Relating to the continuation of undesirable traits through genetic means.
In the long term natural selection selects against traits that are undesirable in the sense of reducing probability of survival of one's genes. However, there are various ways that bad features can be retained by evolution.
One is the tragedy of the commons, where a less than optimal solution to claiming resources is used, simply because the other organisms around are doing the same, and there is no way to agree to forgo immediate benefit for a long term greater benefit. Evolution can not plan ahead.
Another is where a disadvantage comes as the price of an advantage it is linked to. For example, a gene in humans protects against malaria when heterozygous, but when homozygous (inherited in two copies) it causes sickle cell anaemia. The advantage to the large number thereby protected against malaria outweighs the dysgenic effect on the minority who suffer from the double gene.
This explanation might also cover those cases especially in humans where altruism causes dysgenic effects to survive more than they otherwise would. Altruism is a good thing and aids in keeping wisdom and experience alive, so in humans it has been selected for, and we as a species care for those who are physically incapable of independent survival. The simplest (and most energy-efficient) way of encoding this genetically is probably to make it a general injunction to care for fellow humans. So the mentally incapable are also treated as fully human. This is not universal across all societies but is at least widespread.
Some people, since the discovery of genetics in the nineteenth century, have argued that preserving mental or physical weaknesses simply because their bearers are human is weakening the overall human race. This view is called dysgenics, and is part of the movement usually called eugenics, though in fact the historically significant eugenic movements have usually been less concerned with increasing good stock (e.g. by mating Nobel Prize winners) than with removing dysgenic factors (sterilizing the poor and the mentally retarded, or genocide of supposedly inferior races).
On darwinian grounds, this is false. Taken strictly as a problem in natural selection, it would seem the altruism of caring for people is selected over the more brutal efficiency of not doing so. To argue for a dysgenic danger you would have to say that cultural evolution was overbalancing genetic evolution.