EXAMPLE: BANDING PATTERNS IN SNAIL SHELLS

    The snail Cepea nemoralis exhibits variation among populations in the banding patterns on its shells.  In particular, two sets of loci control shell pattern.  One locus determines whether the background pattern of the shell is brown, pink or yellow.  Another set of loci determines the number of dark bands on the shell, which ranges from 0 to 5. Throughout Europe, populations differ in the relative frequencies of the shell color variants, and hence differ in the frequencies of the alternative alleles at the loci influencing color.  A study by Cain and Sheppard (A.J. Cain and P.M. Sheppard.  1954.  Natural selection in Cepea.  Genetics 89-116) examined the relative frequencies of the different color morphs in populations around Oxford, England.  They found that there was a very non-random distribution of color frequencies with respect to the type of habitat the population occupied, as shown by the figure below.  In particular, populations living in wooded habitats tended to have a high frequency of unbanded brown or pink shells (circles in figure), while in more open habitats (hedgerows and fields; squares and triangles in figure), yellow, banded shells predominated.  This non-random distribution can not easily be explained by genetic drift; natural selection is the most likely cause of population divergence.

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

            [ Back to Lecture 6 | Lecture Outlines | Bio 120 Home Page | Department of Biology | Duke University ]