. The elements of genetics. Genetics. SELECTION AND VARIAIHIITY classes must be mainly to favour the more balanced homozygotes at the expense of the less balanced. The favoured individuals wiU produce offspring genetically like themselves; and in so far as the environment is stable, the population must show high agreement with the optimum phenotype. Should the environment change. 2 4 6 8 10 12 SIZE OF LITTER Fig. 72.—The average numbers of young surviving for 3 and 6 weeks from litters of various sizes at birth in pigs. The straight "birth" Hue is the line of no loss. The loss become
. The elements of genetics. Genetics. SELECTION AND VARIAIHIITY classes must be mainly to favour the more balanced homozygotes at the expense of the less balanced. The favoured individuals wiU produce offspring genetically like themselves; and in so far as the environment is stable, the population must show high agreement with the optimum phenotype. Should the environment change. 2 4 6 8 10 12 SIZE OF LITTER Fig. 72.—The average numbers of young surviving for 3 and 6 weeks from litters of various sizes at birth in pigs. The straight "birth" Hue is the line of no loss. The loss becomes disproportionately greater as the size of litter increases. The maximum survival to 3 or 6 v^ceks old is given by litters of interniediate size at birth. (Data for 3 weeks (as dots) from Johansson, 1931, and for 6 weeks (as crosses) from Menzies-Kitchen, 1937.) permanently, however (as sooner or later it presumably must) the population camiot change genetically in the way necessary to give a new adjustment: the variability is frozen in the homozygotic potential form. Such a population, therefore, shows high immediate fitness but no flexibility. The inbreeding system, advantageous so long as the environment is stable, becomes a handicap when the environment changes. A population witli an outbreeding system is in the other case. 284. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Darlington, C. D. (Cyril Dean), 1903-; Mather, Kenneth. New York : Macmillan
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