EVOLUTIONARY MECHANISMS 293 



the advantageous allelomorph has occurred, the whole stock of organ- 

 isms bearing the allelomorph is wiped out, and the general spreading 

 of the gene has to wait till its next occurrence. The theory of this 

 random extinction has been worked out by Fisher and Wright.^ 



Wright's investigations, in fact, have led him to attach considerable 

 importance to chance survival as a principle in evolution. His method 

 is to study the frequencies, in a population, of different gene ratios 



f - or M above j. For instance, if seleaion is very severe, we tend to 



find the whole population homozygous and imiform, i.e. all the gene 

 ratios are either 00 or 0, The rate of evolution is the rate at which 

 genes become "fixed," i.e. uniform, throughout the population. Wright 

 considers mutation and migration as well as selection and random 

 survival as factors in evolution. 



He concludes that the rate of evolution is very dependent on the size 

 of the population. In very small populations it is slow because of the 

 small amount of variation available for selection to work on, and what 

 evolution there is is mainly due to random survival. In large popula- 

 tions, again, nearly all genes are present in fixed ratios depending on the 

 equilibrium between mutation and selection; the only evolution is of 

 the Haldane type depending on the occurrence of new favourable 

 mutations. An intermediate population is not so small that evolution 

 is held up by random extinction of useful genes, but is small enough 

 for the death-rate to produce a random fluctuation of gene ratios round 

 their equilibrium values. 



Wright points out that in assessing the fitness of a population we 

 must consider all the genes simultaneously. For instance, in a popula- 

 tion containing mainly the allelomorphs A and b, it might be useful to 

 change A to a, but only if at the same time B is substituted for b. 

 Wright discusses these changes in terms of probability surface rather 

 Uke the "valley model" of development which was proposed in Part 2. 

 Here each point represents a population of a certain genetic constitu- 

 tion; if we are talking in terms of a three-dimensional model, each 

 point really only pictures the proportions of the two genes, while the 

 third dimension represents fitness. In Wright's model, the fittest 

 populations He at the top of hills, and selection will continually be 

 trying to keep the population there, unless a new mutation occurs 

 which makes some new constitution become fitter; this would be 

 equivalent to raising a new hill nearby, and the population would 

 ' Wright 1931, 1932, 1935. 



