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Vol. LIX, No. 14
July 13, 2007

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Up with Diversity
Talk on Evolution Sheds New Light On Darwinian Theory

On the front page...

Dr. Joan Roughgarden is so bold an evolutionary biologist, she's willing to take on Charles Darwin. "I invite you to consider the possibility that the whole sexual selection theory should be junked, lock, stock and barrel," she said about one of the famous naturalist's tenets to an NIH audience recently. "And we should rethink sex, gender and sexuality from the ground up."

This idea may dismay some people, she conceded during her talk, part of a four-speaker lecture series on Evolution and Medicine sponsored by NIGMS. But she believes challenging the long-held theory can be viewed positively. "It means we are in a time when we can do some research that pertains to the foundational structure of evolutionary biology."


  Dr. Joan Roughgarden takes issue with Darwin.  
  Dr. Joan Roughgarden takes issue with Darwin.  
Roughgarden, a Stanford University professor who published a widely discussed book on this subject - Evolution's Rainbow, in 2004 - said she had no criticism with "the rest of Darwin," and that "there's just one particular topic on which I feel he is mistaken." But because sexual selection is so tied to the way people view gender and sexuality, she said, her views are considered controversial.

"We would have hoped that by taking a scientific look at gender and sexuality, we may find some clarity to settle these troubled times," she explained, adding that instead, the scientific community still has difficulty confronting the diversity in gender and sexuality found among animals. So in her book, and in this lecture, she made a case for a new approach.

When Darwin first developed his theory on sex roles called sexual selection in 1871, Roughgarden said, it was originally posed about a specific issue: the traits on tails of peacocks, called ornaments. But it so motivated him, he began to apply his ideas to the larger animal world. "Males of almost all animals have stronger passions than females," Darwin wrote, while "the female.with the rarest exceptions, is less eager than the male.she is coy." Females, he explained, also choose mates that are "well-armed and attractive," and therefore females breed the traits that males exhibit.

This theory - of the "passionate" male and the "coy" female - still holds sway today, Roughgarden noted. Scientists have recently argued that males try to spread their genes by being promiscuous, while females, because their eggs are harder to produce and therefore more valuable, are more choosy in an effort to find genetically superior males.

The problem with this "narrative," Roughgarden said, is that "once you start taking data from more and more species, these generalizations emerge as no longer true. There are lots of species where the males aren't passionate and the females aren't coy."

Roughgarden outlined where her ideas diverge from Darwin’s theory of sexual selection.
Roughgarden outlined where her ideas diverge from Darwin's theory of sexual selection.
Nonetheless, the theory "continues to have a life of its own, in spite of the need to revise the empirical claims it was supposed to explain in the first place." Now, she said, this idea that started out specifically explaining the traits of one species has extended to become "a system of evolutionary sex, gender and sexuality," with all kinds of interconnected propositions. Because of this, testing one small piece of the theory "puts us in the position of having to take a fairly radical stance.saying you have to change the whole system from top to bottom."

One can start to revisit the ideas behind this system by looking at the many "problematic species" for sexual selection theory, Roughgarden said. Fish, for example, are often both male and female - switching from one to the other or being both simultaneously. Other species of animals exhibit sex-role reversal. In seahorses, when eggs are cared for, this work is done by males: females, in fact, deposit their eggs into a male's pouch making the male, in effect, pregnant. And even though the male "gamete," the sexual reproductive cell or sperm, is much smaller than the female's, it is the male that provides more parental investment, again contradicting ideas of sexual selection.

Another issue Roughgarden cites is the number of "templates" per sex. According to Darwin, there is one template for each sex - the passionate male and coy female. But there are many species that have multiple templates for both males and females. One seabird in Europe includes males with three different types of "ruffs" or collars and mates in different ways; females prefer to lay eggs with pairs of males with different ruffs. This means there are actually three male templates in this species.

Then there is the issue of homosexual mating. Roughgarden said there are 300 species of vertebrates in which homosexual mating is known. "This is a very clear phenomenon," she said, explaining that, of these species, there are few generalizations that can be made. In some, just males engage in homosexual mating; in others, just females. In others, just a few animals engage in it and in still others, all animals of the species do.

All of which points to "a whole panoply of phenomenology that lies outside of the Darwinian narrative," she said, a tip-off that there's a "big problem" with the theory. Providing many other examples of how sexual selection is "falling apart," Roughgarden then outlined the replacement theory she has been working on with colleagues in her lab, which they call social selection. The basic distinction between theories is that under sexual selection, behavior is considered to be a "mating system," she said. "Therefore, natural selection arises from differences in mating success. And in this narrative, females are a limiting resource for males, males compete for mating opportunities and females choose males for genes.

"Our alternative.envisions that social behavior is actually an offspring-rearing system. It's not all about getting mates, it's about producing young. And therefore, natural selection arises from differences in success at rearing young."

In this theory, males and females negotiate "bargains and side-payments" with each other to maximize offspring production - as opposed to genetic quality - and to control the offspring-rearing social infrastructure. Each of the researchers in Roughgarden's lab is working on different projects to advance data on this theory. One, for example, is trying to determine the evolutionary force leading to gametes being different sizes; another is considering "cooperative game theory," and a third is looking at side-payments between birds that increase numbers of eggs.

Roughgarden is proud of these researchers who are "taking a lot of heat," choosing to work on a controversial topic with risk to their careers. She concluded by urging the audience to see the new work in a positive light.

"Sexual selection theory at the moment is acting as a mental straitjacket on us.and I just hope people can appreciate that if we get away from it, we're better off," she said. "It's not as if this is hurting evolution. It's helping evolution by, first of all, making it more accurate, and also by just making it more fun because we have new ideas to think about.

"This is not something to be alarmed about," she added. "It is really a fabulous opportunity." NIH Record Icon

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