Home & Garden Columns
If you visit Mount Diablo this time of year and walk the Fire Interpretive Trail that circles the summit (highly recommended for wildflowers, including the locally rare bitterroot), you’re almost sure to meet one or more of the resident California whiptail lizards. Sometimes they dash across the path from one shelter to another, demonstrating why they’re also called racerunners. But I’ve had some escort me along their personal stretch of trail, keeping a wary eye on me all the while.
California whiptails are fairly normal lizards, if there is such a thing, although as hot-pursuit predators they tend to operate at a higher temperature than the neighboring skinks, alligator lizards, and fence lizards. Normal would describe their reproduction: male meets female, a brusque courtship ensues, eggs get fertilized, the usual.
But they have close relatives in the southwestern United States and northern Mexico—about 15 species altogether—that have evolved a much different approach. Male doesn’t meet female. There are, in fact, no males. These are all-female parthenogenetic species (from the Greek parthenos, “virgin,” as in Athene Parthenos, hence the Parthenon). It’s thought they originated as hybrids between two separate two-sexed species, a mode of speciation that’s rare in animals, although not uncommon among plants. This gives them an extra complement of chromosomes, with sets from both parent species.
These unisexual lizards do pair off and go through the typical whiptail courtship routine. Like Ursula K. LeGuin’s Gethenians, an individual may assume both “male” and “female” roles over her lifetime. Without fertilization, whiptail eggs develop into clones, genetically identical to their sisters and their mothers and, barring the odd mutation, their founding grandmothers.
This is not how vertebrates typically arrange things, of course. There are no parthenogenetic frogs, salamanders, birds, or mammals. But some fish and a number of reptiles do occur in female-only species.
The lizards have gone into it in a big way, with, in addition to the whiptails, some 15 species of unisexual geckoes, night lizards, and representatives of other families. And there’s one parthenogenetic snake: the flowerpot blindsnake, which has traveled all over the world in potting soil.
What’s the advantage of this reproductive mode? Well, if evolution is about maximizing the genes you pass on to the next generation, you can’t get any more maximal than a litter of clones. Parthenogens are great at colonizing disturbed places and remote islands; some of the geckoes that rode Polynesian voyaging canoes all over the South Pacific were all-female species. All it takes to found a new population is one gravid female. If every new addition is a fertile female, you can imagine the shape of the population growth curves.
But, you might counter, if this is such a great deal, why are males (of any species) still around? It has been suggested that there are disadvantages to being a clone. Clones, by definition, have no genetic variety. In most species, males and females endow their offspring with a recombined mixture of their own genes, a brand new shuffle every time. Genetic variety is what makes some individuals more resistant than others to parasites and pathogens. If everyone in the population is a carbon copy, a novel disease might wipe out the lot of them.
Beyond that, variation is the powerhouse of evolution. Genetic recombination gives natural selection something to select among—gene mixtures that may enable the organism to be better at obtaining food, eluding predators, surviving sudden catastrophes or more gradual environmental changes. Without variation, evolution stops. Some biologists have speculated that unisexual species may have fairly short life spans, in terms of geologic time. As good as they may be at exploiting new environments, all those parthenogenetic lizards may be just a flash in the pan.
I wouldn’t want to predict the long-term fate of the whiptails. Note, however, the recent discovery that one group of parthenogenetic animals, the crotoniid soil mites, have gone back to a two-sex reproductive strategy. That’s comparable to a snake re-evolving legs, or a flightless bird re-evolving wings. “Nothing as complex as sex has ever been known to re-evolve,” says mite scholar Roy Norton of the State University of New York, Syracuse. You just never know.
Photograph by Ron Sullivan
A California whiptail lizard on his or her doorstep.