Adaptation and Natural Selection

Selection favorable to hetero-zygote

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On page 60, the famous example of sickle-cell anemia appears. This disease is governed by the type of hemoglobin; if it is the sickle-cell hemoglobin, the holder cannot live long, because of the anemia caused by that type. This is due to the homo-zygote A'A' of a certain gene A'. However, the corresponding hetero-zygote AA' is not lethal, and it produces resistance to malaria, and hence, in a certain region, its holder can be stronger than the holder of the homo-zygote AA of the normal gene. This situation can be represented as follows, in terms of the frequency (p for A and q for A') and the selection coefficient of each genotype (for the blood-cell):

geonotype	AA	AA'	A'A'
frequency	pp	2pq	qq
fitness	1-s	1	1-t

From this table, we can calculate the rate of the change of gene frequencies in one generation (since s and t are not zero, the frequencies in the next generation change). In the long run, the best proportion (equilibrium) for each gene obtains. For more of this, see Futuyma (1991), ch. 6.

Reference

Futuyma, D. J. (1991) 『進化生物学』（原書第二版）、岸由二他訳、蒼樹書房。

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