Researchers object to the theory of sexual selection and replace it with game theory.

Is this cute swan couple in conflict or cooperation? Credit: Luca di Filippo

Darwin’s primary legacy, the theory of evolution, has robustly withstood years of scientific challenges. But now a team of Stanford researchers has published a paper in Science claiming they can top Darwin’s second monster: sexual selection theory.

The Stanford group says sexual selection theory wrongly models interactions between the sexes as competitive. The group has a new theory, social selection, which models mate selection as a cooperative game where parties seek to maximize group welfare.

Darwinian sexual selection is a theory of conflict: It asserts that men and women have different goals in terms of what they look for in a partner. Males want to have sex with several females in order to create as many offspring as possible, while females want to have sex with very few, high-quality males, who will give their eggs the best genes.

According to Stanford grad student Erol Akcay, a coauthor of the study, sexual selection theory first arose to explain features that differ between the two sexes as well as traits that appear to be maladaptive, such as bright coloration that a predator can see easily.

“[Darwin] reasons that females are choosier and choose to mate with only the ‘best armed and most rigorous’ males,” Akcay said via e-mail. “Even though the modern theory is more sophisticated and rigorous in conceptual and mathematical terms, the essence of it still holds true, and it is this central point we are objecting to in this paper.”

Biology professor Joan Roughgarden, the lead author of the paper, said males and females of a species have an equal interest in seeing the maximum number of common offspring reared to adulthood.

The whole reason for sexual reproduction in the first place is to share genes, and a male and female who mate are committing themselves to a common investment at the very beginning,” she said. “So therefore, they embark upon the enterprise of mating from a cooperative standpoint, from a standpoint of common investment and common interest.”

Roughgarden said that pairings are often better explained by creating a viable team than by finding the highest quality genes. Couples are usually genetically similar, she said, and their differences are often complementary: Both members provide the team with the strengths the other lacks.

Using Nobel-winning economist John Nash’s bargaining theory, Roughgarden and her team mathematically modeled reproductive behavior. Prior to this application, scientists had only applied economic game theory to biological conflict, never to cooperation, Roughgarden said.

“To make an analogy with humans, the number of children a couple can raise to adulthood is more influenced by the income of the family rather than the genetic makeup,” Akcay said. “We think that in most species, this is what is going on: Males and females choose each other for ecological benefits rather than superior genetic makeups.”

Reactions to Roughgarden’s work have been mixed. Jerry Coyne, a University of Chicago professor of ecology and evolution, reviewed her 2004 book, Evolution’s Rainbow and told Seed that his comments still stand.

“She is wrong,” he wrote. “[Darwin’s] theory is powerful and largely correct. Yes, there are nuances of behavior that require special explanation, or that we don’t yet understand. But nobody, least of all Darwin, ever claimed that evolutionary biology is characterized by ironclad laws. Our field is not physics. Nevertheless, some generalizations, such as the pervasive competition of males for females, can be powerful and useful.”

Roughgarden said plenty of scientists are happy to accept her theory, provided it is subordinate to Darwin’s, and that cooperation comes out of the sexual conflict that is the basis of reproductive social behavior.

“They’d like it, but they’d like somehow to shoehorn it into sexual selection theory,” she said, “because the whole notion of discarding a chunk of Darwin is really problematic to most evolutionary biologists.”

Robert Dorit, a biology professor at Smith College, said there are problems with sexual selection theory and that Roughgarden’s paper will likely open an important dialogue.

“The value of a new hypothesis is not really whether it’s right or wrong, it’s whether it’s sort of stimulating and productive,” he said. “You can be productive and ultimately turn out to have nailed it, or you can come up with a theory that turns out to be productive but ultimately disproven, and that’s fine.”

Dorit added that Roughgarden’s theory will have to meet an extraordinary burden of proof to pass muster.

“She’s sort of thrown down the gauntlet. She’s not nibbling at the edges of established evolutionary theory. She’s really targeting a pretty important component of the sort of neo-Darwinian argument.”

Originally published March 1, 2006

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