144 Species and Species Change 



mum of physiological imbalance, and (2) phenotypic character- 

 istics bringing a large part of the genetic column into the field of 

 influence of a new environmental situation. Another example per- 

 haps falling in this category is found in rabbits (Hoffmeister and 

 Mohr, 1957). The North American Lagomorpha are terrestrial, in- 

 cluding the cottontail Stjlvilagus foridantis, but the swamp rabbit 

 Sylvilagus aquations spends much time in the water and is an 

 excellent swimmer. Some sharp change in behavior has brought 

 this species into the selective orbit of the aquatic habitat. An almost 

 identical example occurs in North American voles of the genus 

 Microttis of which all but one species are strictly terrestrial or es- 

 sentially so. The Richardson vole or water rat M. richardsoni, while 

 living in meadows, is much like a small muskrat in general behav- 

 ior, swimming and diving well ( Cowan and Guiguet, 1956 ) . In this 

 new environment will the future phylogenetic lines of the swamp 

 rabbit and water rat evolve into distinctive obligatorily aquatic 

 groups such as the muskrats and beavers? 



Another phenomenon may be classed in this category of unusual 

 genetic change. In both plants and animals it is commonplace to 

 find that a cluster of closely related species have approximately 

 the same range or at least that the ranges appear to occur within 

 the same band of climatic conditions. In certain examples of this 

 sort, one species has a range occupying this common band but in 

 addition extends far beyond into a markedly different climate. The 

 American horealis branch of the caddisfly genus Helicopsyche con- 

 tains such an example. This branch comprises 12 or more species, 

 all but one confined to streams in virtually frost-free areas or to 

 outpost springs with warm winter temperatures. One species, H. 

 horealis, occurs in these warmer streams in company with some 

 of its 12 relatives but in addition occurs north almost to subarctic 

 regions (Fig. 60). In some fashion a genetic change must have 

 become established in the northern peripheral populations of this 

 species, a change which produced either winter hardiness or pe- 

 culiar competitive advantages. As a result, this one species ex- 

 tended its range into a climatic zone new to the entire genus. In a 

 sense this is not a great change. The northernmost populations are 

 identical morphologically with the southernmost, indicating that 

 up to the present little has happened beyond an extension of range 

 and that this probably occurred relatively recently. The important 

 circumstance is that the largest part of the species range is now 

 under a set of selection pressures markedly different from the set 

 which has acted on the entire genus up to this time. This should 



