Species and Species Change 147 



established conferring advantages at points lower on the rainfall 

 gradient, say at 19 inches, then 18 inches, and so on, even though 

 these mutations would have been swamped out in the more com- 

 plex homeostatic balance adjusted to the old 20 to 40 inch range. 

 The net result would be a change in the species by which it became 

 adapted to a new climate. Axelrod (1958) cited this as the prob- 

 able mechanism for the origin of the xeric flora of southwestern 

 North America. He suggested that possibly large segments of the 

 ranges of mesic species became trapped in the more mesic high- 

 lands, surrounded by xeric lowlands (Fig. 61) and that subsequent 

 increase in aridity of the whole area brought the mesic-adapted 

 species under extremely strong and inescapable selection pressure 

 for adaptations to xeric existence. 



Among biotic changes, competition is known to restrict the 

 range of a species to less than it would be in its absence. A 

 demonstrated example concerns two European sawflies of the genus 

 Cephus whose larvae bore within wheat stems. Both were intro- 

 duced accidentally into the United States. Cephus tahidus arrived 

 in 1889 and by 1936 occupied a range extending from southern 

 New York to southern Virginia. Cephus pijgmaeus arrived prior to 

 1899, probably in New Jersey, spreading chiefly northward, then 

 westward. Finally it outcompeted C. tahidus where the two species 

 overlapped, and by 1941 C. tahidus was restricted to the more 

 southern and western parts of its former, larger range (Fig. 62) 

 (Udine, 1941; Elton, 1958). Thus competition narrowed the cli- 

 matic selection pressures acting on C. tahidus. 



An inferred example illustrating a different plane of restriction 

 concerns the Canidae (dogs) and Felidae (cats). The common 

 ancestor of the two could have been a rather omnivorous species 

 inhabiting a wide vegetation gradient, from relatively open coun- 

 try to forest edge and into the forest itself. Some of the skunks 

 have essentially these wide ecological tolerances. In such a wide 

 habitat gradient, small mutations favoring swift, short bursts of 

 speed (advantageous in the forest portion of the range) would 

 be mixed continuously with mutants favoring slower but sustained 

 running (advantageous in the open portion of the range). A com- 

 promise between these would be established by the simple process 

 of genetic mixing, with no opportunity for specialization advan- 

 tageous solely for life in one part of the habitat. 



The situation undoubtedly changed after the canid-felid an- 

 cestor had first evolved into many species which then moved into 



