Genetic variation in the genus Littorina
Six British species of the littoral prosobranch
Littorina have been examined for genetic variability at
between eight and twenty-five enzyme loci. The characteristics
and levels of variability observed are similar to those
observed in other taxa of similar somatic complexity.
Attempts to relate patterns of variability with enzynle
structure and function, or with bionomic strategies have
shown no significant trends. A phylogenetic tree has
been constructed and divergence times estimated for "five
members of the genus. The advantages of the littoral
environment, coupled with a poly-specific genus for
examining models of evolution, has been discussed. L. rudis
and L. littorea have been examined in detail at several
sites along the coast of South Wales and variability in
each species has been examined in terms of the physical
separation and the ecological properties of the populations.
Whilst L. littorea is homogeneous and probably
effectively panmictic in South \vales, L. rudis shows a
significant increase heterogeneity as populations become
further separated, indicating substantial population isolation.
A highly significant relationship between heterozygosity
(gene-diversity) and species overlap has been found, neither
of these characteristics could be related to exposure.
In populations sampled at more than one tide level, the
sample from the higher mark invariably showed the higher
heterozygosity. Little relationship could be found between
environmental parameters or heterozygosity and measures of
morphological variation, however it was found that ~ average
older L. rudis and L. littorea were more heterozygous than
younger individuals. Heterozygotes show a lower variance
than homozygotes for a meristic character, shell index, at
some loci. The data havebeen examined for linkage
disequilibrium. Information on some foreign populations is
The data are discussed in the light of modern
evolutionary theory, and a balancing selection model for the
maintenance of genetic variability is proposed.