On Wednesday, March 24, biology professor Jordan Price presented an explanation of the sexual selection of expressed traits. The lecture, which took place in Schaefer Hall, was part of the Natural Science and Mathematics Colloquium series.
After an introduction from College biology professor John Ramcharitar, Price began his lecture by explaining the definition of sexual selection. “Sexual selection is the evolution of traits that evolve solely because they enhance reproductive success,” he said. Unlike natural selection, a theory of evolution conceived by naturalists Charles Darwin and Alfred Wallace in the mid-1800s, sexually-selected traits do not necessarily increase the fitness of an individual; instead, the trait is preferred by the opposite sex.
“Differences we see in the sexes are always presumed to be due to sexual selection,” said Price. This includes size dimorphisms (one sex having a larger overall size than the other), dichromatism (differences in color between the sexes), and even behavioral dimorphisms (one sex exhibiting behaviors different from those exhibited by the other sex). These dimorphisms allow for intraspecific competition between members of the same sex (usually between males) and intersexual mate choice between the sexes (usually by females).
“What is less understood is female choice,” said Price. “There are some questions we don’t understand yet about how it works.”
Price discussed two theoretical models used to try to explain this phenomenon, the “good genes” models (in which females prefer males with higher genetic quality, as exhibited by environmentally-influenced and energy-costly traits) and the Fisherian “runaway selection” models (in which genes for the trait and preference for that trait both exist, leading to a multitude of potential genetic outcomes).
From there, Price began a discussion of his research, a culmination of phylogenies (a tree-like diagram showing the evolution of species based on specific traits) and an extensive collection of bird data. He analyzed three assumption of sexual selection: that males are usually more elaborate and conspicuous than females (meaning that males change more often than females); that changes occur more rapidly in polygynous (one male mating with many females) than in monogamous (one male mating with one female) species; and that Fisherian selection cannot be tested empirically.
“There are different ways that dimorphism can evolve,” said Price as he addressed the first assumption. The difference could be due to the gain of a male trait, or loss of the female trait, and later differences in the expression could be due to the gain or loss of the trait in either sex.
After studying patterns in bird song and color patterns of New World blackbirds, the phylogenies seem to show the loss of bright colors and singing in females rather than a gain of these traits in males, questioning the validity of this assumption.
To verify the second assumption, Price analyzed two groups of blackbirds, the polygynous oropendolas and mostly monogamous caciques. Comparing genetic with behavioral observations, he determined that oropendolas seem to have evolved dichromatism and changes in song much more rapidly than did the caciques, validating the second assumption.
While Price has studied song patterns and plumage in these birds and the monogamous orioles to observe relationships among species, the validity of the third assumption has not been proven or disproven. “We’re not sure, and we still need to accumulate evidence for that,” he said.
It seems that while the monogamous orioles follow the “good genes” model through the repeated convergence and reversals of selected traits, the polygynous oropendolas seem to follow the Fisherian model in that new traits accumulate with little to no convergence, leading to a possible conclusion that the models each explain different relationship styles.
“I really enjoyed Dr. Price’s lecture,” said sophomore Sam Berry. “He did a really good job with the topic and tied it all together well with the three assumptions about sexually selected traits and two theories about the evolution of sexually-selected traits.”
“Dr. Price presented an animated and easy-to-follow overview of sexual selection,” said Ramcharitar. “It is clear that he continues to be a highly productive and innovative scientist.”
The next NS&M lecture will be given on Mar. 31 by College mathematics professor David Kung.