Natural intersectional hybridization between North American species of Populus (Salicaceae) in sections Aigeiros and Tacamahaca. III. Paleobotany and evolution

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Abstract
Fossilized leaves resembling those of contemporary intersectional hybrids occur together with leaves assignable to sections Tacamahaca Spach and Aigeiros Duby in Miocene and Pliocene sediments in western North America. They are not referable to any particular extant hybrid species and are assigned to the extinct Populus × parcedentata Axelrod. Together with other evidence, these ancient hybrids raise questions concerning the evolutionary role of hybridization between species of the two parent sections. Present evidence about hybridization as a bridge for intersectional gene flow is contradictory. The apparent absence of backcrossed individuals in most studied hybridizing populations is offset by morphological pecularitics shared by sympatric cottonwoods and balsam poplars that are not shared with their cladistic sister species.

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