THE NEUTRAL ASSUMPTION AND MAINTENANCE OF COLOR MORPH FREQUENCY IN MANGROVE SNAILS

The leaf-living mangrove snail Littoraria pallescens is polymorphic for shell colour, and morph frequencies vary little throughout the species range. Constancy may be due to stabilizing selection or large effective population size. Using variances of morph frequency obtained from sample data, estimates of 4N(e)(v + w) have been made (N(e) is effective population size and v and w mutation rates for two shell colour morphs). On the assumption of neutrality, the estimates are 12.2 or 25.2, depending on dominance, suggesting that N(e) is probably of the order of 10(6) or more. Stabilizing selection could, however, have inflated these values. If a set of estimates from isozyme data indicated a much lower effective population size, then the supposition of stabilizing selection would be reinforced. Conversely, if they indicate a similar size there would be no reason to reject the neutral hypothesis. However, apparent mutation rate depends on detectability of phenotypic differences, and may vary by orders of magnitude between classes of data. The relation of apparent phenotypic conversion to base mutation is outlined. Instead of looking at frequency distributions of multiple alleles at allozyme loci, or the number of alleles present, the alleles may be pooled to give the same number of morphs as the colour locus. This avoids the problem of comparability of mutation rates. Published allozyme data from a congeneric species give estimates of effective population size similar to those for L. pallescens morphs.