How do we know that interactions between plant species affect their evolution? While there is abundant evidence to suggest that plant-pollinator interactions influence the evolution of floral traits, there is little direct evidence that interactions between plant species shape the evolution of such characteristics.
Evolutionary biologists Robin Smith and Mark Rausher of Duke University recently examined this question in a study of two morning glory species that commonly co-occur in the southeastern United States.
By measuring patterns of selection on floral traits of the focal species (the ivyleaf morning glory) in the presence and absence of flowers of a second species (the tall morning glory), they found direct experimental evidence that reproductive interference between closely related plant species can alter patterns of selection on floral traits that influence the mating system and contribute to reproductive isolation.
In the summer of 2004, Robin Smith and Mark Rausher conducted a field experiment in which they planted over 1,350 morning glories (~650 individuals of each species) checkerboard fashion in an abandoned agricultural field in central North Carolina. Using hand-sewn bags made of bridal veil mesh to cover the plants and control whether pollinators could transfer pollen between the two species, their aim was to determine whether reproductive interference from the tall morning glory alters the pattern of selection on floral traits of the ivyleaf morning glory.
"The advantage of this experimental design is that it allowed us to manipulate only one variable--the presence vs. absence of flowers of the second species--while keeping all other environmental conditions constant," says Robin Smith, the study's lead author.
They found that selection acts to increase clustering of anthers and stigma (the parts of a flower that send and receive pollen) when flowers of the second species are present. "Since clustering of anthers and stigma is known to promote self-fertilization, this suggests that one way for plants to avoid pollen deposition from the wrong species is to increase the rate of self-pollination," says Robin Smith.
"In a broader sense, however, these results are important because they provide direct evidence for what ecological and evolutionary theory has long predicted, which is that species interactions can influence selection on morphological traits and drive phenotypic divergence."
This research was published in the January issue of the American Naturalist.
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