Evolutionists have long recognized the role of reproductive isolation in speciation, but the relative contributions of different reproductive
barriers are poorly understood. We examined the nature of isolation
between Mimulus lewisii and M. cardinalis, sister species of
monkeyflowers. Studied reproductive barriers include: ecogeographic
isolation; pollinator isolation (pollinator fidelity in a natural mixed
population); pollen competition (seed set and hybrid production from
experimental interspecific, intraspecific, and mixed pollinations in
the greenhouse); and relative hybrid fitness (germination,
survivorship, percent flowering, biomass, pollen viability, and seed
mass in the greenhouse). Additionally, the rate of hybridization in
nature was estimated from seed collections in a sympatric population.
We found substantial reproductive barriers at multiple stages in the
life history of M. lewisii and M. cardinalis. Using range maps
constructed from herbarium collections, we estimated that the different
ecogeographic distributions of the species result in 58.7\%
reproductive isolation. Mimulus lewisii and M. cardinalis are visited
by different pollinators, and in a region of sympatry 97.6\% of
pollinator foraging bouts were specific to one species or the other. In
the greenhouse, interspecific pollinations generated nearly 50\% fewer
seeds than intraspecific controls. Mixed pollinations of M. cardinalis
flowers yielded >75\% parentals even when only one-quarter of the
pollen treatment consisted of M. cardinalis pollen. In contrast, both
species had similar siting success on M. lewisii flowers. The observed
99.915\% occurrence of parental M. lewisii and M. cardinalis in seeds
collected from a sympatric population is nearly identical to that
expected, based upon our field observations of pollinator behavior and
our laboratory experiments of pollen competition. F, hybrids exhibited
reduced germination rates, high survivorship and reproduction, and low
pollen and ovule fertility. In aggregate, the studied reproductive
barriers prevent, on average, 99.87\% of gene flow, with most
reproductive isolation occurring prior to hybrid formation. Our results
suggest that ecological factors resulting from adaptive divergence are
the primary isolating barriers in this system. Additional studies of
taxa at varying degrees of evolutionary divergence are needed to
identify the relative importance of pre- and postzygotic isolating
mechanisms in speciation.