TEST OF ASSOCIATION OF MORPHOLOGICAL VARIATION WITH HETEROZYGOSITY INTHE SNAIL CEPAEA-NEMORALIS

It has been suggested that the polymorphism for shell colour and patte rn in Cepaea may be unrelated to the visible appearance of the shells, but maintained by heterosis or heterozygote advantage. A possible way to test this supposition is to examine whether phenotypic variance is lower in the morph exhibiting the dominant phenotype than in that wit h the recessive phenotype. This has been tested in two samples of shel ls involving totals of 1792 and 5423 individuals, respectively. Overal l, there is a significantly greater variance in the recessive than the dominant class (P<0.05). Correlation between measurements on the same shell was also examined. No evidence of difference between phenotypes was found. If the difference in variance is real it may indicate eith er: (i) that visible appearance is not important and the polymorphism is maintained by a heterozygous balance; or (ii) evolution of such a b alance in a polymorphism maintained over a long time span by selection acting on visible attributes of the shells.