604 



ECOLOGY AND EVOLUTION 



breed readily. Mutation pressure of the 

 white phase toward the blue may be low. 

 The fox population has striking varia- 

 tions in numbers, with a peak every three 

 or four years, followed by famine, epidemic 

 disease, or emigration. In 30,000 years 

 there would be about 8000 periods of 

 scarcity. According to Elton, if the fox 

 population is reduced during the depres- 



Numerous cases in nature beside the 

 white and blue foxes show the estabhsh- 

 ment of seemingly nonadaptive variants in 

 local populations. The high incidence of 

 black leopards in Asia and the low inci- 

 dence in Africa, melanic garter snakes on 

 the shores of Lake Erie, and black wolves, 

 muskrats, foxes, and sheep in the central 

 Yukon valley may be examples. Of course, 



o, 



B 



.r^i 



m**-- 



D E F 



Fig. 229. The field of gene combinations occupied by a population within the general field 

 of possible combinations under specified historical conditions indicated by the relation to the 

 initial field (broken contour) and arrow; A, increased mutation or reduced selection (4 NU, 

 4 NS very large); B, increased selection or reduced mutation (4 NU, 4 NS very large); C, 

 qualitative change of environment (4 NU, 4 NS very large); D, close inbreeding (4 NU, 4 

 NS, very small); E, slight inbreeding (4 NU, 4 NS medium); F, division into local races 

 (4 nm medium). N is population number (species); U is mutation rate from gene per gen- 

 eration; S is selection coefficient; n is population number (race); m is population exchange 

 with the rest of species. ( From Allee, 1938, and Wright, 1932. ) 



sions to a small fraction of the population 

 at the peaks, even a rare blue mutation 

 might become established by chance in 

 local populations, which would then expand 

 at the periods of abundance, giving rise to 

 a greater incidence of blues. During an- 

 other depression, in a few cases the blues 

 by chance might be the only survivors, 

 thus establishing a local population com- 

 posed entirely of blues. Elton thus suggests 

 random extinction and random establish- 

 ment as important factors in evolution. 



selection might rapidly increase the inci- 

 dence of certain established genes, as prob- 

 ably happens when adaptation is indi- 

 cated—for example, black rodents on black 

 lava in New Mexico (pp. 627, 650, 668; 

 Fig. 245). 



Although behavior resulting in region- 

 ality often has a genetic basis, yet within 

 that framework, homing (individualized 

 tradition) may tend to isolate populations. 

 Study of individual birds through banding 

 over several years reveals a strong tendency 



