Variation
As an example of the survival effects of variation, we take
the case of sickle-cell anemia. Sickle-cell anemia is a gentic defect in a gene
which codes for the production of hemoglobin, the reddish colored, oxygen carrying
protein in blood. The normal form of hemoglobin is here labeled A.
The mutant form of hemoglobin which causes sickle-cell anemia is here labeled
S.
Most people inherit 2 normal hemoglobin genes from their parents
and thus have an AA makeup.
Some people inherit an A gene
from one parent and an S gene from another. They
have the AS makeup. Most of their hemoglobin is
normal, but some of it is abnormal, especially if the person exercises very
hard or is exposed to low air pressures (like at high altitude).
Some people inherit an S gene
from each other parents. We say these people have an SS
makeup. All of their hemoglobin is abnormal, so their blood cells are sickle-shaped
and thus may stick in the capillaries forming debilitating clots. Many of these
people die before they are able to reproduce.
Long & Slichter