Ecological speciation is driven by divergent natural selection. I discuss tests of ecological speciation mechanisms using the plants Ipomopsis aggregata and I. tenuituba. Reproductive isolation is incomplete with little hybrid breakdown in fitness, but hybrid fitness depends on the environment and the parent contributing cytoplasmic genes. High hybrid fitness in dry areas outside of the parental habitats is explained by the ability of hybrids to increase water-use efficiency and flower production under experimentally imposed drought. Some flower traits are under divergent selection by hummingbird and hawkmoth pollinators. Differences in hawkmoth behavior in response to flower scent can explain the geographical variation in reproductive isolation. Pollinator-mediated and habitat-dependent selection together contribute to ecological speciation.