The ecology and evolution of elevation range limits in monkeyflowers (Mimulus cardinalis and Mimulus lewisii)

Living organisms inhabit an incredible array of environments across the planet, but any particular species occurs in only a subset of habitats and geographic areas, an observation so fundamental that its cause is rarely questioned. Nevertheless, the ecological and evolutionary forces that give rise to species' distribution limits remain poorly understood. To determine whether species are maladapted to the environment at and beyond the distribution boundary, I investigate how fitness changes across the elevation ranges of closely related species of monkeyflower, Mimulus cardinalis and M. lewisii (Phrymaceae) in the Sierra Nevada Mountains, California. I use transition matrix models to estimate asymptotic population growth rates and find that population growth rates of M. lewisii are highest at the range center and reduced at the range margin. Population growth rates of M. cardinalis are highest at the range margin and greatly reduced at the range center. Because observations of natural populations cannot determine fitness beyond a species' present distribution, I reciprocally transplanted M. cardinalis and M. lewisii within and beyond their present elevation ranges. For both species, I find the greatest average fitness at elevations central within the range, reduced fitness at the range margin, and zero or near-zero fitness when transplanted beyond the present elevation range limit.;To identify the underlying causes for changes in fitness versus elevation, I examine plant performance in growth chambers simulating low and high elevation temperature regimes and show that temperature alone generates patterns of differential survival and growth similar to those observed in reciprocal transplant gardens. Mimulus lewisii and M. cardinalis differ in photosynthetic physiology under temperature regimes characterizing their contrasting low and high elevation range centers, suggesting that the species' elevation range limits may arise, in part, due to metabolic limitations on growth that ultimately decrease survival and limit reproduction. To measure natural selection on physiological and phenological traits within and beyond elevation range limits, I transplanted interspecific hybrids to low and high elevation and find that selection favors early flowering at high elevation and increased leaf photosynthetic capacity in warm temperatures at low elevation. I also find that hybrids selected at high elevation display reduced biomass when grown in temperatures characteristic of low elevation, suggesting that adaptation to the environment within the range may entail a cost to adaptation in other environments that places evolutionary constraints on range expansion.