Most of us are so used to the male+female=baby system of reproduction that we practice that it doesn’t even occur to us that there are other options. Sure, there’s the occasional instance of parthenogenesis in some megafaunal species, but that seems like the exception rather than the norm. And we do recognize that a lot of the microbes that make up the majority of life on Earth reproduce asexual. But, when it comes to sex, we’re stuck on male+female=baby.
Kurt Vonnegut thought otherwise. In his classic novel Slaughterhouse Five, he introduced the Tralfamadorians and their understanding of human reproduction:
The Tralfamadorians tried to give Billy clues that would help him imagine sex in the invisible dimension. They told him that there could be no Earthling babies without male homosexuals. There could be babies without female homosexuals. There couldn’t be babies without women over sixty-five years old. There could be babies without men over sixty-five. There couldn’t be babies without other babies who had lived an hour or less after birth. And so on. It was gibberish to Billy.
Sure, that’s science fiction or fantasy, but there are some animals that utilize reproductive systems nearly as complex as Vonnegut’s version of humans. A very accessible review of those crazy reproductive systems can be found in this Slate article entitled “Why are there two sexes?”. Besides taking a romp through some of the odder reproductive systems (ie, organisms with three sexes), there is also a brief treatment of why the majority of sexually reproducing organisms only have two sexes.
In addition to the examples of odd-ball sexual systems presented in the Slate article (clam shrimp with ZZ males in addition to ZW and WW hemaphrodites, or harvester ants with two types of males and two types of queens), there’s another odd sexual system found in a common model organism. You’d think that the animal models would resemble humans in many ways. Well, Drosophila sex chromosomes are a bit different than human sex chromosomes, but Caenorhabditis elegans are even stranger. These roundworms have hermaphrodites (XX) and males (XO). The hermaphrodites are capable of mating with themselves, while the males (which make up less than 1% of all progeny when a hemaphrodite gives birth) must mate with hemaphrodites in order to pass on their genes. Why males are even maintained is up for speculation.
The Slate article also discusses why two sex systems are so prevalent in sexually reproducing organisms. We know that systems with more than two sexes can evolve, so why aren’t they more common? Laurence Hurst has done some work trying to figure out the answer to this question (see here and here). It basically comes down to mitochondria and other cytoplasmic goodies. In many sexually reproducing species the males only pass on their genetic material while the females contribute organelles in addition to DNA. This requires an either or system (either you pass on the other stuff or you pass on just DNA), so it must involve only two sexes. The arguments in favor of this hypothesis are further fleshed out in Nick Lane’s book Power, Sex, Suicide.