THE MEDIONIGRA GENE IN THE MOTH PANAXIA-DOMINULA - THE CASE FOR SELECTION

Analyses of changes in frequency of the gene medionigra in colonies of the moth Panaxia dominula, begun by R. A. Fisher, E. B. Ford and P. M . Sheppard, have long been regarded as a model study of natural select ion under field conditions. Recently, their conclusions have been crit icized, on the grounds that phenotypes have been improperly scored and that population structure has been misunderstood. The results are re- examined here, including recent unpublished collections. It is argued that the colonies studied are distinct populations, as usually defined , and that the results could not arise as a result of migration. Fluct uation in population size from year to year, large variance in fecundi ty and some features of mating behaviour probably reduce the effective number to less than half the estimated population size. Variable expr essivity and consequent subjective variation in scoring, casts some do ubt on earlier claims that selection fluctuates significantly from gen eration to generation, and on a reported case of increase in frequency from a very low starting point. However, the one natural and thr ee a rtificial colonies studied are consistent in providing estimates of se lection of 7 per cent or more against medionigra, despite the variatio n introduced by small population size and scoring difficulties. There appears to be an equilibrium at a low frequency, and the rate of appro ach to it suggests that the selection is frequency dependent. One arti ficial colony, at West Kirby on the Wirral, Merseyside, U.K., does not show an equivalent decline. Selection cannot be as strong as indicate d elsewhere, but since the population size is small some disadvantage to medionigra cannot be rejected.