CHROMOSOMAL MUTATIONS 



demonstrated, just as the corpuscular genes have not yet been demon- 

 strated. 



Difficulties of Neo-Darwinism. Goldschmidt believed that the neo- 

 Darwinian type of evolution, by accumulation of micromutations under 

 the influence of natural selection, is largely restricted to subspecific differ- 

 entiation within species, and that the decisive step in the formation of 

 new species must involve an altogether difiFerent genetic process, the sys- 

 temic mutation. Only a few of the reasons which led him to this conclusion 

 can be indicated here, and these only briefly. If neo-Darwinian evolution 

 gives rise to new species, then new species should come only from the 

 terminal members of a Rassenkreis, and the Rassenkreise of closely re- 

 lated species should blend into one another. But actually this does not 

 happen. He believed that good species are always separated from their 

 nearest relatives by a bridgeless gap. Controversial cases he believed de- 

 pend in part upon purely morphological definitions of species which do 

 not take the genetic facts into account. Goldschmidt believed that inter- 

 breeding, or potentially interbreeding, populations should be treated as 

 a single genetic unit, a species, from an evolutionary point of view, even 

 if other factors may make it advisable for taxonomists to break it up into 

 several species. On this basis (which is not acceptable to most geneticists 

 and taxonomists ) , many diflBcult cases can be resolved in accordance with 

 his ideas. 



Another major neo-Darwinian tenet is that isolation of a subspecies is 

 essential if it is to accumulate enough gene difiFerences to become a good 

 species, distinct from the parent species. But there are known instances 

 in which long isolation has failed to produce more than subspecific dif- 

 ferentiation. Thus there is a race of Lymantria dispar (Gypsy moth) 

 which has been isolated on the island of Hokkaido (North Japan) since 

 the early Tertiary period, yet in the intervening 60,000,000 years only sub- 

 specific differentiation has occurred. Again, the seasonal variation within 

 a single race of Papilio ( butterfly ) may be greater than the variation be- 

 tween races at any one time. 



Finally, Goldschmidt believed that the neo-Darwinian theory places 

 too great a burden upon natural selection. The theory demands that only 

 very minor mutants be significant for evolution, and these should be sub- 

 ject to very slight selection pressures. One may define selection pressure 

 in terms of loss of survival value. Thus, if 1000 individuals of genotype AA 

 survive to reproductive age while only 999 of genotype aa survive, the 

 selection pressure against a is said to be 0.001. Haldane calculated the 

 results where a selection pressure of this magnitude is operating in favor 

 of a new gene which is present in a population to the extent of one allele 

 in a million. If the favored gene is dominant, it would require 11,739 

 generations to raise the frequency to two in a million. But if the favored 

 gene is recessive, 321,444 generations would be required! Selection oper- 

 ates more rapidly when the gene frequencies are higher. But it is not self- 

 evident that a process operating as described above could produce the 

 existing species of plants and animals, even given time on a geological 

 scale. And there are many examples like the Lymantria case mentioned 



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