While some complex animals are hermaphroditic, most have two sexes. Why is this? And if two sexes are generally “better” than hermaphrodites (and I don’t know if this is true), why are there still some animals that are hermaphroditic?
Evolution isn’t directed unless you’re a Teilhard de Chardin fan. Sexual reproduction works better than asexual reproduction because it involves gene exchange and mixing; there are species for which hermaphroditism works well such as most plants (note that in animals it often isn’t simultaneous, there are species which “become” one gender or the other at different times depending on the external situations) and others for which fixed sex works just fine, but nobody went and said “well, ok, this hermaphroditism thing snails got going on works fine for snails but next I’m going to try fixed-sex”. The more complicated the sexual setup, the more rearrangement a change would involve: rearranging a snail’s organs (invertebrated and quite… malleable) is not as complicated as rearranging those of a fish (vertebrated but external-breeder) or a mammal (vertebrated and internal-breeder).
IOW, neither hermaphroditism nor fixed-sex is inherently “better”, both work and different species get different cases.
One reason that comes to mind is that each sex is generally less complex then everyone having all equipment. It also leads to a natural division of labor, so occupying a slightly different niche and allows sharing of territories between m and f that may be problematic without such differences.
Specialists are generally better at their specialty than generalists; a specialized male and female are going to be better at being male and female than a hermaphrodite. A pure male human for example can do without the large, resource hungry female reproductive system and used the saved resources for other purposes like increased size and larger muscles; a hypothetical human herm wouldn’t have that option. By the same token, a pure female can sacrifice size and muscle mass, in return for being able to go though pregnancy without being sucked dry by the resource demands and killed.
Also, part of gender appears to be avoiding the mixing of mitochondria; when reproductive cells mix mitochondria from both parents the result tends to be destructive competition between the two strains. A sperm-and-ovum system where the ovum supplies all the mitochondria prevents this.
For a more detailed explanation, see Matt Ridley’s The Red Queen, particularly Chapter 4.
Yes, consider the fine points of physiology - muscle mass takes extra calories to maintain, even at rest. That’s why the body starts consuming its muscle mass when you don’t eat enough… Similarly, women store a lot more body fat under the skin - hence the “softer, curvy” look. This is a cushion for pregnancy; it’s not like a woman used to be able to just change her mind before the nine months are up, so there needed to be a cushion.
Note that these are not “directed” choices. The male unnecumbered with pregnancy was best suited for the “heavy lifting” - hunting, fighting, etc. The ones with teh best compromise between muscles and available food survived. Similarly, a female with too much muscle ran a higher risk of starving before pregnancy finished.
Based on those competing factors, a single specimen trying to do both job would be worse than either.
Of course, this sort of selection happened long ago, before any species got as complex as humans or mice. After all, where’s the advantage in hermaphrodites - even in a 2-sex species, all genetic material is in play for mix-and-match. There is no great advantage in being able to do both jobs, and some disadvantages. When one sufficiently outweighs the other, that method wins.
The thing you have to keep in mind here is that humans are actually a pretty extreme example of sexual dimorphism. In most other species, the sexes are much more similar to each other than they are in us.
What’s the advantage to being able to produce both male and female gametes?
Typically, female gametes are more expensive than male gametes. They cost more to make. Yet only one male gamete is needed to fertilize a female gamete.
So consider a population of hermaphrodites. One has a mutation that causes it to only put all of its energy into male gametes, and none into eggs. It mates with its hermaphrodite neighbors, and produces more offspring because it can produce twice as much sperm as a hermaphrodite. Since it is more successful, the genes that cause maleness spread. Pretty soon there are lots of cheating males in the population and the species is awash in sperm. Yuk.
Now notice what happens. When maleness is rare, it is advantageous to be male. But as maleness increases in frequency, it becomes less and less advantageous. And when 50% of the population is male, it becomes equally advantageous to be female or male, and when over 50% of the population is male, it is better to be female. So the hermaphrodites give up producing sperm and concentrate on producing eggs.
Because it’s not always necessary to be better. It only necessary to be good enough for the environmental niche involved.
Just think how much more stressful dating would be if there were three!
There are no organisms that require three types of gametes for fertilization. You get all the benefits of sexual reproduction with two, and so there is no further benefit with three. In fact, the optimum number is probably less than two, which is why so many organisms use a mix of sexual and asexual reproduction.
Note also that most organisms don’t have much sexual dimorphism–often the only difference between males and females is that females produce eggs and males produce sperm. The gametes are released into the water and find each other. Internal fertillization is fairly common, but carrying the developing eggs inside the body of the female is not. So the arguments for differentiation into males and females based on the idiosyncratic mammalian reproductive system don’t make any sense. Our ancestors were differentiated into males and females long before mammals evolved internal gestation.
Another issue that leads to sexual dimorphism is that biologically speaking, males are expendable. A population that loses half of its females is in trouble, but a population that loses half of its males can rebound easily. So males in most species tend to specialize on the jobs that are more risky. In humans, for example, hunting tends to be risky, and so most human hunters are male. By comparison, for lions, hunting isn’t all that risky, and the females do it, but the male gets the job of fighting off hyenas.
Whoosh!
It turns out there are species with three sexes, and with more (how about eight :)). Mentioned here (about 1/3 down the page) and in this Straight Dope thread (posts # 25, 27, & 29.) The mating is done in pairs – pick any one sex besides your own to mate with.
But those fungi with “mating types” aren’t really males or females, they are isogamous–meaning they produce one type of gamete rather than sperm or eggs. The single gamete can fertilize/be fertilized by any other gamete, as long as the other gamete is of a compatible mating type.
It is really misleading to say that these fungi have three, or eight, or a hundred sexes. Especially since the most common setup is that a gamete can join with any gamete that is of another mating type. So if there are two mating types, each gamete could merge with only half the other gametes, but if their are 8, they can mate with 7/8ths of the other gametes.
As for the three sexes, of course I knew you were making a joke, I just thought I’d explain why, even though they are logically possible, there aren’t in fact any species that have three sexes. The mating types above don’t increase the number of sexes or gametes required, they actually reduce them.