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More than One Way to Do It

Posted by | November 29, 2008

Those of us who’ve been around for a few decades have seen friends or family engulfed in bitter custody disputes. There’s nothing like denouncing the other parent of your children in open court (and paying steep legal bills for the privilege of doing so). Custody clashes are clearly only the tip of the iceberg of the total cost we sometimes pay for sexual reproduction.

Fom a scientific point of view, there’s another problem with sex, and it’s a doozie.

Some primitive animals can reproduce either sexually, with a partner, or all by themselves. This means that in some species, females can mate with males, while other females simply reproduce all by their lonesome. Snails can do exactly that trick. If a female creates baby snails by herself, the offspring are identical clones of the mama snail.

Wait a minute, you say, you’ve never heard of natural “clones” like that, only special man-made clones like Dolly the sheep.

But actually, you yourself may well have cloned a creature in the plant kingdom, simply by taking a cutting of a plant and letting it root. The new plant is genetically identical to its parent, making it a natural clone.

Now think of a lake full of snails. If a particular female mates with a male snail and produces 100 eggs, she might have 100 babies that are 50 percent male and 50 percent female. Her genes will be in all the babies, along with the papa’s genes.

But if a female snail reproduces by herself, she can create 100 babies that are all female and all carry only her genes. Basically, this route puts more of the female’s genes into her offspring. Way more!

If evolution is all about surviving and getting your genes into the next generation, it looks like females reproducing on their own should have taken over the whole planet long ago.

After all, it’s not so easy to reproduce sexually. You have to find a partner, and that can take some doing (if you are a rare bird, for example). Attracting a mate can be a very big deal, with a big investment of time and energy (think of the peacock). Beyond that, both male and female must do the right things and both must be fertile at the same time.

Compared to that, self-cloning starts to look easy and much more likely to be successful. Why are we not all cloned females reproducing identical copies of ourselves?

Prof. Mark Dybdahl of Washington State University is a biologist who investigates that fundamental question.

“There’s got to be something in ecology or evolution that favors sexual reproduction,” he said to me recently.

This thought experiment can help. Think for a moment of an Iowa cornfield, planted with a particular strain of corn, all the plants being genetically identical to one another due to inbreeding. Because the plants are the same and genetically programmed to create high yields, the farmer may well get many, many bushels of corn from his field.

But if a disease or parasite arises that can feast on that particular strain of corn, disaster results.

Here’s the key concept: sexual reproduction is unlike cloning because it creates offspring that are different from one another. It can be well and good to be a female snail that reproduces exact copies of her own snaily essence. But that’s useful from an evolutionary point of view only if nothing comes along that can wipe out all her identical descendents.

“A female snail that reproduces sexually creates offspring that are varied. That increase the chances that some of her offspring can resist problems like parasites or disease,” Dybdahl said.

Dybdahl’s work has nailed down quite a bit, at least in the realm of snails.

“The snails that reproduce sexually – and pay the costs of having sex – do well only when parasites are beating the clones down,” he said.

I don’t know about you, but nobody taught me any of this material in middle school sex ed class. You have to keep learning well into middle age to really get to the basics – like the fundamental reasons that lie behind family court.