Pardon me for boasting, but on the way home from the gym this morning I seem to have solved a long-standing problem in evolutionary theory. I don’t think I’ve read this idea anywhere — it just sort of popped up.
The problem is, why does sexual reproduction exist? Evolution is ruthlessly economical. Any behavior that doesn’t “pay off” in terms of reproductive fitness will sooner or later get weeded out. Sexual reproduction is expensive in behavioral terms — all the trouble of finding a mate, fighting off rivals, and so forth. What’s the payoff?
A theory that I have read is that the payoff is protection from microbes. The microbes in your immediate environment, which includes the millions of tiny critters living on your skin right at this moment, breed much faster than you and I do, so they can evolve faster. There’s a kind of arms race going on. Any advantage they gain (in making you sick, which will increase their numbers radically) has to be fought off by your immune system. Sexual reproduction, according to this theory, jumbles up the genes of the next generation, which essentially confuses the microbes. They have to start over, trying to figure out how to make the next generation sick.
I’m sure that’s a fine theory. I’m not a cell geneticist, so I’m not equipped to evaluate it. But here’s a different idea.
When we talk about evolutionary fitness, we’re not really talking about the fitness of a big, strong animal. We’re talking about the fitness of the genes that encode information with which to build a big, strong animal. It’s the fitness of the genes that is crucial in evolution.
In asexual reproduction, the mother passes all of her genes on to her daughter, and so on, unto the nth generation. Because evolution is ruthlessly economical, it will tend to trim away redundant genes. An asexual creature would quite likely have, for instance, only one gene to produce an essential digestive enzyme, because if there was ever a second gene that did that, when the second gene fell apart or got mis-copied, it could never be reconstructed.
Genes do occasionally mutate. Not often, but it happens. And there are, in your genetic makeup and mine, thousands of genes that are essential for the organism to remain alive. You have genes that constructed your heart, your lungs, your skin, and so on. If you had only one copy of each of these genes, any mutation (in the portion of your own developing body that produced egg cells) would be fatal to your offspring. You would never produce any viable children.
But when you have two copies of these important genes, one from your father and one from your mother, a defect in one of the copies is not necessarily fatal to you or to your offspring. If you only have one copy of the gene that makes that essential digestive enzyme, you may never even know it — and half of your children won’t inherit it. True, fatal mutations can still occur. But the redundancy of the genetic information lowers the rate of fatalities due to mutation.
This is all Biology 101. But the essential point is this: From the point of view of your genes (anthropomorphizing a bit here — genes obviously have no point of view), sexual reproduction protects all of them against the occasional fatal “traitor” gene. The healthy genes work together, producing a sexually reproducing species, in order to protect themselves from those occasional traitors.