The NSS Bulletin - ISSN 1090-6924
Volume 47 Number 2: 109-117 - December 1985

A publication of the National Speleological Society

Evolutionary Reduction by Neutral Mutations: Plausibility Arguments and Data From Amblyopsid Fishes and Linyphiid Spiders
Thomas L. Poulson


Patterns of change among the six species of Amblyopsid fish are used to argue that evolutionary reduction is based on accumulation of neutral mutations whereas troglomorphic traits are selected. Net changes, for 14 reducing and 17 troglomophic traits, were calcuated for the trogloxene-troglophile pair of species, the troglophile vs troglobite with least reduced eyes, and so on along a sequence of increasing eye reduction. The patterns of decrease for reducing traits did not parallel the patterns of increase in troglomorphic tratls as expected in reduction were due to indirect selection by pleiotropy. Pattern of change was consistent for torglomorphic traits with moderate net increase for both -xene to -phile and -phile to -bite 1 and virtually no further increase among any of the three troglobite species pairs. In contrast, patterns of decrease for reducing traits were quite variable but all showed continued high to moderate decrease among all the pairs of troglobites.

Natural experiments and calculations of energetic savings for spiders are used to argue that neither direct energy savings nor indirect material compensation are likely bases for evolutionary reduction. There is no trend with more food limitation from low to high trophic levels or from food-rich to food-poor caves at global, regional or local scales. Development, growth, and maintenance cost savings for structural reduction are less than 5 percent of whole organism cost and much less than 1 percent of savings due to troglomorphic decrease in routine metabolism in spiders.

The general discussion deals with alternative hypotheses relevant to structural reduction and troglomorphy, assesses the evidence for loss of physiological compensation for environmental variation, and relates trends in morphological and genetic variation to patterns of reduction and troglomorphy.

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