Sni.ECTION AND VARIABILITY 



generation to generation. A fixed proportion of heterozygotcs in 

 the population can be maintained only by means of crossing or 

 outbreeding, and any given amount of outbreeding will maintain a 

 corresponding proportion of heterozygotcs. The breeding system, 

 in determining the proportion of heterozygotcs, detcrniincs the 

 rate of flow of variability between the different states. Random 

 mating gives virtually the maximum outbreeding and the maximum 

 flow, while close inbreeding gives no flow at all. Outbreeding 

 means genetical lability, inbreeding genetical fixity. 



Fitness and Flexibility 



The importance of the difference in effect between inbreeding and 

 crossbreeding becomes clear when we consider what natural 

 selection is doing. If we observe a character in which a population 

 is freely and continuously variable, as for example stature in man, 

 we find that most individuals show it to an intermediate degree and 

 the extreme expressions are rare. As Galton put it, the majority are 

 mediocre. 



The significance of this is shown by Bumpus' observations. He 

 found that a number of characters, bone lengths, wing spread and 

 skull conformation, varied in a sample of sparrows in the same way 

 as stature in man, the mediocre again being the most common. 

 Now these sparrows had been disabled in a rainstorm, but some of 

 them later recovered and flew away. The birds which thus overcame 

 the ill effects of the storm proved to be those which approximated 

 most closely to the form characterized by means of the various 

 measurements. Those that died were the more extreme individuals, 

 regardless of the direction, positive or negative, in which they were 

 extreme. Natural selection was operating in favour of the mean, 

 and against departure, whatever its direction, from the common 

 type. So far as the variation of these sparrows was heritable (and 

 while there is no evidence on this point in regard to the sparrows, 

 it is the general rule derived from a wide variety of organisms that 

 a portion of such continuous variation is polygenically determined), 

 the effect of selection was to favour the more balanced combinations 

 of genes. 



Natural selection has been observed in operation in this way on 



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