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Species Concepts and Reproductive Isolating Barriers

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Floral Isolation

Floral isolation (pollinator isolation) occurs when gene flow between flowering plant species is reduced due to mechanical and ethological differences in pollination systems. Mechanical isolation occurs when the transfer of pollen between individuals of different species is impossible because of different flower structures. Ethological isolation occurs when pollinators preferentially visit one type of flower at the exclusion others, even if they exist in the same geographical location.

Two species of columbine provide an example of floral isolation. The two species, western columbine (Aquilegia formosa) and hairy columbine (Aquilegia pubescens) have different flower structures and different pollinators: western columbine is pollinated by hummingbirds, and the hairy columbine is primarily pollinated by hawkmoths. The two species have morphologies that have evolved to suit a particular kind of pollination. The western columbine has long, nectar-containing tubes that only can be reached by the beak of a hovering hummingbird. The hairy columbine provides a landing platform, and is fragrant in the evening to attract its moth pollinators. The two species also are ecologically isolated: the hairy columbine is generally found at higher elevations than the western columbine.

Floral isolation is a premating isolating barrier as well as a prezygotic isolating barrier.