The NSS Bulletin - ISSN 1090-6924
Volume 47 Number 2: 123-127 - December 1985
A publication of the National Speleological Society

A Reconsideration of Ludwig's Differential Migration Theory of Regressive Evolution
Daniel W. Fong and David C. Culver

Abstract

Ludwig's differential migration model of regressive evolution in cave organisms was examined. Our reformulation of the model shows that only differential emigration can lead to fixation of small-eyed morphs in cave populations, at equilibrium, while differential immigration cannot. However, the rate of approach to equilibrium is very rapid. We specifically studied two predictions of the model. This first is that of a cline of larger-eyed individuals outside caves to smaller-eyed ones inside caves. We found no evidence of such a clinal variation in eye facet number in a population of Gammarus minus which spans the entrance zone of a cave in distribution. The model also predicts a negative correlation between eye size and light avoidance behavior. We found extensive phenotypic variation in eye size and phototactic behavior in a population of Gammarus pseudolimnaeus, but no significant correlation between the two traits. Analysis of the genetic parameters underlaying these traits showed that a large portion of the osbserved variation is genetic in origin for eye size, but is not genetically determined in phototaxis. Our data strongly suggest that Ludwig's differential migraiton model cannot explain regressive evolution of cave organisms.

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