Hawaiian honeycreepers (birds), consisting of 50 species, some of which are now extinct, represent a much more diverse lineage than Hawaiian thrushes. Honeycreepers have a remarkable variation in bill morphology, whereas Hawaiian thrushes exhibit far less morphological variation. Bill morphology is strongly linked to the resources available for these birds to eat. This suggests that _______ in honeycreepers may have occurred as a result of _______.
a) sexual dimorphism:leksb) allopatric speciation;hybrid zonesc) sexual selection; monogamous pair bondsd) adaptive radiation;dietary specializatione) adaptive radiation; poor dispersal ability