ILLUSTRATION BY TARA JACOBY
What we can learn about evolution from species who thrive without sexual reproduction.
Decades ago, behavioral neurobiologist David Crews read a strange report about the desert grassland whiptail, a small, slender lizard that lives in the sagebrush of the American Southwest. The paper claimed that the species was entirely female, and reproduced by cloning. It tested the limits of what Crews felt to be biologically plausible in higher vertebrates. “I didn’t believe that such a thing existed,” he says. But he was curious, and a friend who was going to New Mexico offered to collect some from the wild. Crews, who at the time was at the Harvard Museum of Comparative Zoology, installed a half-dozen whiptail lizards in glass tanks in his animal room. One day, he noticed a lizard biting at her cagemate’s rear legs and tail, and soon after that, riding atop her. Crews instantly recognized that they were doing what lizards do when they have sex. But why would two females simulate the act of mating?
“I literally fell out of my chair trying to get to my camera,” Crews says. “In those days, you’d keep the film in the freezer, and I’m trying to cram it into the camera so I could document it, because at that time I thought it was rare—just weird.” He snapped photos as the two adopted the “donut” pose—a contorted mating posture in which the top lizard twists around and bites the belly of the other.
Crews, now a professor of zoology and psychology at the University of Texas at Austin, looked beyond the possibility that the whiptails were just having fun. What he saw that day was crucial to his decades-long exploration of the neural basis of sexual behavior in these lizards. For him, that day in the animal room opened up a whole new world of biological understanding. He and other researchers have since shown that parthenogenesis—the ability to reproduce without sex—turns out to be more relevant to our understanding of the relationship between reproduction and evolution than anyone would have believed.
Sex is far from a perfect way to reproduce. It imposes a huge cost on a species, and that cost is called “males.” If roughly 50 percent of a species is made up of males who are incapable of producing babies, it is at a serious reproductive disadvantage relative to another species made up mostly of females capable of reproducing on their own.
And an animal that reproduces by herself has a big advantage when moving into new territory, because she doesn’t need a partner to be fruitful and multiply. Every single one of her babies will also produce its own offspring. Sexual reproduction “seems like a simple thing, but from an evolutionary perspective, it’s so inefficient,” says Rob Denton, who studies unisexual salamanders at Ohio State University. “It’d be so much easier if everyone were female.”
Brains don’t come pre-wired to act male or female, but are organized by sex chromosomes, hormones, environment, and experience.
Despite its drawbacks, sex does seem to offer a species an incontrovertible advantage. It recombines individuals’ genes so that the species as a whole can maintain the diversity of traits it needs to survive whatever challenges—faster predators, changing climate, giant comet impacts—the future may throw at it. That way the species as a whole can benefit from any useful mutations that randomly pop up in one individual. Sex also protects any one individual from inbreeding—being burdened by too many recessive genes, which often are dangerous. “Genetic diversity has always been seen as the way to adapt to changing conditions,” says Warren Booth, an evolutionary biologist and geneticist at the University of Tulsa.
By this logic, parthenogenesis is an evolutionary cul-de-sac. A population of unisexuals with essentially a single genome between them should be catastrophically unprepared for the challenges of survival. And yet the desert grassland whiptail lizard thrives. It’s also not alone. Since DNA testing has become cheap, parthenogenic reproduction has begun to reveal itself like some dark secret that certain species have been hiding from biologists since before Darwin’s voyage on the HMS Beagle. And some researchers have begun to suspect that it might not be such a bad way to survive…