EXAMPLE: COORDINATE VARIATION IN PGI FREQUENCIES IN CRICKETS

    In a study on two species of crickets, Gryllus pennsylvanicus and G. veletis, Richard Harrison (R. A. Harrison.  1977.  Parallel variation at an enzyme locus in sibling species of field crickets.  Nature 266: 168-170) quantified the gene frequencies at the phosphoglucose isomerase (PGI) locus at eleven localities in eastern North America where both species occurred.  The figure below plots the frequency of one of the alleles, , for both species in these populations.  There is a strong correlation across populations.  Because these two species are reproductively isolated, evolution at this locus is independent in the two species.  The chance of obtaining this correlation by chance (e.g. by genetic drift) in independently evolving species is exceedingly small, indicating that natural selection is most likely responsible for the divergence among populations within a species for gene frequency at this locus.

Figure from D. J. Futuyma.  1998.  Evolutionary Biology.  Sinauer, Sunderland, MA.

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