Azoospermia. I'm just saying.

Here's a weird idea to mull over, a strange cocktail of evolutionary forces, medical advances, logical necessity, and a tiny little bit of sperm. If your kids can handle it, invite them on in to the discussion.

It goes like this:

1. Evolution favors animals that can reproduce, obviously. The more you reproduce, voila, the more of your genes are represented in the next generation.
2. Infertility can be inherited, such as azoospermia (inability to produce sperm) resulting from cystic fibrosis or other congenital problems.
3. Thanks to the advent of in-vitro fertilization (IVF) and similar treatments, "infertile" people can now have whole passels of biological kids tearing around their back yards.
4. These kids may carry on the "infertility" genes, and as adults they may need assisted reproduction as well.
5. And so on over generations and generations.
6. Here's where things go off the rails a bit. What's to stop infertility becoming the norm? And what is "infertility" if everybody who is azoospermic, for example, has as many kids as the next guy? And will we evolve to a species that can't reproduce without extraordinary medical intervention? And would everyone's life be better that way?

As I swig this cocktail, I can hear the objections already.

"But Kate, the prevalence of infertility is low, and almost nobody has access to those fancy IVF procedures. This will never have any large effect on the course of human evolution, and genes for infertility will never rise to any appreciable proportion in the population!"

To this I say a resounding "Perhaps!" But cast your mind into the future. Infertility affects at least 10% of the population (and probably much more, don't get me started on that point.) That's a lot of uteruses we're talking about.

And in only about thirty years, IVF and other such machinations have gone from high-media spectacles to routine health care that's increasingly covered by health insurance across the country. In Europe, infertility treatments have been fully covered for decades. In Denmark, nearly 4% of all births in 2006 resulted from IVF. Four percent! That number can only get higher, I think. IVF is becoming increasingly common even in low-resource populations with high fertility norms, like India and sub-Saharan Africa.

Also, success rates for infertility treatments are ever-rising. These days, an "infertile" couple getting IVF now has a far greater chance of conceiving than a "fertile" couple going at it the old-fashioned way for the same amount of time. Now who's infertile?

So I'm thinking: a good proportion of the population is using common, affordable means to overrule their own infertility. Over the long term, this is not negligible. Drink up.

Here's another objection I can hear:

"But Kate, medical intervention has slowed down human evolution in a thousand ways already. People who never would have survived or reproduced 100,000 years ago now can do both. Your example is just one of many!"

Again I chug my cocktail and say "Maybe!" But don't you think there's something truly strange about a gene for infertility no longer being selected out? You can't get much more paradoxical than that.

Anyway, azoospermia and IVF. I'm just saying.