help me on carrier advantage/evolved via sexual selection - (Dec/12/2012 )

Stuck on this study guide question. Any help would be fantastic.


Given the potential advantage to carriers, discuss how this advantage might have evolved via a sexual selection (SS) mechanism. Be sure to define SS, put it in the context of natural selection (NS), and clearly state what the advantage to carriers is. Remember that evolution by NS takes place in the context of specific environments so be sure to include the role of the environment.

Hint: sickle cell

Makes sense, western Africa and sickle cell carriers are at an advantage since they fight off malaria better. But how would I incorporate the terms sexual selection and natural selection. Sexual selection would be that humans who are carriers are more likely to survive and therefore mate to produce more carriers? Natural selection would be that the sickle cell benefits people in a population so they have better chances to survive>?

Actually sickle cell affected individuals will at some point in their life have a crisis where the blood clots abnormally due to the odd shape of the cells. These may or may not be survivable, which means that the individuals have a shorter life span than non-affected individuals. So natural selection is against the disease.

However as you noted the people with sickle cell have a protective mechanism against malaria that means that they are less likely to get the disease, and hence have a better chance of surviving and/or have a shorter disease course meaning (in the absence of modern technology/shops etc.) that they are more likely to survive to hunt for food again. Note the incomplete recessiveness - partial expression of the recessive gene is enough to confer malaria resistance... there's your mechanism