Population Genetic Consequences of Local Colonization: Evidence from the Milkweed Beetle Tetraopes tetraophthalmus

Abstract

It is argued that host plants with patchy spatial distributions can organize herbivorous insects into highly localized populations. These insect populations could then be subjected to the effects of genetic drift or founder effect during colonization events associated with patch turnover. This would provide an ecology or population structure favorable to the operation of Sewall Wright's Shifting Balance mode of evolution. Three lines of evidence are provided that this is likely to be the case in the association between the milkweed beetle Tetraopes tetraophthalmus and its host plant Asclepias syriaca. 1) Mark-recapture and life-history studies have shown that the great majority of beetles reproduce in the milkweed patch into which they were born. 2) Gene frequency variation was detected among groups of beetles collected from milkweed patches separated by only a few miles. 3) The number of adult beetles colonizing artificial, uninfested milkweed patches over 2 years was small; often as few as 1 or 2 inseminated females.