It's all natural: biologist and transwoman Joan Roughgarden takes on Darwin and science's traditional view of sexuality.

Joan Roughgarden had a revelation as she marched in San Francisco's 1997 gay pride parade. The Stanford University professor of biological sciences--who was herself then transitioning to life as a woman--was struck by the overwhelming physical diversity of the tens of thousands of people lining the parade route. Male, female, gay, straight, and everything in between: The variety seemed endless.

"What struck me was how many people there were," Roughgarden recalls. She realized that by its very size, the crowd contradicted the rules of traditional biology, in which sexual and gender variance are ignored, disparaged, or discounted.

"Basically," she says now, "if there's a theory that there is something wrong with so many people, it's the theory that's wrong and not the people. So that's what planted the seed."

That seed grew into Roughgarden's new book, Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People (University of California Press, $27.50), which not only throws open the animal kingdom's closet doors but also rebuts Charles Darwin's classic theories of sexual selection, where male and female roles are strictly defined.

"Males are supposed to be these passionate, ardent warriors, basically competitors; and females are supposed to be choosy, coy, discerning damsels," Roughgarden explains of Darwin's theory, which has formed the basis of sexual-selection study for generations. "He imagines that females are choosing the males who are the winners at male-male competition."

The problem is, numerous examples abound that show females choose not for the best genes, but for the best mate. In her book Roughgarden points to many species, including lions, that mate partially as a defense mechanism. "If a male is potentially going to kill a female's offspring, she makes sure she mates with him so that he shares partial paternity of those offspring and therefore has no incentive to kill them," Roughgarden argues. "Another main reason has to do with whether or not a male will actually deriver on the promise of parental care."

And as for the "unnatural" act of homosexuality, Roughgarden gives detailed descriptions of the wide range of same-sex sexual behavior in everything from fish to swans to chimpanzees, something never reported in nature documentaries or at zoo exhibits. "I think it's fair to say almost everybody among professional biologists realizes [that if we label it 'unnatural'] we've got a problem on our hands with the amount of homosexuality that occurs," she says, "because 300 species is not loose change."

But as every good scientist knows, there has to be some biological reason for homosexuality, or it wouldn't exist at all. Roughgarden doesn't know for sure, but she has some good ideas about why it exists. "It's beneficial because it leads to building relationships," she suggests. "And that's a precondition for all kinds of social interactions throughout the day and throughout the animal's life. And so it's really valuable as a mode of communication."

Critics have charged that, as a transwoman and a supporter of LGBT rights, Roughgarden is pushing her own agenda--a claim she embraces, since it doesn't change the evidence. "The factual issues that provoke this discussion are still there, and they know it," she argues. "And then, of course, there's the further issue of whether they have an agenda. So while I'm up-front with what mine is, they are just being secretive about theirs."

Roughgarden is on something of a mission, using her book not only to challenge Darwin but to get the scientific and research community to open their eyes to the homosexual behavior they have been ignoring throughout their entire careers. She also hopes that her work opens up the world of natural history to LGBT people who never felt they belonged before.

"Animals are kinder to one another than we have been led to believe," Roughgarden says, "and nature has more space for people like us than we have been led to believe."