doesn't a gene pool 'fizzle' out at some point...?
noop only it is not suitable for the current environment
Without genetic pressures, the pool will stay constant - Hardy and Weinberg established this in around 1923.
A deleterious gene will hang around for astounding amounts of time if there is (a hidden) advantage - SIckle cell (malaria) springs to mind, and also the cystic fibrosis gene.
and finally there are a group of diseases which dont fizzle
but you can show ARE present in olden times Huntington's Chorea springs to mind for the New England centre, because one of the Pilgrim fathers was observed at the time to be twitching - this is called a founder effect and is observed in the English Royal Family and AIP, Tay Sachs and the Polish Jewish families of 1680-1700 and porphyria in South Africa when it was first observed in Jan van Niekerk's time (1660)
and I havent even got up to the bit why Ellis van Crefeld Syndrom persists in the Australian Aboriginal population