1HH 



PHYSIOLOGIC GENETICS 



Table 43 

 Comparison of evolutionary processes in homogeneous and subdivided populations 



Homogenous population 



Population subdivided into numerous 

 partially isolated demes 



Entity selected 



Source of varia- 

 tion 



Process 



(1) Conditions 



static 



(2) Conditions 



change 



Gene, differing from alleles 

 in net selective value 



Gene mutation 



Selection among individuals 



Progress restricted to exten- 

 sion of currently control- 

 ling peak 



Progress up most available 

 peak in new surface of 

 mean selective values 



Set of genie frequencies, characterized 

 by harmonious genie effects, con- 

 trolled by a selective peak 



Shift in controlling peak in a deme 

 (by random drift and selection 

 toward new peak) 



Selection among demes (by differen- 

 tial growth of population and 

 migration) 



Continual shifts in prevailing selective 

 peak 



Interdemic selection relative to all 

 available selective peaks in the 

 new surface of mean selective 

 values 



A major evolutionary step under the third theory, as discussed above, simulates 

 fixation of a favorable mutation in the first or second, but in this case is a byproduct 

 of a continuous process of intrademic and interdemic selection that is occurring below 

 a superficial appearance of near-uniformity in the species. The theory helps account 

 not only for occasional major steps in evolution less miraculously than in the other 

 theories but also for the extraordinary perfection of fine details that we find in the pro- 

 ducts of evolution. The essential difference is between a theory in which selection can 

 operate in the virtually infinite field of interaction effects of recombinants instead of 

 almost exclusively on the net effects of each separate gene. 



DISCUSSION 



Dr. Burdette: Thank you, Dr. Wright. The discussion of Dr. Wright's paper 

 will be opened by Dr. Herman Chase. 



Dr. Chase : This is an excellent and exhaustive paper, leaving nothing to be taken 

 away and very little to add. Compared with the guinea-pig work of Dr. Wright, there 

 has been relatively little done with mice in the way of genie interaction. Since this is 

 a symposium supposedly dealing with methodology, I would like to point out what I 

 consider to be the four main approaches or general methods in physiologic genetics. 

 They are: first, genie substitution; second, genie interaction; third, comparison of 

 genetic constitutions; and fourth, teratogens. Simple genie substitution with resulting 

 analysis of the varying phenotype is certainly used the most and is basic in work with 



