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Trends in Geographical Variation in Palaerctic Members of the Genus Parus

D. W. Snow
Evolution
Vol. 8, No. 1 (Mar., 1954), pp. 19-28
DOI: 10.2307/2405662
Stable URL: http://www.jstor.org/stable/2405662
Page Count: 10
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Trends in Geographical Variation in Palaerctic Members of the Genus Parus
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Abstract

1. In the genus Parus in the Palaearctic Region geographical variation is closely related to climate. Bergmann's rule is general, but it is modified to a greater or lesser extent by a 'latitude effect,' size being greater at lower than at higher latitudes for equivalent winter temperatures. In nearly all the species, relative tail-length increases and the relative lengths of beak and tarsus decrease in the colder parts of the range (Allen's rule). In continuous populations colour variation follows differences in temperature and humidity. 2. The rules for variation in colour and tail-length seem to carry with them no concomitant disadvantages, but Bergmann's and Allen's rules may at times conflict with other ecological requirements. 3. Until the geographical variation of a whole genus has been analysed with respect to climate, subspecific differences cannot be safely interpreted as adaptations to habitat or to the presence or absence of competitors, nor can they be used as clues to past history.

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