Why do we have sex? Part 3

In the competitive game of natural selection, the winning organism is the one whose DNA is most prevalent and longest lasting.

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When a bacterium divides, the offspring carry the same DNA as the parent, as do the offspring in the third and fourth generations, and every generation after.

Sexually reproducing organisms lack this sort of genetic continuity. Your children will each carry genes that come in part from you and in part from their other parent. When your children breed, the portion of their DNA that you contributed will be further diluted. In the course of a few generations, there's not likely to be much of you left in your descendents. On the bright side, genetically speaking, at least your descendents and your species as a whole, can survive in the face of extreme stresses, while an asexually reproducing species could be wiped out easily. In fact, the scarcity of asexual species suggests that they hardly ever survive for long in nature.

I know what you're thinking. If two sexes are better than one in dealing with stress and mutations, wouldn't three be better still? The fact that there don't seem to be any suggests the answer is no.

Nevertheless, physicists have developed mathematical models of hypothetical creatures that breed in sets of three. (The researchers who wrote the papers examining the three-sex models, unfortunately, didn't bother to explain how these creatures would get their three-way groove on.)

Three-sex creatures and their offspring are triploid, with three complete sets of genes rather than the two of diploids like us, or the single genes of haploid amoebas and other simple animals and plants.

The numbers work out poorly for three-sexed creatures. For one thing, it's much more complex to get a reproductive trio together. The simple fact that it takes more parents to produce the next generation means that the population will grow slower than that of competing diploids just as diploids are out-bred by asexual haploids. Triploids, however, have one thing going for them - they are less susceptible to random mutations than haploids or diploids, thanks to even greater genetic redundancy than two gene animals like us. Unfortunately, they lag behind when it comes to adapting to other sorts of stress. Like so many cases of competition in nature, too much of a good thing turns out to be bad.

If triploid genes work in the same way ours do (which seems like a good guess) then each of the three genetic sequences has a genes for every trait, but it's the dominant one that wins out, or the trait ends up being a blend of all three. If the blue eye gene is dominant in a triploid population, the fact that each member of the population has three shots at getting a blue eye gene means that it is much more likely that everyone will have blue eyes.

It's easier to grasp the problem if we take it a few steps further. Imagine a population with ten sexes (and ten sets of genes per cell), and blue eye genetic dominance. Even if nine out of ten genes code for non-blue eyes, the one remaining blue eye gene wins. In cases of incomplete dominance and co-dominant genes, blending more and more versions of a certain trait leads to genetic uniformity instead of diversity, just as mixing more and more colors from a painter's palette results not in brilliant new colors, but to ever muddier shades of brown. While single-gene haploids have essentially no genetic diversity, increasing the numbers of complete genes in an organism beyond the two of diploids also leads to steadily decreasing diversity, which means less and less opportunity for evolutionary adaptation.

For most organisms, at least the larger and more structurally complex ones like humans, two-sex genetic diploids have an optimal combination of diversity, adaptability, reproductive efficiency, and resistance to genetic errors.

When humans rely solely on the tools nature provides us, we reproduce as most two-sex creatures do - a male provides semen, a female provides an ovum, and another generation begins. With a little medical and scientific intervention, however, we have lots more options.

Artificial insemination of course is one of the oldest and simplest alternatives to actual intercourse. Infertility treatments involving insemination in a Petrie dish are much like the external insemination practiced by fish and other aquatic and amphibious creatures.

While it is not triploid sex, when a woman serves as a surrogate mother for a fertilized ovum she is part of an interaction much like the three part male-female-female mating model.

Human cloning, however, is perhaps the most controversial method that may soon be among our potential reproductive options. Setting ethics aside, humans who reproduce via cloning would gain many of the asexual benefits that bacteria enjoy. Presuming that people who choose to clone are women who carry their own fetal clones in their wombs, and tend to have the same numbers of children over their breeding lifetime as other women, they would be able to increase their numbers much faster than sexual human couples could.

Men who opt to clone themselves need to seek out a woman to host the fetus in her womb, which means the process still takes a man and a woman. So that's really not an advantage numerically. In addition, the woman host would have to agree to waste precious reproductive time and effort to bear a child of no genetic relation to her. Surrogate mothers do that today for couples who can't, or choose not to, carry their own children to term, but surrogates usually require fee in exchange for the rented womb.

In a community where male-female couples and cloning women each choose to raise two children, the numbers of mating couples stays constant with each passing generation, but the numbers of clones double from one generation to the next. The clones' numbers could grow exponentially. Again, it's the males that are the reproductive liability in mating couples.

Other asexually reproducing creatures are highly susceptible to changing stresses in their environment, which favors sexual populations molded by natural selection. Humans, at least those living in more highly developed nations, tend to deal with stresses through controlling the environment and counteracting the stresses rather than through evolution.

If it's too cold, there's no need to evolve fur; we turn up the heat or put on a coat. If it's too hot, we turn up the air conditioning. We no longer adapt natural immunity to diseases; instead we develop vaccines, antibiotics, and prevention methods. As a result, people who are reasonably well suited to surviving in modern society, and who reproduce via cloning, would face few, if any, drawbacks from their asexual reproduction while gaining all the benefits.

Scientists have managed to clone many types of animals including cattle, cats, sheep, and monkeys. As of this writing, there have been no confirmed human clones produced from adult DNA. The complexity of cloning and the risks of severe birth defects mean it may be years before human cloning is perfected. But it will happen eventually.

If cloning ever becomes accepted practice, society could rapidly become populated primarily with women who reproduce by strolling down to the corner pharmacy to pick up the Acme Home Clone kit, rather than wasting time and energy looking for a male mate.

Clones won't evolve, so there is no reason for them to lose their sexual urges. Although they will have to learn to rely on lesbian sex to fulfill their needs, because men will eventually die out as sexually reproducing people lose ground to the clones.

When clones come to rule the world, sexual intercourse will be nothing more than a source of recreation, relaxation, and social bonding. Sex will be useless for procreation.

Of course, that's already true 99.99% of the time anyway.