For a single co-dominant locus in two populations of blue oak, I am given genotype frequency data (genotypes are AA, AB, BB) for adult trees (at least 150 years old) in both populations (pop 1 and pop 2) and from sampling of seedlings taken in fragmented woodland from the same populations (pop 1 and pop2) very recently. I am asked to detect any difference in Fst among adults and seedlings, and propose a hypothesis explaining how any difference observed could have come about.
To tackle this question, I calculated the Fst value between Pop1 and Pop2 using the adult's genotypic frequencies and then Fst between Pop1 and Pop2 for seedlings using the respective genotypic frequencies given for the seedlings.
The Fst values for the adult's was 0.004 and the seedling' was 0.113.
Clearly there is genetic differentiation between population 1 and 2 in seeds, but not much in adult trees.
I'm mostly stuck on the explanation for why this would happen. My gut response was that there is some selection against certain adult genotypes in population 2 in that they are unable to produce seeds? Or maybe that adults in population 2 are mating non randomly and therefore produce seeds that do not match the genotypes of the adult population?