Sickle cell anemia in natural selection?

Answer 1

The genetic 'mutation' (or phenotype/genotype) that causes sickle-cell anemia didn't originate as a response to malaria. It happened and happens randomly, by simple chance. As malaria epidemics would strike some area, the people who didn't have the mutation would be less likely to survive. Thus the percent of people that had the mutation would increase, and (since it is genetic/hereditary) the percentage would quickly increase. In places where malaria was a regular hazard, the people who lacked the mutation would be rapidly selected out as the percentage of those with the mutation would have so strong a survival advantage.

The advantage was and is profound. Granted, the disease comes with a truly dreadful down-side. Regrettably, since the mutation doesn't kill you off before reproductive age, the selective pressure is simply "positive" so it only grants what seems like a genetic advantage.

Answer 2

Sickle cell anemia could be adaptive in the way that it offers protection against malaria. Sickle cell anemia is dominant genetic disorder whereas if you inherit it from both parents (AA alleles) then you die early in life because your blood cells are unable to carry the required amount of oxygen. However if you only inherit it from one parent (Aa alleles), then only part of your blood cells shows the characteristic sickle shape while the other part is fully functional. We have not yet discovered the reason behind this, but it plays a part in protecting against the malaria parasite.



Some literature that may help you research this topic more thoroughly (some are free access at www.pubmed.com):

1. McKenzie, E; Killeen, G; Beier, J; Bossert, W. 2001. Seasonality, Parasite Diversity, and Local Extinctions in Plasmodium falciparum Malaria. Ecology 82(10) pp: 2673-2681.

2. Dajer, Tony. Blackwater Fever. 1992. Discover Magazine pp: 47-52.

3. Burns, E; Pollack, S. 1988. P. falciparum infected Erythrocytes are capable of Endocytosis. In Vitro Celular & Developmental Biology. 24(5). pp: 481-486. 4. O'Donnell, A; Weatheralla, D; Taylorc, A; Reederb, J; Allenab, A. 2005. Muscle cell injury, haemolysis and dark urine in children with falciparum malaria in Papua New Guinea 5. Suarez, C. 1985. Plasmodium Falciparum: Invasion and Development in highly Parasitized Cultures. 21(3). pp:161-164.

Answer 3

The heterozygous condition confers some resitance to malaria, especially in children. So, those who possed this condition would tend to survive more often in malaria country and live to have more children, some of which would inherit the trait.