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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Solved Problem

1. | The polymorphisms for shell color (yellow or pink) and for the presence or absence of shell banding in the snail Cepaea nemoralis are each the result of a pair of segregating alleles at a separate locus. Design an experimental program that would reveal the forces that determine the frequency and geographical distri-bution of these polymorphisms. See answer

Solution
a. A description is made of the frequencies of the different morphs in a large number of populations covering the geographical and ecological range of the species. Each snail must be scored for both polymorphisms. At the same time, a description of the habitat of each population is recorded. In addition, an estimate is made of the number of snails in a population.
b. Marking a sample of snails with a spot of paint on the shell, replacing them in the population, and then resampling at a later date enables migration distances to be measured.
c. If broods are raised from individual snails, the genotype of male parents can be inferred, so nonrandom mating patterns can be observed. The segregation frequencies within each family will reveal differences between genotypes in probability of survivorship of early developmental stages.
d. Further evidence of selection is sought from (1) geographical clines in the frequencies of the alleles; (2) correlation between allele frequency and ecological variables, including population density; (3) correlation between the frequencies of the two different polymorphisms (are populations with, say, high frequencies of pink shells also characterized by, say, high frequencies of banded shells?); (4) nonrandom associations within populations of the alleles at the two loci, indicating that certain combinations may have a higher fitness.
e. Evidence of the importance of random genetic drift is sought by comparing the variation in allele frequencies among small populations with the variation among large populations. If small populations vary more from each other than large ones, then random drift is implicated.
See question

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