A Reevaluation of the Derivation of Quercus margaretta from Quercus gambelii

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Abstract
An earlier hypothesis deriving Q. margaretta, of the Gulf Coast, from Q. gambelii, of the Rocky Mountains, via an extensive eastward migration of the latter at the close of the Tertiary and during the Pleistocene, is shown to be untenable. Detailed study reveals a number of morphological differences between these oaks, the most distinctive of which argue against a close phylogenetic relationship. A comparison of biotic associates reveals no clear evidence that any of the principal plant or animal associates of Q. gambelii (including its gall wasp parasites of the genus Cynips) made such an eastward migration. This would cast doubt on the postulated migration of Q. gambelii itself, since any climatic change which permitted the latter to migrate might be expected to have permitted similar migrations by associated species with similar ranges of tolerance for climatic factors. The absence of these associates (or closely related derivatives) in the range of Q. margaretta today suggests that the derivation of these oaks from a common stock was sufficiently remote in geologic time to have allowed elimination of virtually all close common associates. It is suggested that they may have diverged from a common ancestor of northern Mexico during early to middle Tertiary time.

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