A well-known sociobiologist, Trivers is most renowned for his revolutionary theories on the evolution of social behavior-specifically, how genes influence the behavior of parents and offspring, how people choose mates and how they interact to get what they want from others in their social groups.
"[Trivers'] papers in the early '70s really came to define the way we think about natural selection," said Professor of Biology William Zimmerman as he introduced Trivers.
"When I first started working in biology in the late '60s, the field of social biology was just getting over some stultifying notions," said Trivers. "We found a whole field of social relationships that opened up a whole new area once we got back to basic Darwinian ideals."
During yesterday's lecture, entitled "The Evolution of Selfish Sex Chromosomes," Trivers addressed the natural phenomenon of species with more females than males. Rather than the usual 50-50 female-to-male ratios, these species exhibit anywhere from 60-40 to 90-10 female-to-male ratios. This phenomenon occurs because of the production of X-drive chromosomes, which have "the tendency to replicate themselves faster … which is a huge evolutionary advantage," he said. This can result in "a situation where the male ends up producing X-drive chromosomes instead of the normal 50 percent X and Y-bearing sperms," he continued.
Trivers explained that the existence of these driving genes has been "discussed since way back in the '20s, but it was viewed then as a slight deviation from Mendel's laws," he said.
There are 13 known species in which the X-chromosomes drive against the Y-chromosomes so that the ratio of females to males becomes unbalanced, according to Trivers. As far as biologists can tell from the evidence available, the tendencies of these species have been stable for millions of years.
"Unchecked, the driving X-chromosome would lead to extinction, with the population collapsing thereafter. So what is keeping the driving X in check?" said Trivers.
Trivers answered his own question by pointing out the "spitefulness" of the driving X-chromosome, not only toward other types of chromosomes but also towards itself. As the number of males decreases, the more each individual male must mate in order for the species to keep up a steady population level. This increases proportion of X-drive chromosomes in the next generation, which in turn increases the level of spitefulness. This keeps the species from going extinct.
Trivers continued his lecture by giving examples of specific species in which scientists have noted imbalanced ratios of females to males, "A most interesting system of feminization has evolved in lemmings," said Trivers.
The lemming phenomenon, as Trivers explained, is the result of the X* chromosome, a mutant X-chromosome. Any lemming that has the chromosome will automatically be female. When a female lemming with the X* chromosome mates, the possible offspring chromosome combinations are X*X, X*Y, XY and YY. The YY combination is inherently unstable, however, and so only three possible combinations are left, so that X* is now in two-thirds of the population instead of half.
"All progeny of X*Y females are female, because they only pass on X*X or X*Y-all females," said Trivers. "[It is] a very curious phenomenon that, when ordering creatures in terms of reproductive success, X*Y is better than the other types of females-X*X and XX-and by best, we are comparing them in mature life." He noted, "X*Y females mature earlier, produce more litters per unit time and reproduce larger litters."
Trivers named several reasons for the increased reproductive capacities of the X*Y females. "X* is always found in females only so it evolves to become a purely female-benefiting chromosome whereas X is in males one-third of the time and so it will evolve some constraints on the females," said Trivers.
During his lecture, Trivers also commented on the possibility of driving Y-chromosomes within species. However, he explained that this phenomenon is virtually nonexistent as "We can show that above 60 percent, Y drive is not advantageous because it suffers too much through the loss of YY ... Paradoxically, a high Y drive may make the female bias higher," he said.
Trivers concluded his lecture by giving examples of several species in which X- and Y-chromosomes have expressed themselves in species other than through the gender ratios. "With red deer, the Y-chromosome tends to like dominant females and X-chromosome tends to like subordinate females," according to Trivers. With birds, the Y-chromosome tends to be linked with attractiveness. "[With humans,] the X-chromosome is more interested in breasts and hips and Y in clear skin and traits for both sexes," he said.
Trivers has written many papers and books on the evolution of altruism, spite and deceit, including the "Social Evolution," a well-known sociobiological textbook published in 1985. He is currently pursuing studies regarding self-deception by analyzing voice recorder data from plane crashes, people's responses to questions about homophobia and the testimony of NASA employees before congressional committees. He is also studying Jamaican schoolchildren for "fluctuating asymmetry," examples of which are an eye, hand or foot that is larger than the other. Trivers is taking what he learns about fluctuating asymmetry and looking at how it correlates with social traits such as the choice of sexual partners, aggressiveness and academic achievement.
Trivers earned his B.A. from Harvard in U.S. history in 1965 and illustrated children's books for two years before returning to Harvard, where he received his Ph.D. in biology in 1972.