Bankers, keen for you to invest your money with them, speak often and in glowing terms of the power of compound interest. Take $200, they say, invested in 1901 at 5 percent interest and compounded annually; today that account would stand at more than $26,000 dollars.

True. What is seldom or never mentioned is that in 1901 a person could live for a year on $200. Inflation can easily keep up with, and often exceed, the increase in value provided by compound interest.

Inflation, however, does not apply in nature, where the long-term effect of compounding can be spectacular. Let's take a simple example. Suppose we have two organisms that compete for the same environmental niche. Suppose that at some point in time there are equal numbers of each. However, let one of them have a slight competitive advantage over the other: for every million surviving offspring produced by Organism B, suppose that Organism A produces a million and one. It is easy to see that over time there will be more and more of Organism A, compared with the number of Organism B. How many generations will it take before members of Organism A outnumber Organism B by a thousand to one?

The math is not difficult, but you may find the answer surprising. It will take less than seven million generations. For humans, with an average generation time of maybe 15 years (we used to breed a lot younger than we do today), the result is not very interesting. Seven million generations would take more than a hundred million years, and we have been around as a species for only a small fraction of that time. For other organisms, however, the result is far more striking. Some bacteria can reproduce every twenty minutes. Such creatures run through seven million generations in a few hundred years. It is not surprising that we have such trouble wiping out strains of bacteria, when specimens with a small natural advantage in resistance to our antibiotics can multiply so fast in the population.

The law of compound interest explains such things as the speedy development of disease-resistant strains of bacteria, but it can also lead to results that are at first sight totally baffling. Consider, for example, an organism that can reproduce itself both sexually and asexually. There are many of these, both plants and animals. Suppose that when the organism multiplies sexually, it produces equal numbers of male and female offspring. When it reproduces asexually, of course, it has no choice but to produce exact copies of the same gender as itself. Let's call that gender female, since we expect the female of the species to produce the young.

What happens in the next generation? Well, if it requires the same investment of time and energy to produce a male as it does to produce a female, asexual reproduction will lead to twice as many females, each themselves able to reproduce. The sexual males, by definition, do not themselves produce offspring.

This is compound interest with a vengeance. Rather than a one-in-a-million advantage in each generation, there is now a reproductive advantage equal to two. After just twenty generations, there should be a million times (two to the twentieth) as many offspring produced by asexual reproduction as by sexual reproduction. Sexual reproduction loses, and so strongly that even if it had appeared, it ought to have long ago vanished from the world.

It hasn't. Look around you. Sexual reproduction is alive and doing well. It is fair to say that with rare exceptions it is the preferred reproductive mode of all higher animal forms. Sex, it is clear, confers some other competitive advantage in the struggle for existence - and it must be something truly major, to outweigh the factor of two per generation that favors asexual reproduction. What could that advantage possibly be?

No one is really sure, but one way to approach the inexplicable is to match it with something equally confusing. The sexual reproductive process itself is baffling, in that it seems unacceptably dangerous to the organisms that practice it. Think of it in personal terms. Your DNA has been developing for three or four billion years. It is precious stuff, the material that absolutely defines you as an organism. Yet the sexual process randomly mixes and matches this invaluable genetic material with that of another creature about which you know little or nothing, except that the two of you are capable of producing offspring. From your worried DNA's point of view, there is no such thing as safe sex. The offspring may be anything from sterile to super, and no one can predict which.

Now we introduce two factors so far unmentioned: reproductive copying errors, and natural selection. In spite of error-correcting procedures, occasional genetic copying mistakes occur in both sexual and asexual reproduction. In the case of asexual reproduction, offspring are stuck with whatever their single parent gave them, plus any new copying errors. Natural selection will cull unfit specimens, and is the only mechanism at work to reduce the amount of unfavorable genetic material.

Sexual reproduction, with its mixing of parental material, can and will produce a much wider diversity in the offspring. Unlike in the case of asexual reproduction, some offspring will have less unfavorable genetic material than either parent. Of course, other offspring will be unlucky and inherit badly from both parents. However, natural selection will work strongly here to weed out those with worse genes, and favor those with better ones.

The final result is a contest between two opposing forces: on the one hand, the numerical advantage in each generation of asexual reproduction; on the other, the long-term cumulative effect of superior culling by natural selection of the diverse offspring resulting from sexual reproduction.

It is tempting to say that we know who must win the contest, because we, and anything remotely like us, reproduce sexually. That, however, may be too simple-minded a point of view. Creatures that reproduced asexually were around long before their competition. They remain plentiful today. It could be that, far in the future, Earth's environment will change and the doubling advantage in each generation of asexual reproduction will prove more important than anything else.

The "sexual revolution" will then be over. Sex, with all its near-infinite diversity, will vanish from the Earth. And whatever life-form rules the planet at that time will have to find a different obsession.

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