If the condition (probably in heterozygous state) is benefitial for some reason, it will be kept in the population. Look for sickle cell disease
http://en.wikipedia.org/wiki/Sickle-cell_disease

The important thing is, that you’re talking about people. People do a lot of unreasonable things, sometimes not benefitial and sometimes even hurting…

yes, sickle cell is a very good example of a homozygous recessive disease. In Africa, the gene is kept in circulation through the evolutionary force of balancing selection, more specifically balanced polymorphism.

Africa is infested with malaria carrying mosquitos, which as we all know is a very deadly virus, especially where quality of life is so low. Those with the heterozygous form of sickle cell are free from the harmful side effects of its homozygous recessive form. Not only that, but they are granted immunity to the malaria virus(because the blood cells become smaller, so the parasite cannot live). With that being said, those with the heteozygous form are able to reproduce and therefore pass on the gene with the chance of passing down either the homozygous or heterozygous form.

As for the second question, I am not 100% sure, but I am pretty sure that cultural evolution is a form of natural selection; therefore, look at how religion effects eating habits… Ramadon(or however it’s spelt), eating coshier(however it’s spelt), or even becoming a vegan and going against mankind’s omnivorous traits.

Hope that helped.