Kin Selection: this is where any gene that promotes the survival of all ‘kin’ is selected because it will mean the greater likelihood of all genetic relatives surviving and thus passing on common genes shared amongst the relatives.
Natural selection works at the individual level whereas kin selection works at the level of a group where members of the group share common genes.
Eg: altruism can be argued to contradict natural selection because it costs the individual and, therefore, reduces their chances of survival and reproduction. However, if the altruistic act improves the chances of survival and reproduction of a genetic relative, then, by the mechanism of kin selection, it can be argued the altruistic act is an adaptive response.
Klinefelter’s Syndrome: affects about 1 male in every 500 to 1,000. They have an extra X chromosome – ie: XXY. Consequently their testes are small and their fertility is usually very weak – many are effectively sterile. Small, low functioning testes means they have low testosterone levels. Usually the penis is small too and they may have slightly increased breast tissue. There is no growth of body hair. See Chromsomal Problems in Gender Development for more on this condition.