The Adaptation of Flowering Plants to Nocturnal and Crepuscular Pollinators

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Abstract
The evolutionary significance of the adaptation of flowers to distinct classes of pollinators is considered and an attempt is made to assess the frequency with which the mutual adaptations of flowers and particular night-flying flower-visitors have become so close as to provide reproductive isolation for the plants from those adapted to other visitors. Probably the obligate relationship shown by Yuccas represents a nearly unique situation which may have reduced rather than promoted rates of speciation in both the Yuccas and the Yucca moths to which they are adapted. A description is given to the general characteristics of flowers which are adapted to pollination by evening, night, and early morning visitors (including birds). The flowers of many tropical and subtropical trees are pollinated by bats, and two flower-types are discussed in this context. The restriction of bat pollination to the tropics and adjacent subtropics appears to result from a lack of suitable plant material in temperature regions. Adaptations to combinations of flower visitors are considered in relation to risks of failure in seed-production. Such "bridging" may also have considerable significance in the evolution of adaptation to new pollinators. Finally, it is concluded that, as yet, there is no evidence that adaptation to any crepuscular or nocturnal pollinators is sufficiently complete to initiate speciation, although the partial reproductive isolation which it provides may complement the effects of other more primary factors in the maintenance of specific distinctions.

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