Over at Dispatches a poster asked:
Is it not logical and optimal to breed beneficial characteristics in humans and breed out unbeneficial ones? ...What are the grounds of promoting or assisting [nonbeneficial traits] when it seems to run counter to "survival of the fittest"?
This is a fairly common conclusion that some draw from evolutionary theory, but it is sadly very wrong. At basic philosophical level, it is deriving an "is" from an "ought", a problem that, despite the best efforts of Ayn Rand and others, has really never survived Hume's critique. Science is about what is. Ethics and morality are about what ought to be. Evolution is science. It says that the cumulation of random genetic changes over time has produced all life on this planet from some distant common ancestor. The short version is "survivial of the fittest", which is somewhat misleading. It provides insight into the flaws in the question above to note Darwin's preferred phrase: descent with modification.
It is simplistic and misleading to talk of "fittest", because it implies that "fittest" is an absolute objective trait of a being that we can measure and use to predict survival. Fitness is completely dependent on the environment. For example, take dogs and wolves. By any objective, contextfree analysis, wolves are more fit than dogs: they are, on average, larger, faster, tougher, and smarter. Yet dogs greatly outnumber wolves, due in no small part to forming a semi-symbiotic relationship with the most dangerous species on the planet: us.
Once evolution is understood in this context, it becomes clearer why promoting "beneficial traits" isn't going to mean in evolutionary terms what we mean when we normally use those terms. Nature, not our values, determines fitness, and nature changes all the time. There is also a more generic argument against social Darwinism. If we were able to successfully breed ourselves into some sort of supermen, we would reduce the genetic variation of the species. This would greatly increase our exposure to catastrophic events, such as disease. If there is any normative value from evolutionary theory, it is that heterogeneity is good.
It should also be emphasized that evolution is stupid. It only moves incrementally from where it is. It is incapable of looking ahead. Were it a mountain climber, it would climb, always upward, but in otherwise random directions and distances, until it came to a point with no possible upward step. It's odds of getting to Everest are remote. Likewise, it simply produces more of whatever survived more and reproduced more. It can't look ahead to potentially filling up the planet, or exceeding available food stores, or running out of oil, or the many other problems facing humanity. There is a good reason 99% of all species who ever existed are now extinct. If we simply let evolution guide our decisionmaking, we can expect the same fate.
In the end, even if we could objectively and absolutely identify a way to improve our species and its survivability through selective breeding, Hume can still ask "So what? It does not logically follow that because you can do these things you should do these things", and indeed he would be correct. Evolution is a scientific theory. It explains why many facts are what they are. It can never tell us what we should do about it.