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Metachromism or the Principle of Evolutionary Change in Mammalian Tegumentary Colors

Philip Hershkovitz
Evolution
Vol. 22, No. 3 (Sep., 1968), pp. 556-575
DOI: 10.2307/2406880
Stable URL: http://www.jstor.org/stable/2406880
Page Count: 20
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Metachromism or the Principle of Evolutionary Change in Mammalian Tegumentary Colors
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Abstract

Metachromism is the principle of saturation, bleaching (or reduction), and elimination of integumental pigments along similar and irreversible pathways, in all mammals, irrespective of the environment. Hair or coat color advances from the primitive and concealing agouti pattern to either blackish or reddish through the saturation process. It then bleaches along the eumelanin pathway (brown-drab-gray) to white, or the pheomelanin pathway (reddish-orange-yellow-cream) to white. There is limited interchangeability between pathways, and there are many tones in the chromatic scale between each color grade. Some or all the tones between saturate blackish or reddish and white may be skipped in the bleaching process. Metachromic processes are stimulated or accelerated by non-selective inbreeding within a small and newly isolated founder colony. As a rule, the more distant the colony of a chain of founding colonies or the longer its reproductive isolation from the primitive or parental race or population, the greater the dilution or loss of its tegumentary pigmentation. Multiplication and spread of the colony entails establishment of a dynamic equilibrium between metachromic pressures and natural selective forces. Consummation of the degenerative processes of bleaching, depigmentation or depilation results in extinction unless the population occupies or evolves into a niche where color or pelage have no survival value. Marmosets and tamarins, small South American primates of the family Callithricidae, serve as models for illustrating the metachromic processes of saturation and bleaching of pelage, and depigmentation of skin. The same media are used for illustrating the related evolutionary processes of hypertrichy and depilation. Metachromism in marmosets is then compared with the same phenomenon in mice inhabiting the white sand dunes and the black lava beds of the Tularosa Basin of New Mexico. Social selection based on imprinting appears to be the controlling factor in stabilizing metachromic processes in the marmosets, while predator selection for concealing coloration operates on the desert mice.

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