In “Are Human Beings Still Evolving?” Cecil implies that sickle-cell anemia and Tay-Sachs disease are caused by inbreeding. Actually, in both cases the genes that cause the traits give partial immunity to diseases: malaria in the case of sickle-cell and tuberculosis in the case of Tay-Sachs. In both cases, one copy of the gene gives immunity, while two copies cause the genetic illness. The populations prone to these genetic conditions evolved in areas where the diseases are prevalent: malaria was (and still is) common in Africa, and tuberculosis was common (and easily spread) in the cramped ghettos where Jews were forced to live during the middle ages.
In both cases these genetic diseases were not caused by inbreeding. They are examples of genetic traits that spread through their populations because they led to improved fitness - i.e. the carriers of these genes were more likely to survive and have children. In other words, both sickle-cell anemia and Tay-Sachs disease are the result of normal evolution by natural selection. I would expect that, in places where protection from malaria and tuberculosis are not important (e.g. the United States), the sickle-cell and Tay-Sachs traits will eventually disappear from the populations that carry them (again, because of natural selection).
Cecil says that Tay-Sachs and Sickle Cell Anemia are noticable in certain populations that are relatively isolated and breed within themselves. e.g., Jewish people may be prohibited from marrying outside of their faith, so any traits specific to the pouplation would tend to manifest themselves within that population.
That would seem likely. Given that Africa has many regions where malaria is prevalent, it makes sense that humans would tend to develop a genetic coping mechanism.
Why would this mutation manifest itself in the Negroid population and not in the Amerinds of South America or the Mongoloids of the mosquito-infested parts of Asia or the inhabitants of the Indian Subcontinent? I don’t know.
Cecil is saying that rapid evolution occurs in isolated populations that inbreed. He’s not specifically refering to the detrimental effects of inbreeding Isolated populations - ghetto Jews or Sub-Saharan Africans - can adapt to their environment through mutation in ways that the general interbreeding population (if such a thing exists) cannot.
No, they don’t. But if a gene developed that gives an advantage to the organism carrying it, for example, resistance to malaria, then the organism is more likely to succeed. Of course in this case it’s a double-edged sword. Net gain? Net loss? Is it a wash?
Well, if you are looking at an inbred population I think that slaves(in the US) would qualify(in comparison to the population of subsaharan africa where most slaves were taken). It would only take a small population with proven resistance to thrive in malarial conditions, isolated from the greater gene pool.
I don’t think Cecil was suggesting in-breeding in the pathologic sense, just relative isolation.
IIRC, Thalassemia Beta offers malaria resistance to some mediteranian populations.
Sickle cell trait, caused by one recessive gene causes malarial resistance, while sickle cell disease is caused by a double recessive, therefore, if I have this right, if both your parents have a single gene, you have a 1 in 4 chance of sickle cell anemia, a 1 in 4 chance of malarial vulnerability, and a 2 in 4 chance of malarial resistance. If mortality is less than 100% in the first 2 cases, you have a net gain
As far as Amerinds go, I would speculate that the trait lost out during the artic crossing and hasn’t had time to reappear in the malaria zones of the Americas
It was probably never there in the first place. Africa has a vast genetic diversity, much of which never left with the small number of early emigrants who moved outwards to other parts of the world. (Some scientists believe that, if there is any real genetic basis to the concept of race, humans can be divided into about 8-12 “African” races plus an “everbody else” race.)
I was not aware of the percentages you’ve noted. However, having said that I will repeat that Thalessemia beta is found in Italian and Greek heritage, offers protection from malaria, and can cause disease in the same double recessive pattern as sickle cell anemia, maybe there’s a connection
Larry