Mendelism and evolution / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
1931
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Mendelism and evolution / by E.B. Ford. Source: Wellcome Collection.

    99/144 page 83
    EVOLUTIONARY PROBLEMS 83 izes polymorphic species is further indicated by the fact that it may sometimes persist for long periods of time. Thus the banding of certain land-snails is inherited on simple Mendelian lines. Diver (1929) showed that the proportion of banded and unhanded forms in living colonies does not differ from those of Pleistocene age. It is evident, however, that where stability depends upon ecological factors, the proportions of the different forms may vary widely in different localities. I am much indebted to Mr. C. S. Elton for supplying me with the information on foxes which ülustrates this and other points. The Arctic Fox {Vulpes lag opus) is subject to a winter polymorphism, the individuals being either ' blue ' or white. In Kamchatka only the white type is found, and on certain islands off the coast of Alaska only the blue ; while on the mainland of Alaska a north-to-south gradient in the proportion of the two occurs, the blue being commoner in the south. Stability may further be upset, or rather it may have a slightly different meaning, if the species concerned is one whose numbers are given to fluctuation. For the whole ecological situation may be different when the population is at its maximum and at its minimum. Elton has studied the fluctuations which occur in the Red Fox [Vulpes fulva) of North America. He ñnds that it is subject in some places to a regular ten-year cycle in numbers, with smaller irregular changes. Systematists have divided the Red Fox into several species, but many are characterized by a polymorphism involving the same types : red, ' cross', and silver. The first two are the common forms, though on Prince Edward Island the ' cross ' type does not occur. Silver foxes are rarer, and Elton has found that in a given locality the proportion of this form varied over a com-
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