Kreitman [2] took advantage of these ideas to provide additional insight into whether natural selection was likely to be involved in maintaining the polymorphism at Adh in Drosophila melanogaster. He cloned and sequenced 11 alleles at this locus, each a little less than 2.4kb in length. If we restrict our attention to the coding region, a total of 765bp, there were 6 distinct sequences that differed from one another at between 1 and 13 sites. Given the observed level of polymorphism within the gene, there should be 9 or 10 amino acid differences observed as well, but only one of the nucleotide differences results in an amino acid difference, the amino acid difference associated with the already recognized electrophoretic polymorphsim. Thus, there is significantly less amino acid diversity than expected if nucleotide substitutions were neutral.
Does this settle the question? Is the Adh polymorphism another example of allelic variants being neutral or selected against? Would I be asking these questions if the answer were ``Yes''?