THE MATING CONTINUUM 



way, each individual will pass on some of its genes to, on the average, 

 two genetically distinct offspring and so may be regarded as initiating 

 a number of lines of descent. The lines of descent, of which any 

 individual represents either the culmination or the initiation, are 

 obviously not independent of those which are represented by other 

 individuals of the same generation. Sooner or later, if we trace the 

 ancestry of the members of a population at one time, or if wc trace 

 their descendants, we shall find the lines merging. In this sense an 

 outbreeding group represents a mating continuum: a continuum 

 within which any member may have received genetic materials 

 from over the whole area which the group occupies. Looked at in 

 another way, from the standpoint of the distribution of the group, 

 the various sub-groups into which it may be divided spatially, or 

 perhaps in other ways, can be regarded as exchanging chromosomes 

 or exchanging genes. The individuals in one sub-group will have 

 lines of ancestry tracing to other sub-groups, and will in turn leave 

 lines of descent in these others; though the number of lines joining 

 sub-groups will be less than those staying within any one of them. 



In so far as the members of a continuum cover an area of territory, 

 they must be occupying a variety of ecological slots or niches, 

 distinguished by altitude, nutrition, moisture and so on. Each 

 individual, or each local sub-group of individuals, must be under 

 the action of selective forces tending to adjust its genotype to the 

 local environment (Fig. 76); and to the extent that these adjust- 

 ments are to different environments, exchange of genes between 

 the local sub-groups must lead to maladjustment. A mating 

 continuum, therefore, is genetically flexible, but it does not permit 

 the best local adaptation where differences occur within the common 

 environment. 



Local maladjustments of an outbreeding species in a differentiated 

 environment will always tend to increase, for the relations between 

 environment and genotype are not constant. No environment can 

 be stable in the long run, and almost any change must be for the 

 worse in respect of an existing organism. Furthermore the environ- 

 ment will not only be worsening for each individual: it will be 

 worsening in different ways for individuals in different ecological 

 circumstances. The need for local adjustment will not only be 

 growing : it will be growing in different directions in different parts 



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