| phylogenetic and molecular evidence for allochronic speciation in gall-forming aphids (pemphigus). | sympatric populations can diverge when variation in phenology or life cycle causes them to mate at distinctly different times. we report patterns consistent with this process (allochronic speciation) in north american gall-forming aphids, in the absence of a host or habitat shift. pemphigus populi-transversus riley and p. obesinymphae aoki form a monophyletic clade within the north american pemphigus group. they are sympatric on the eastern cottonwood, populus deltoides (salicaceae), but have di ... | 2004 | 15119438 |