Keeping Good Mutations
In examining genomic differences between humans and chimps, researchers noticed a difference in the caspase-12 gene. The mutation creates a "pseudogene", which means the gene is still present in the genome but the mutation results in the production of an inactive protein. The original form of the gene is involved in activating the immune system, so one would think that inactivating it would be dangerous and be subject to negative selective pressures. It turns out that inactivating caspase-12 actually increases the chances of surviving a severe bacterial infection, referred to as sepsis. This provides a positive selective pressure for the mutation, and as a result it has been “fixed” in most of the human population, which means that most people carry the mutation.
The researchers were interested in demonstrating the role pseudogenes might play in evolution, so they compared the gene form found in various populations. They found that the mutation is much less common in African populations, which scientists consider to be ancestral to other human lines. They conclude that the mutation was present in a small subset of the ancestral African population, and as humans carrying the mutation migrated through the world positive selection for the gene fixed it in the population outside of Africa.
Links:
Mutant caspase-12 fixation in population. Public Library of Science article on "pseudogenization"
