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In six bird species convenient for study, niches are known to be broader on some islands than on the mainland. In every case there is also greater variation in a bill measurement on the islands, except for one species in which the niche on the Canary Islands is narrower than that on the mainland. The adjusted variances average about twice as great in the broader niche. These and other results suggest that continuous variation within local populations is often adaptive in itself and is not part of the genetic or phenotypic load.
Leigh Van Valen (Wed,) studied this question.
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