Each one of us carries with us a gene that, while unique to ourselves, is also the result of several hundred-thousand years' worth of mixing different DNA strings in such a particular combination that it now defines us as the persons we are. Any tiny difference in the way your genes had been put together in all that time, and you wouldn't be you any more, in other words.
So the gene that we carry has survived up to this point in time, and our genetic imperative is to ensure this gene we carry will continue to survive by combining it with someone else's gene, thus passing it on to the next generation. And speaking in evolutionary terms, we hope that the new combination will improve the new genetic host, making it stronger and fitter to survive into the following generation after that, and so on.
Natural selection tells us that often, gene-lines come to an end because the genetic mix was flawed and was thus unfit to be passed on. These genetic hosts are therefore literally dead-ends, and deserve to be removed from the gene-pool because they were defective. These genes will not contribute to develop the species any further as the stronger, fitter genes that do get passed on to the next generation pass them by.
But although the biological drive to have offspring is strong -- after all, the more offspring there are, the more likely it is for the parents' combined genes to continue to survive another generation -- these days, the human species is making evolutionary history by making a conscious decision not to procreate any further.
Considering the falling birthrates that today's ST is pointing out in Taiwan (politics too unstable, therefore we'll not have kids), and throughout the developed nations of Asia, people are declaring themselves as evolutionary dead-ends, thereby voluntarily removing themselves from the gene-pool. Thus, they summarily end several hundred-thousand years' worth of their own gene's evolution, not because the genes are themselves defective, but because the current hosts are "exercising a choice" not to pass them on any further.
It's almost like saying that we are the epitome of our gene-line's evolution, and this is as far as it goes... because we said so.
We human beings think too much. We intellectualize our environment by assessing and analyzing risk factors and cost-benefit ratios. We imagine worst-case scenarios and visualize perfection, and are even prepared to hold out for it even though the ideal is only ideal because it is unattainable. We have developed over-complicated mating relationships, and possibly childcare duties as well.
Our big brains that we're so proud of, the one that has led us to evolve into the dominant species on the planet, could perhaps also be our greatest impediment to evolutionary progress. Evolving smarts has helped us immensely, but evolving too smart could in itself be a genetic defect. I can already imagine the gene-pool lifeguards yelling, "OK, everybody, out of the pool!"
Those of us who do have children, we sometimes think of in animalistic terms. We say that they "breed like rabbits," and we are aghast at men who have voracious sexual appetites, calling them "beasts," "wolves" and "buaya," but chances are, in the long-run, their genes have a better chance of advancing our species than those of the more conservative persuasion. If our wholesome couples persist in choosing not to have kids, we can be assured that the future will only belong to the horny bastard.
Now, there's a future to contemplate!