Every species occupies a restricted geographic distribution, but it is unclear why natural selection at the range margin fails to increase
tolerance to limiting environmental variables and thereby allow
continual range expansion. Models indicate that the interplay of
demographic asymmetries, dispersal, divergent natural selection, and
adaptive trade-offs across spatially varying environments can give rise
to stable range limits. Here we examine sister species of the
monkeyflowers Mimulus cardinalis and M. lewisii to identify traits that
might contribute to the evolution of the species' ranges and to ask
whether adaptive trade-offs between environments can limit their
geographic distribution. In the Sierra Nevada Mountains of California,
M. cardinalis is found from low to mid elevation and M. lewisii is
found from mid to high elevation. We transplanted segregating
populations of interspecific hybrids to low and high elevation and
cross-pollinated those that survived to flowering to create selected
populations that evolved at low or high elevation. When grown in a
common environment, the progeny of hybrids selected at high elevation
flowered earlier compared to a greenhouse control population, whereas
hybrids selected at low elevation displayed increased warm-temperature
photosynthetic capacity. If adaptation to one environment entails a
cost to adaptation in other environments, then selected hybrid
populations should display reduced fitness, relative to an unselected
control population, when grown in an environment in which they were not
selected. Two such trade-offs were observed in this study, where
hybrids selected at high elevation displayed reduced biomass when grown
in temperatures characteristic of low elevation and hybrids selected at
low elevation showed reduced resistance to freezing. These results
identify traits under selection for range expansion and suggest that
adaptive trade-offs can contribute to limiting the geographic
distribution of species.