In terms of microevolution; what exactly does a change in frequency of alleles in a population mean?
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Change in frequency of alleles refers to the change in the combination of all genotypes in a population or the gene pool. For example take the Drosophila melanogaster (the common fruit fly) it has a well known mutation that causes vestigial wings (recessive allele - vg). Since this is a recessive allele for a vestigial winged phenotype possible alleles are vgvg. And the normal phenotype is known as the wild type (dominant allele - vg+) .: possible genotypes are vg+vg+ or vg+vg. Wing flapping is an important part of being able to secure a mate. Vestigial flies may not be able to undergo mating rituals therefore it may not pass on its alleles to next generation.
The point is so if we complete an experiment where we have 10 vestigial wing flies (vgvg) and 10 wild type pure bred (vg+vg+) the allele frequency of vg would decrease over time (at a selective disadvantage), till the wild type phenotype becomes fixed, but they may be vg+vg or vg+vg+. So because vestigial winged phenotype is selected against, the allele frequency of vg would decrease and the allele frequency of vg+ would increase.
Or another way of thinking about it is, say we have population A in a field and population B over the hill (of the same species). If we let them interbreed there would be an exchange of alleles between the 2 populations. Hence, the allele frequencies in each population would change due to gene flow. A new allele may be introduced or an allele may increase in frequency or decrease.
Hope that made sense.
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