Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [44]
If you remember high school biology and a bit about sex chromosomes, you might think the answer is a consequence of how sex is determined when sperm meets egg. Humans and other mammals all possess two sex chromosomes: in females, two copies of an X, and in males, a single copy of the X and a smaller Y chromosome. Since only one of the sex chromosomes goes into each sperm or egg cell, half the sperm make daughters, with two Xs, and half make sons, with one X and one Y. Ipso facto, you get an equal distribution of males and females. Although birds, butterflies, and a few other animals have the situation reversed, with males produced from two Z chromosomes and females from a Z and a W, the same process takes place. The analogy with tossing a coin has been made so often that you have to wonder whether, if we commonly had more than two sexes, we would all be using three-sided coins.
But satisfying though this explanation may seem at first glance, it is ultimately not the answer to the question of equal sex ratios, as Mike Majerus points out in his book Sex Wars. First, it's quite possible that the mechanism for determining which chromosome goes with which sex evolved as a means to get to the optimal sex ratio, not beforehand; as with the coins, it would be fallacious to argue that we have the two options of heads and tails on our coins so that we can calculate the sex ratio. Second, as I will describe in more detail below, many animals have the same sex determination mechanisms we humans have but still produce wildly biased sex ratios. And finally, even though "our" XX/XY (or ZZ/ZW) system is widespread, numerous other ways to determine maleness or femaleness are found throughout the animal kingdom, including special genes and the temperature at which eggs are incubated (in many kinds of turtles, for example, cooler nest temperatures yield male babies and warmer ones, female babies). So the equal sex ratios in turtles, at least, must arise from some other source.
As with so many other ideas in biology, Charles Darwin both identified the problem and proposed a plausible solution. Many biologists overlook his original suggestion because it appeared in an early edition of The Origin of Species and was retracted in the later ones. Sir R. A. Fisher, a British geneticist and statistician, is generally credited for the breakthrough. Either way, the key is to think about things, not from the standpoint of what is most efficient for the population or species as a whole, but from a gene's eye view. What will make it more likely that a gene will be perpetuated in future generations, the key to evolution?
Say that, indeed, fewer males than females are produced in a hypothetical population of animals, as it was in Jan and Dean's 1963 song "Surf City" ("Two girls for every boy"). Each male fertilizes multiple females, so more copies of each male's genes appear in subsequent generations. That means that having sons is advantageous, so any tendency to do so will be favored by natural selection. Eventually, though, more males than females appear in the population, and then those males are not in nearly as happy a circumstance. Because each child can only have one mother and one father, some of the surplus males must go without mates, and then parents who produce daughters, rather than sons, are at an advantage. That bias then leads to an abundance of females, with the same benefit to being a male that we started with. This kind of seesaw evolutionary process across the generations is called frequency-dependent selection, and it is easy to see that