the only way i can think of, or i think as far as anyone else can think of, deciding whether it has in fact been important in speciation, is to look for pairs of closely related species which differ in some striking morphological trait, but are still sufficiently similar genetically for you to be able to carry out a genetic analysis, i.e. to cross them, to get offspring, to get f two's and it's then possible, it's obviously not — i'm not going to explain the details of the technique to you now — but it's possible to work out whether the difference is due largely, or in part, to some single large gene, or whether it's on the whole due to quite a lot of small ones.