Understanding Asexuality

History

The story of life begins asexually, and for most of Earth’s natural history (about two-thirds of it), asexual reproduction has predominated (Cowen, 2005). It has only been in the last twelve hundred million years, out of the thirty-eight hundred million years that life has existed on Earth, that sexual reproduction has flourished—a mere fraction of geological time. Now it is the predominant form of reproduction for species on Earth.

The exact date of the emergence of sex—call it, ahem, the little bang theory—is a bit in question, however. Twelve hundred million years is a ballpark figure, give or take a few hundred million years. The first sex likely occurred in simple organisms, eukaryotes (Cowen, 2005). Eukaryotes are simple single- and multicell organisms, although they are more complex than the first single-cell organisms that predominated for about 2.5 billion years. Eukaryotes owe their complexity to sophisticated structures that the first forms of planetary life did not have, such as nuclei.

There are also other important dates in the evolution of sex beyond its first appearance. A second milestone in the natural history of sex was when this form of reproduction first occurred in the genealogical line leading to modern animals, perhaps about five hundred million years ago. This prize may belong to a worm-like creature called Funisia, the fossilized remains of which were originally found in Australia (Droser & Gehling, 2008). A third milestone was reached when the first “penetrative” sex appeared, around three hundred million years ago in a species of fish. These fossils were also originally found in Australia (Long, Trinajstic, & Johanson, 2009). Evidently, this fish’s pelvic fin was used in penetration during copulation.

Note that not all sex involves penetration/copulation, as the definition of sexual reproduction is the combination of genetic material from two parents to form offspring. Sexual reproduction involves two processes: meiosis, the splitting in half of the parents’ complement of genes, and fertilization, the recombination of genes by the melding of the parents’ gametes (i.e., sperm and egg). Fertilization can be done in a variety of ways, one of which is internal, making penetration a common technique, but there are other ways of achieving this outcome.

It might be asked: If asexual reproduction was the way it all began, why is sex even here? This is an especially pertinent question, given that sex is a complex and costly way of reproducing one’s genes. For example, there is considerable time and risk involved when it comes to finding a mate. Moreover, only 50 percent of an organism’s (unique) genes are replicated in traditional sexual reproduction, versus 100 percent in asexual reproduction.

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).

The above brief discussion of the natural history of asexual and sexual reproduction only indirectly relates to the main subject of this book: asexual humans. The type of reproduction of a species— sexual or asexual—is somewhat different from the phenomenon of asexual beings, including human beings, within an exclusively sexually reproducing species. There is also a distinction between sexuality and the capacity for reproduction. For example, the vast majority of asexual people can still reproduce sexually (i.e., are still part of the sexually reproducing species of humans), even if they are not interested in the sexual mechanisms of it. Finally, in modern humans, sexuality is often divorced from reproduction, so asexuality is (partially) a different phenomenon than asexual reproduction.

More to the main point of this book, then, are there examples of asexual animals within a normally sexually reproducing species? There are. For example, a chorus line of sexual variability exists in farm and lab animals, even though they are often bred—and sometimes genetically altered—to be nearly identical food-producing or lab-friendly machines. As in humans, you can find some that are studs or sexual dynamos. You can also find some having “homosexual” tendencies, in that they prefer sexual relations with the same sex; for example, males not only affiliating with other males but also actively preferring to mount other males rather than females. You can also find animals that have no sexual interest whatsoever in other animals. This pattern of sexual variation among animals is often most clearly observed in males, as it is sometimes harder to determine female sexual tendencies, in part because they, relative to males, are less likely to initiate mating, often having a more subtle sexual response associated with receptivity (see chapter 6 on gender).

Male rodents raised in experimental labs (e.g., mice, gerbils, guinea pigs) often demonstrate wide sexual variability, with behaviors ranging from hypersexual to asexual. These extremes are often called “studs” and “duds,” respectively. Such variability in rodents may have parallels to human sexual variability, including asexuality. Yet there are a number of unknowns. First, some non-mating males may have sexual attraction to other males, and so a lack of mating may not mean asexuality. A second, related unknown is that the conclusions rely on the