A major reason for the difficulty is that at the genetic level there are quantita- tive differences

It is difficult to prove that traits seen to differ are truly the result of different selective pressures, that is, genetic adaptations. A major reason for the difficulty is that at the genetic level there are quantitative differences in frequencies of genetic variants, not qualitative genetic differences, among populations. When multiple forms of a DNA sequence, either a coding se- quence or a noncoding sequence, are present, the sequence is referred to as polymorphic and the forms as alleles at the