466 INTRODUCTION TO EVOLUTION 



(homozygous "normals"). The small proportion of sickle-cell anemia 

 patients produced in each generation is a price the population pays for 

 increased ability to survive the ravages of malaria. 



Genetic Homeostasis 



Closely linked to heterozygote superiority is the concept of genetic 

 homeostasis. While this term has unfortunately been used in more than 

 one sense (Waddington, 1953), the usage pertinent to our discussion is as 

 a name for the self-regulating ability of a population which enables it to 

 survive environmental change or diversity. The population is said to be 

 "buffered" against change. The idea is analogous to that of the physiologi- 

 cal homeostasis of the human body. If one kidney is destroyed or 

 removed, for example, the other kidney will adjust and compensate for 

 the lost organ, and the body will continue to function normally. This 

 ability to adjust to change and so to continue normal life is homeostasis. 

 The idea of genetic homeostasis is that populations possess a capability of 

 self-regulation so that they can continue normal existence under a variety 

 of environmental conditions and so survive environmental vicissitudes 

 which would destroy populations less capable of "rolling with the punch." 



We may expect that natural selection will favor the development and 

 maintenance by a population of such homeostasis. We note that genetic 

 homeostasis has much in common with the Baldwin effect (pp. 420-425) 

 and with the canalization of embryonic development (pp. 421-422). All 

 three refer to self-regulating properties permitting normal existence in 

 varied environments. Natural selection favoring the development of such 

 properties has been termed stabilizing selection (Schmalhausen, 1949), 

 or canalizing selection (Waddington, 1953). 



Evidence is accumulating that populations heterozygous for many gene 

 pairs are better buffered against change than are populations composed 

 of homozygotes. This evidence comes both from experiments with such 

 laboratory forms as Drosophila (see Beardmore, Dobzhansky, and 

 Pavlovsky, 1960) and from experiments with domestic animals (largely 

 summarized in Lerner, 1954). Apparently both wild populations and 

 successful breeds of domestic animals have a high degree of hetero- 

 zygosity for genes which are deleterious when homozygous. If attempts 

 are made to rid the population of these genes the population becomes less 

 viable, or fertile, or able to withstand environmental change. Lerner, for 

 example, described a series of experiments with a hereditary abnormality 

 of chickens known as "crooked toes." The genes for this are probably 



