Natural selection and the scarlet tiger moth, Panaxia dominula : inconsistencies in the scoring of the heterozygote, f. medionigra

Abstract

In 1989 we rediscovered an artificial colony of the scarlet tiger moth Panaxia dominula , started in 1961 by Philip Sheppard at West Kirby in the Wirral (well outside the normal range of the moth) and it had been undisturbed for many years (Clarke et al . 1990). The founding stock of 13000 back cross caterpillars came from Cothill, near Oxford, and in 1989 at West Kirby the proportions of the three phenotypes (typical, medionigra and bimacula , figure 1) were in the same Hardy-Weinberg equilibrium as they would have been had there been random mating and no selection (see table 1). This was in marked contrast to what had been reported at Cothill, a much bigger colony, researched for fifty years and characterized by great fluctuations in gene frequency and usually strong selection against medionigra (summarized by Jones (1989)). In the present paper we suggest that one factor contributing to the Cothill findings may have been variability in the scoring of medionigra by different observers over many years. The evidence for this comes first from two Cothill experts, and second from our own test matings using Wirral Way material.