Effects of Competition and Temporal Variation on the Evolutionary Potential of Two Native Bunchgrass Species

Ecological release exposes genetically based niche variation

The evolutionary trajectories of ecological niches have profound impacts on community, population and speciation dynamics, yet the underlying causes of niche lability vs. stasis are poorly understood. Here, we conducted a field experiment to quantify the effects of competition and, conversely, competitive release on the microevolutionary processes driving microhabitat niche evolution in an annual plant population restricted to California vernal pool wetlands. Removing competitors generated a strong increase in mean fitness, the exposure of genetically based niche variation and directional selection for niche evolution in the experimental population. In contrast, genetic variation in the microhabitat niche and directional selection for niche evolution were not detected in individuals growing with competitors. These results indicate that ecological opportunity (here, the removal of competitors) can trigger the immediate expression of latent, heritable niche variation that is necessary for rapid evolutionary responses; conversely, competitors may restrict niche evolution, contributing to niche conservatism in saturated communities.

Genetic variation among mainland and island populations of a native perennial grass used in restoration

Genetic marker studies can assist restoration practice through selection of seed sources that conserve historical levels of gene diversity and population genetic differentiation. We examined genetic variation and structure within and among mainland and island populations of Elymus glaucus, a perennial bunchgrass species native to western North American grasslands that is targeted for grassland restoration. Island populations of E. glaucus represent sensitive sites and potentially distinctive seed sources for reintroduction, and little is known of their genetic composition. Genetic diversity and structure were estimated using amplified fragment length polymorphism markers for 21 populations and 416 individuals distributed across two coastal California mainland locations and three California Channel Islands. Eight primer combinations resulted in 166 markers, of which 165 (99.4 %) were polymorphic. The number of polymorphic bands was significantly greater among mainland populations relative to island sites, and locally common alleles were present for each sampled island and mainland location. Population structure was high (62.9 %), with most variation (55.8 %) distributed among populations, 7.1 % between mainland and island locations, and the remainder (37.1 %) within populations. Isolation by distance was only apparent among islands. Using marker data to recommend appropriate seed sources for restoration, E. glaucus seeds are best derived within islands with collections representing a large number of individuals from matching environments. Given the limited gene flow and prior evidence of adaptive divergence among populations of this species, regional collections are recommended in all cases to maintain diversity and to avoid long-distance introductions of highly differentiated plant material.

Fitness variation and local distribution limits in an annual plant population

Understanding how genetic variation shapes species' distributions involves examining how variation is distributed across a species' range as well as how it responds to underlying environmental heterogeneity. We examined patterns of fitness variation across the local distribution of an annual composite (Lasthenia fremontii) spanning a small-scale inundation gradient in a California vernal pool wetland. Using seeds collected from the center and edge of a population, paternal half-sib families were generated and transplanted back to the center and edge of the original population. All transplants were adapted to the conditions at the center of the population. The effect of the environment on the opportunity for selection depended on the model of selection assumed. Under a model of hard selection, variance in absolute fitness was lower among transplants at the edge of the population than at the center. Under a model of soft selection, the variance in relative fitness was similar between center and edge microhabitats. Given that this population is likely well-mixed, differences in habitat quality between center and edge microhabitats will likely cause selection at the center of the population to dominate the evolutionary trajectory of this population.