GEOGRAPHICAL VARIATION AND EVOLUTION IN PSEUDOSINELLA HIRSUTA.

Abstract

Cave collembola furnish excellent material for the study of evolution. This is the result of the limited nature of geographical movement between caves and the limited selective pressure of cave life resulting in the persistence of primitive forms in caves. Early studies have established both the nature of large scale evolution in cave forms and the relationship between behavior and form in this process (Christiansen, 1961; 1965). All of these studies have dealt with large scale changes involving clearly separated forms. In order to examine the small scale changes which have occurred in the caves, the geographical variation of Pseudosinella hirsuta (Delamare) and P. violenta (Folsom) in caves was analyzed in detail. An additional study of surface population variation in P. violenta is in progress. Pseudosinella hirsuta is the most variable troglobitic form of the genus in North America. It has been found in caves of Kentucky, Tennessee, Alabama, Georgia, and Virginia (see Fig. 1). The variation is extensive, so that some forms superficially resemble P. argentea (Folsom) and others resemble P. christianseni (Salmon) with all gradations in between. The species is highly cave-adapted and apparently has undergone extensive evolution within caves. Pseudosinella violenta is a cave troglophile that has invaded extensively the caves of the Edwards Plateau and adjoining regions of Texas. It has reached neither the stability nor the adaptations of P. hirsuta. It is in the early phases of the second stage of cave adaptation, as described by Christiansen (1961). P. hirsuta is in the beginning of the third and final phase of cave adaptation. In this first publication of these studies we shall deal with the variations in P. hirsuta. Subsequent papers will analyze the variation in P. violenta.