size of the two sexes.  The dimorphism persisted even though natural selection favored an intermediate shape and size in this lake; more crucial, however, is their survival, because the sticklebacks are an endangered species in this area.

In Paxton Lake, another lake on Vancouver Island studied by Boughman’s lab, four distinct clusters for both shape and size were found, meaning that two different species still thrived.  Here, she found that limnetic males were larger than limnetic females, but benthic females larger than benthic males. This goes back to the question of whether natural selection acts with or opposes dimorphism. Most importantly, the differences reflect each species adapting to what provides it the best fitness, which indeed makes the prediction on a case-by-case basis as to whether the two forces act together or oppose one another.

Boughman has plans to study the causes of reproductive isolation and the relationship between color, size, and the overall gene pool in sticklebacks.