150 Species and Species Change 



immediate environment, and close to the extinction point of the 

 population. Under these conditions it was supposed that adaptive 

 genetic mutants which might be swamped by a larger population 

 could, by rare happenstance of matings, suddenly become homo- 

 zygous and thus phenotypically effective. The events suggested in 

 this chapter would achieve the same result through ecological re- 

 striction without necessarily reducing the population to a figure 

 which could be described as small. Nor would the populations pass 

 through an "inadaptive" stage because they would never be less 

 well adapted to their environment than they were initially. 



If population size were to reach the extremely low levels of the 

 Wright-Simpson model, it is possible that interspecific hybridiza- 

 tion would follow. Certainly in many groups of animals it is likely 

 that two closely related species would be in the same range and 

 that extremely low population density would result in stress mat- 

 ing. The peculiar similarity of head markings in the mice Peromysciis 

 manicuhtus triangularis and Microtus townsendi cowani, both re- 

 stricted to tiny Triangle Island off the coast of British Columbia 

 ( Guiguet, 1955 ) , suggests that small populations have here resulted 

 in intergeneric hybridization. As has long been appreciated by 

 botanists but generally disregarded by zoologists, the unpredict- 

 able results which are possible in hybrid progeny could account 

 for a great deal of the observed shifts in the adaptiveness of species 

 (Stebbins, 1959). 



DIRECTIONAL FACTORS IN ADAPTATION 



After a species has moved into a different environment and come 

 under the influence of a new set of selection pressures, these pres- 

 sures will continue to select adaptive changes, as was explained in 

 the preceding chapter. From the standpoint of the long-term phy- 

 logenetic line, two types of consequence are of special interest: the 

 evolution of characters prerequisite for later changes, and the oc- 

 cupation of new adaptive situations. 



PREREQUISITE CHARACTERS (PREADAPTATION) 



It is evident that certain characters could not have evolved imtil 

 some other character had evolved previously. For example, insects 

 evolved from a multilegged, centipede-like ancestor and became 

 six-legged creatures in which the locomotor function was centered 

 near the anterior part of the body. Because of this position of the 

 legs, later insects were able to evolve a rapid gait and the ability 



