“Is plant fitness proportional to seed set? An experiment and a spatial model”

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Diane R. Campbell, Alison K. Brody, Mary V. Price, Nickolas M. Waser, and George Aldridge

Relative fitness is modified by sib interactions taking place between seed dispersal and juvenile recruitment

Does making twice as many seeds make a plant twice as fit?

Scarlet gilia (Ipomopsis aggregata).
(Credit: Diane R. Campbell)

Ever since Darwin, scientists have studied natural selection on the extraordinary diversity of flower traits seen in nature, often using the number of seeds to estimate fitness, or at least that portion of fitness obtained as a mother. It stands to reason that a plant that makes more seeds would contribute more offspring to the next generation. But is that contribution to the next generation actually proportional to seed production? Little is known about the link between the number of seeds produced by an individual and number of successful offspring, and there are reasons to suspect that seed production might provide a biased estimate of maternal fitness. In a paper in The American Naturalist, Campbell, from University of California, Irvine, and colleagues Brody, Price, Waser, and Aldridge addressed this gap in knowledge. Using a plant species, scarlet gilia, that they have made a “model system” in four decades of work at the Rocky Mountain Biological Laboratory, they asked, “Does a plant that produces twice as many seeds as its neighbor contribute twice the offspring to the next generation?” In field experiments, they used their natural-history knowledge to mimic natural seed dispersal, and genetic markers to follow the fates of seeds from individual parents. Spatial models showed that relative fitness based on seed number is modified, depending on the extent of competition between sibling seeds versus non-sibling seeds, which in turn depends upon the extent of seed dispersal. Plants that produced twice as many seeds as their neighbor produced fewer than twice the number of recruiting juveniles, as expected from the model. Thus, there was a tradeoff between high seed production and offspring survival, making the intensity of selection on a floral trait likely weaker than investigators have previously thought. Competition and other interactions among siblings may have general effects on evolution of plant traits, and may be particularly important for plants with little multiple mating, low adult density, and low seed dispersal. Read the Article