Sni.rCTION AND VARIAIUIITY 



tlirough their effects on Darwinian fitness, the competitive power 

 of the individuals comprising the species. How will inbred and 

 outbred populations differ in the genotypes and phenotypes of their 

 individuals? 



To answer this question we must look at the implications of 

 elementary Mendclism, not now for experimental families, but for 

 massed populations. The static must be translated into the dynamic. 

 And in doing this, to understand crossbreeding and inbreeding, we 

 shall fmd that we come to understand the organization of 

 heredity in the wider sense of the interdependence of the relations 

 between the genes in the nucleus and between the individuals in 

 the population. 



The States of Variability 



Homozygotes and heterozygotes always differ in one respect, the 

 respect by which they are recognized, i>iz. that heterozygotes give 

 segregation in their progeny while homozygotes do not. If we 

 breed a homozygote, or a group of like homozygotes, the offspring 

 will be genetically and, apart from non-heritable fluctuations, 

 phenotypically identical both with their parents and with one 

 another. If, however, we breed heterozygotes (whether for one 

 gene or for many), the offspring arc neither all alike nor all like 

 their parents, genetically or somatically. Variation has appeared as 

 a result of segregation. Now the germ of tliis variation must have 

 existed in the heterozygotes, even though they themselves showed 

 no variation. Thus we may distinguish between the latent or 

 potential variability which heterozygotes themselves contain, and the 

 variation or free variability which wiU make itself apparent in the 

 phenotypes of their offspring when segregation has occurred. 



Let us consider a simple theoretical situation on a mendelian 

 basis and, in the first place, in the absence of selection. A continued 

 programme of inbreeding, by self-pollination or the mating of 

 close relatives, when applied to the offspring of heterozygotes, will 

 eventually result in the establishment of a population of homo- 

 zygotes. Half of these will carry one (AA) and half the other {aa) of 

 the two allelomorphs o(^ any gene for which the original ancestors 

 were heterozygotes (Aa). The variability due to such a gene wiU 

 then have changed from being entirely potential in the heterozygous 



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