Understanding Asexuality

Although it is still debated, the most widely cited explanation for sexual reproduction concerns the creation of genetic variability, or the shuffling of the genes, which allows a species to survive in new and changing environments. This shuffling of genes may be particularly relevant for keeping ahead of various parasites that can take advantage of a gene pool lacking in diversity (Bell, 1982; Ridley, 1995; Van Valen, 1973). This explanation for the origin of sex is sometimes called the “Red Queen hypothesis,” after the character in Lewis Carroll’s Through the Looking Glass who exclaims, “It takes all the running you can do, to keep in the same place” (Carroll & Tenniel, 1960, p. 345). In other words, sex may have evolved as a weapon to win—but just barely so—the arms race against new environmental threats, including parasites.

This shuffling of genes may also be characterized as a type of “bet-hedging” strategy, allowing sexual reproducers to keep one step ahead of unknown threats in constantly new and changing environments. It is as if sexual reproducers are evolutionary dart players, throwing a variety of genetic arrows at the game board of life and seeing if one or more sticks.

The Red Queen hypothesis begs the question: If the shuffling of genes is often good for the health and vigor of organisms, why aren’t there more than just two sexes? Surely a mating among more than two sexes would shuffle the genes even better than a mating between just two sexes. For example, why don’t we see tri-sexual species—exotic characters belonging, seemingly, in a sci-fi movie—all over Earth? Well, actually, there are a number of species on Earth with three or more sexes (Roughgarden, 2004), but such examples are rare in Earth’s natural history. Perhaps the costs associated with three or more sexes—such as having to find one another, and only reproducing a third of one’s genes—is not sufficiently offset by the advantages of genetic diversity that combining three sexes would bring in producing offspring.

Most multicellular organisms are exclusively sexual reproducers. Yet there are spectacular exceptions to this rule, including some species larger than we are. Some sharks, for example, have the capacity to reproduce asexually. Scientists discovered this recently by accident when a female hammerhead shark in captivity in a zoo, without the company of any males, suddenly gave birth. It turned out that the mother hammerhead used a form of parthenogenesis, in which an unfertilized egg develops into an adult without any contribution from a male. The offspring in parthenogenesis is genetically identical to the mother and thus is always female. Subsequent biochemical analysis indicated that the offspring in the hammerhead case was genetically identical to the mother shark; thus, the scientists ruled out the possibility of stored sperm from a sneak fertilization with a male shark years earlier (Chapman et al., 2007; Eilperin, 2007, May 23). Talk about a virgin birth!

Humans are mammals, appearing relatively late on the planetary stage, so this ancient capacity to reproduce asexually still lingering in some very old (phylogenetically speaking) species does not exist in humans. Sharks have been around for a very long time, about 350 million years, as compared to humans (i.e., the genus homo), who have existed for—blink and you will miss it—a mere two to three million years.

There are also examples of seemingly exclusively asexual reproducers that, surprisingly, sometimes reproduce sexually. The fungus-farming ant, once thought to be an exclusively asexual reproducer (indeed, the species consists of all females), has been found to have telltale signs of sexual reproduction: some of the offspring were not 100 percent identical to the queen in one of the subpopulations of these ants, and the queen herself had—aha!—storage organs in her body that were filled with sperm (Rabeling et al., 2011; Ghose, 2011, July 18). These authors argue that sexual reproduction in this subpopulation may provide certain advantages (e.g., the ability to extend its range to new and complex environments) over those exclusively asexual subpopulations of ants. Relatedly, the authors argue that such glimpses of sexual reproduction in normally asexual species may provide an important view of how the evolution of sexuality occurs: “If you come back in five million or ten million years, there’s a good chance the asexual lineages go extinct, but the sexual lineages are still existing” (Ghose, 2011, July 18).