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.
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Ed Subitsky did a great comic-strip piece for the National Lampoon called "Saturday Night on Antarius (the Planet with Thirteen Sexes)", about the difficulties of getting thirteen people together (one of each sex) for a bit of hanky panky.
The above comic strip sheds more light on the matter than the profs quoted, I think. Especially when individuals of a multi-sex type are rare and just appearing, it seems they would be doomed to extinction, and couldn't invade a two-sex pop.
One of the profs used the wrong analogy to show the inefficiency of a two-sex system -- i.e., that with more choices available, it would be easier to find a mate. But the real question is, Why aren't there more than two necessary sexes for mating? Why don't they make zippers with three or more interlocking rows of teeth?
Another reason is conflict among the selfish genes of the parents: the potential conflict increases exponentially as the number of parents increases. With two parents competing, the only conflict is A vs. B, but with three parents, there is A vs. B, A vs. C, B vs. C, A and B teamed up vs. C, A and C teamed up vs. B, B and C teamed up vs. A, and A vs. B vs. C (war of all against all).
The multinomial theorem is a bitch for multi-sex systems! Has this "genetic conflict" issue been addressed in the lit?
Ah, now it's obvious.
Other systems have been explored by for example Piers Anthony. In one novel he introduces a tripartite sexual system, where the three partners each must contribute material. (Supposedly also genetic.) Such a society could naturally be more diverse and complex, and Anthony portrayed that among other things.
Of course, there's a bit of an "animal" bias here - plants are much more imaginative when it comes to breeding biology.
About 20 years ago there was a special issue of Science on sexual reproduction. I recall a slime mold or someting with 12 sexes. Maybe more properly called reproductive types. A can mate with B or C. B can mate with A, C or D. C can mate with A or B. D can only mate with B.
Torbjörn, That was my thought too. But the book is by Isaac Asimov, not Piers Anthony, called The Gods Themselves. Its a fantastic read!
Its a parallel universe where the sun is dying out and this race has 3 genders: Rationals, Emotionals and Parentals. Each with its own role in sex. I won't spoil the rest, its a good read.
hmmmm....more than two sexes eh. I guess that puts to bed (sorry) the whole Noah and the ark, two by two myth ;)
I suspect that Ed Subitsky's piece is derived from:
Phil Klass' "Venus and the Seven Sexes" under the psuedonym William Tenn.
Reprinted in various anthologies, first:
Ted Dikty (whom I knew), who wrote to Frederik Pohl. January 18, 1951, re: obtaining Phil Klass' Venus and the Seven Sexes, for The Great Book of Science Fiction.
Sex in the 4th dimension is explicitly the subject of:
"The Sex Sphere", a novel by Dr. Rudy Rucker, and to a somewhat lesser extent, his "Spaceland: A Novel of the Fourth Dimension."
Whoops, I've run out of time for the serious comment that I intended on rate of evolution in sexual versus asexual Genetic Algorithm simulations.
kevin:
I was thinking of Piers Anthony's Vicinity Cluster series, where in Sphere Spica the hero Flint meets Impacts, Undulants (water world) and Sibilants in the first novel. These beings corresponded to the sexes "Sire", "Catalyst" and "Parent".
From your description it seems PA (1978) may have taken the idea from IA (1972). And that I should read IA's book. :-)
Btw, I see from Wikipedia when I checked the year of IA's book that the Emotionals provide energy, not genetic material. So they don't fit as well with the posts model.
Oops. It was the Cluster series, the first novel was Vicinity Cluster.
I go offline for 4 days and I come back to two topics that are near and dear to me. Sex (on Slate; thanks, Evolgen) and Giardia (in Science; see Sandwalk). And I have so much other work to catch up on....
Of course, there's a bit of an "animal" bias here - plants are much more imaginative when it comes to breeding biology.