122 I The Process of Evolution 



its peak, though remaining in the vicinity. This is a trial-and-error 

 situation which may lead to the capturing of higher and higher 

 peaks, although under the described conditions progress would be 

 extremely slow. 



In Fig. 6.13/ a large species is subdivided into numerous semi- 

 isolated populations. The part of the field occupied by each sub- 

 population shifts continually in a largely nonadaptive fashion and at 

 a much faster rate than in the preceding case ( since it is dependent 

 on the amount of intermigration rather than mutation rate). With 

 the rapid movement in the general neighborhood of one peak, 

 sooner or later one subpopulation will cross the lower slopes of a 

 higher peak and ascend it. This subpopulation will then expand in 

 numbers; by migration its genes will flow into the other subpopula- 

 tions, and the whole species will be brought into the field of influ- 

 ence of the new peak. This situation, featuring intergroup selection, 

 permits trials of new combinations with a smaller risk to the species 

 than a situation involving only intragroup selection. Such a sub- 

 divided population, then, provides the best opportunities for low- 

 risk evolutionary change. 



SUMMARY 



In this chapter an attempt has been made to give a brief introduction 

 to evolutionary processes from the viewpoint of population-genetic 

 theory. One of the fundamental concepts of biology is the Hardy- 

 Weinberg law which states that, in an idealized population and in 

 the absence of evolutionary forces, the gene frequencies of auto- 

 somal alleles in the population will not change and, after one- gen- 

 eration, the proportion of genotypes will reach an equilibrium. The 

 ways in which mutation, migration, drift, and selection may cause 

 deviations from this equilibrium have been formulated mathe- 

 matically. The effects of these forces depend not only on the inter- 

 actions among them but also on the structure of the population and 

 the feedback effects of this structure on the forces themselves. 

 Theoretical descriptions of possible responses of populations to 

 various combinations of factors have been developed. Considering 

 populations as shifting and interacting arrays of gene frequencies has 

 given the evolutionist tools (however crude) for analyzing his ob- 

 servations with some degree of rigor and precision. Population- 

 genetic theory is extremely useful in describing what may happen 

 in natural populations and in interpreting data gathered from such 

 populations. 



