The PANY Bulletin

Psychoanalytic Association of New York
Volume 41, #3 Fall 2003

Psychoanalysis and Biology
Evolutionary Biology of Sex
I. Sex is a Lousy Way to Reproduce
Lawrence Kunstadt, Ph. D.

A fundamental finding of evolutionary biology is that every organism is in a competition to leave more of its genes (or, according to some biologists, its offspring) in the next generation than do other members of its population. One often hears that animals reproduce in order to perpetuate their species. Biologists do not believe this any more; this is a nineteenth century point of view.

Reproduction is a competition by an individual against other members of its population. This means that not only are all members of a population competing against each other but even when a male and female get together to make a baby, while in a sense they are cooperating, each is also trying to maximize his or her own reproductive success. Using the word "cooperation" to describe mating in many animals is a misnomer. It is often quite a violent, coercive affair.

The idea that reproduction is a competition is really the central idea. It is a competition among members of a population, not an absolute competition. So if you have three offspring and the average for your population is 3.5, you lose. If you have 3 offspring and the population average is 2.5, you win. The reason this is so important is that it has been shown that even a slight increase over the average will result in your genes penetrating through the population in a few generations. This change in gene frequency is evolution, or, at least, one way to look at evolution.

There are three ways to get your genes into the next generation. The first one, the one most people think of immediately, is by making babies. The second way was discovered about 40 years ago by William Hamilton and elaborated by Robert Trivers. It is called inclusive fitness. Fitness is a measurement of reproductive success. It refers loosely to the idea that if you "fit" into your environment you can survive and reproduce, whereas individuals that do not fit well into the environment do not survive and reproduce.

"Inclusive" refers to the following: 50% of the genes in each of your sons or daughters comes from you. The other 50% comes from whomever you made the baby with. Each of your brothers or sisters also has 50% of your genes, since you come from the same parents. So each of your nieces or nephews has 25% of your genes.

From an evolutionary point of view this means that having two nieces or nephews is just as good as having one son or daughter. Evolution does not care how you get your genes into the next generation, it only cares how many of your genes make it compared to your competitors' genes. If you have one child and your sister has 2 children, that is equivalent to your having two children and your sister having none. And so on.

The third way to get more of your genes into the next generation than other members of your population was discovered about 30 years ago by the anthropologist Sara Hardy. Studying langurs, she discovered that when a young male leaves his natal troop, that is, when he leaves the troop he was born into and grew up in, and enters another troop in order to copulate with a female, if she already has babies from a previous mating he will kill them.

When this was first discovered there was quite a bit of resistance to the idea, but it has since been confirmed and expanded. The idea is that you can get more of your genes in the next generation either by making babies or by having relatives that make babies, or by decreasing the number of genes that your competitors get into the next generation by killing their offspring. Either you increase the numerator or you decrease the denominator. They have the same effect.

Now killing another male's offspring is not common but it has been found that there are lots of ways to decrease the number of other individuals' genes in the next generation. For instance, the penis of a damselfly has scales that are used during copulation to remove the sperm that were left in the female by a previous male.

Mammals use another technique. In many lower mammals, after the male ejaculates into the female, the semen coagulates. This is called a seminal plug, and what it does is plug up the female so another male cannot enter her.

Just a few years ago it was discovered that human semen also goes through a temporary state of coagulation. Since there is no known physiological function for this, we assume it is something left over from evolution, suggesting rapid multiple copulations by our female ancestors, something still seen in other primates. I assume I don't have to review the many ways in which males behaviorally prevent other males from copulating with their females.

Sex and reproduction are not the same thing; in fact, in a sense they are opposites. Sex in biology refers to a recombination of genetic material resulting in a decrease in the number of individuals. So when a sperm and egg combine, forming a zygote, that is sex.
Reproduction refers to an increase in the number of individuals. If you start with a male and a female and end up with a father, mother and baby, that is reproduction.

There are two general types of reproduction: sexual and asexual. Asexual reproduction refers to what amebas do. They grow to a certain size and since surface area increases by a factor of two and volume increases by a factor of three, at some size the surface area will not be large enough to support the functions of the volume, so the cell splits in half.

If an organism reproduces asexually, 100% of the genes in its offspring comes from the "parent". In a certain way of thinking, the parent's offspring are the same organism, they're just half the original size.

In addition, we also know that if you grow asexually, reproducing organisms in a flask and keep giving them nutrients and keep removing waste products, they can keep dividing indefinitely. There does not seem to be any intrinsic limitation in the cells.

So if you reproduce asexually, not only do 100% of your offspring's genes come from you, and in a sense they are you, but you can also live forever. What could possibly be more narcissistically gratifying than that?

But let's look at sexual reproduction. If you reproduce sexually, two things happen. One, each of your sons or daughters has 50% of your genes, your grandchildren have 25% of your genes, your great-grandchildren have 12.5% of your genes, and so on. So after a few generations the percentage of your genes in your descendents gets diluted down to practically nothing.

And if that weren't bad enough, it is also a fact that no member of a sexually reproducing species has ever been found that doesn't have a lifespan and die. What could possibly be worse than that?

So despite the fact that almost all species reproduce sexually, from an individual's point of view, sex is a lousy way to reproduce. Of course, this raises a host of important questions, the leading one being that if sexual reproduction is so terrible, why did it evolve in the first place and, even more importantly, why does it still exist and not just exist but why is it overwhelmingly the most common form of reproduction? The brief answer is that we don't know. But biologists are as bad as psychoanalysts and when we don't know something we love to speculate, so there are dozens of theories. What many of them have to do with is either maintaining good genes or getting rid of bad genes.

So we are left with a mystery but also with the understanding that every step of the way in sexual reproduction is an obstacle to be overcome. The advantages of reproduction need to be seen as flowing from generation to generation, not as specific advantages to either the individual or the species.

In the next column I will continue with sex differences deriving from evolutionary biology and then finish this three-part series with a speculation on the evolutionary origin of the Oedipal complex.