ANGERT, A. L.; SCHEMSKE, D. W.; Michigan State University: Trade-offs and the evolution of altitude range limits in monkeyflowers

Every species occupies a restricted geographic area. Sometimes geographic distributions stop at obvious barriers such as land � water interfaces, but more frequently, distributions stop at seemingly arbitrary points along continuous environmental gradients. Ecologists often correlate range boundaries with climate to identify environmental determinants of distribution, but such associations present a conundrum because natural selection should increase tolerance to limiting environmental variables and thus allow distributions to expand. To examine evolutionary constraints on distribution, we are studying sister species of monkeyflowers, Mimulus cardinalis and M. lewisii, across their altitude ranges in California. Both species occur in riparian habitats, but M. cardinalis is found from low to mid altitude (0 � 1600 m) whereas M. lewisii occurs from mid to high altitude (1200 � 3100 m). We conducted reciprocal transplant experiments at four altitudes distributed across the ranges of the two species, and find that each species has highest fitness at altitudes central within its range, reduced fitness at its range margin, and extremely low fitness when transplanted beyond its present altitude limit. To measure natural selection across the altitude gradient, we established experimental gardens in which F3 hybrids were exposed to low and high altitude environments. Random tissue samples of F3 individuals that survived to flower were genotyped and compared to samples from an unselected control population to identify genetic loci under selection at each altitude. We have identified one region that is strongly selected towards the M. cardinalis allele at low altitude and towards the M. lewisii allele at high altitude. This pattern of opposing selection suggests that trade-offs between performance within versus beyond ranges constrain the ability to respond to selection for range expansion.