Most of us are probably familiar with the concept of reproduction. In nature, this usually involves a female organism mating with a male organism to produce fertilized eggs, which then develop into offspring. However, females of some species of animals, especially insects, can produce viable, fertile offspring without needing any sperm, the male sex cell, to fertilize her eggs. This process is known as parthenogenesis. Some vertebrates can reproduce this way as well—for example, female hammerhead sharks, Komodo dragons, and Californian condors can all produce fertile offspring on their own, though they can also reproduce with a male. Intriguingly, there also exists species that reproduce by “obligate” parthenogenesis, which means they only reproduce without a male gamete. In fact, there are no males in these species at all, and the entire population is female.

The desert grassland whiptail lizard, aptly given the Latin binomial name Aspidoscelis uniparens, is one such example. They are a species of relatively small reptiles, around ten centimeters or three inches long, with a thin tail longer than their entire body length. They are an all-female unisexual species and likely arose from hybridization events, like how a donkey and a horse can cross-breed to form a mule, only that this cross-breeding happened multiple times in the case of the whiptail lizard. And unlike the sterile mule, the offspring produced by these hybridization events can reproduce. This convoluted origin led to some interesting genetic composition—organisms usually have one (haploid) or two (diploid) sets of chromosomes, but these lizards are triploid, meaning they have three full sets of chromosomes. 

But what about the lizards’ genetic diversity? In most other organisms that can reproduce via parthenogenesis, the offspring are genetically identical to their mothers, which can be disastrous for the survival of the species. Any detrimental genetic weaknesses from the mother will 100% be passed on to their offspring, as there’s no chance of it being “masked” by the chromosomes from another gamete since there is no other gamete. The species as a whole will also have catastrophically low genetic diversity, which means they are less able to adapt to changes in the environment—genetic diversity is the raw material of evolution. However, in the case of desert grassland lizards, because they arose from many hybridization events between distinct species, they come naturally equipped with high genetic diversity. Additionally,  a unique form of chromosome behavior during the formation of their eggs allows them to recombine their own chromosomes with each other, hence resulting in offspring that are genetically different from the parent. 

“That doesn’t make them lesbian, you clickbaiting liar,” I hear you complain, “all-girls colleges exist but that doesn’t mean everyone is gay!” Well, guess what—individuals of this whiptail lizard species do in fact engage in courtship behavior with each other! This act is dubbed pseudocopulation and involves one individual displaying a more male-like behavior and mounting the other, similar to how males and females behave in some closely related sexually-reproducing species. No genetic exchange actually occurs between the two lizards—after a while, the one that was mounted simply lays eggs through parthenogenesis. This process has been shown to enhance the ovulation of the lizard being courted. Increased ovulation benefits the reproduction of this species, which explains why this seemingly pointless behavioral trait was maintained in evolution. It’s really quite amazing how what may well be described as a series of accidents led to the creation of such a uniquely functional species!