ECOLOGY AND ISOLATION 



605 



for the individuals of certain species, such 

 as the song sparrow (Nice, 1934, 1937, 

 1941), to return to the vicinity of their 

 origin, with consequent inbreeding within 

 a small population. The evolutionary ef- 

 fects of such tradition would be similar to 

 the drastic reduction in numbers of a 

 fluctuating population, or the geographic or 

 ecologic isolation of a small population 

 from a larger one (Emerson, 1943; Thorpe, 

 1945; also seep. 619). 



Monogamy, polyandry, polygamy, and 

 the like, resulting from conditioned be- 

 havior, also affect the breeding structure 

 of populations and doubtless produce evo- 

 lutionary results. Degrees of inbreeding 

 through various breeding structures within 

 small neighborhood subgroups affect dif- 

 ferentiation of populations (Wright, 1946; 

 see also p. 608). 



We conclude that the breeding struc- 

 ture of populations may be influenced 

 by numbers and by mating behavior, that 

 variations in numbers of interbreeding in- 

 dividuals may be produced by a variety of 



factors, and that such variations a£Fect the 

 chances of the estabhshment of mutations 

 and gene or chromosome frequencies. 

 Population genetics is thus of concern to 

 the ecologist interested in evolution. 



SUMMARY 



In summary, ecologic factors influence 

 genetic variability. Hereditary mechanisms 

 have undergone adaptive evolution toward 

 efficient internal balance, and the external 

 environment has exerted selection pressure 

 in this direction. The environment may also 

 affect mutation pressure in certain in- 

 stances. Mutation rate, although important 

 for genetic variability, is not alone respon- 

 sible for rapid evolution. Variations in the 

 size of breeding populations exert an im- 

 portant influence upon reassortment of 

 genes and chromosomes, genetic fixation, 

 and gene frequency in populations. The 

 unit of selection, even in the primitive or- 

 ganisms, often must have been the popula- 

 tion group (pp. 602, 683, 684, 695) as 

 well as the individual organism. 



32. ECOLOGY AND ISOLATION 



INTRODUCTION 



Divergent evolution— the branching of the 

 phylogenetic tree— depends upon reproduc- 

 tive isolation. Reproductive isolation in- 

 cludes any factor that prevents gene flow 

 from one population to another such as 

 spatial and ecological separation, as well 

 as reproductive physiological or psycholog- 

 ical incompatibility. Even if the characters 

 of the two populations are the same at the 

 time of separation, and even if the environ- 

 mental conditions are the same for the 

 separated populations, they will gradually 

 drift apart genetically through random 

 changes in gene frequency, genetic fixation, 

 and mutation. 



Most of the hereditary differences be- 

 tween human races result from differences 

 of gene frequency rather than from pres- 

 ence or absence of qualitatively different 

 genes (Strandskov, 1944). In all proba- 

 bihty the same rule holds for the majority 

 of animal and plant races (p. 602). We 

 presume that mutations in reproduc- 

 tively isolated populations accumulate in 

 time, thus qualitatively distinguishing sep- 



arated populations, even though muta- 

 tions of genes influencing the rate ol 

 growth of different structures may only 

 produce quantitative differences between 

 the phenotypes. 



It follows that species would be expected 

 to diverge in relation to the degree of isola- 

 tion (see VagiHty, p. 213), the breeding 

 structure of the population, genetic fixation, 

 the amount of mutation pressure, and the 

 time lapse since isolation. Selection hastens 

 the process and is believed to be the pri- 

 mary factor in divergent adaptation (pp. 

 655, 664). However, much nonadaptive 

 evolution at the infraspecies and species 

 level may take place through the action ot 

 isolating factors in the absence of selec- 

 tion. 



Random genetic divergence in isolated 

 populations will in time probably result in 

 intrinsic inhibition of cross breeding 

 through regressive evolution (pp. 672, 

 676). Also, if cross breeding becomes 

 harmful to the diverging groups, selection 

 may speed the evolution of intrinsic, i.e., 

 genetic, isolating mechanisms. There are. 



