120 I The Process of Evolution 



ADAPTATION AND GENE 

 COMBINATIONS 



As Sewall Wright pointed out, sexual organisms have available to 

 them a tremendous number of possible gene combinations. A species 

 with only 1,000 loci, each occupied by a series of 10 alleles, could, 

 through recombination, produce 10^*^°*^ different genotypes (a num- 

 ber inconceivably greater than the number of electrons in the uni- 

 verse ) . While many of the theoretically possible combinations would 

 be inviable or would yield identical phenotypes, such a species obvi- 

 ously has a very large capacity for genotypic variation. Within this 

 vast field of possible combinations, there must be a large number 

 of highly adaptive combinations and also many less highly adaptive 

 or even lethal combinations. Wright represented this field as a con- 

 tour map in two dimensions, with adaptive peaks and nonadaptive 

 valleys, and stated that "the problem of evolution ... is that of a 

 mechanism by which the species may continually find its way from 

 lower to higher peaks in such a field." A population of mosquitoes 

 selected to avoid insecticides by not landing on poisoned surfaces 

 might be considered as occupying an adaptive peak. A higher 

 adaptive peak might be the development of a method of physio- 

 logical resistance (allowing, perhaps, better access to houses). In 

 order to acquire the more efficient physiological resistance, con- 

 siderable reorganization of the genotype could be required; this 

 reorganization might result in the loss of the behavioral resistance. 

 The species would then have to cross an adaptive valley (little or 

 no resistance) in order to attain the higher peak. 



Figure 6.13a to / illustrates what might happen to certain kinds 

 of populations occupying the adaptive field under different specified 

 conditions. The field is represented as a topographic map with con- 

 tour lines indicating different levels of adaptation. The heavy 

 broken line represents the initial position of the population, and the 

 arrow the direction of subsequent change. In Fig. 6.13a one sees the 

 effect of increasing mutation rate or reducing selection pressure, a 

 general increase in the variance and lowering of the average adaptive 

 value of the population. If it spreads far enough, a portion of the 

 population may occupy the lower slopes of an adaptive peak that is 

 higher than the initial one; if this occurs, the entire population will 

 move over and occupy the new peak. The effect of increasing the 

 selection pressure or decreasing the mutation pressure is shown in 

 Fig. 6.13b. The average level of adaptation increases at the expense 



