Speciation in Mimulus , or, Can a simple flower color mutant lead to species divergence

The general pattern of speciation in nature has been clear for a long time the divergence of portions of a population, usually small (Levin 1993), usually in geographic isolation (Mayr 1976), and the accumulation of genetic changes by selection and/or genetic drift (Crow and Kimura 1970) that produce reproductive isolation and normally character divergence as well. The critical step is reproductive isolation, and yet that step except for polyploid formation which in itself is not always effective (DeWet 1980) has rarely been observed actually happening in nature. A promising group in which to study speciation events in progress is section Erythranthe of the genus Mimulus (Vickery 1978). The six species of monkey flowers comprising section Erythranthe are moisture-requiring, herbaceous perennials 1-10 dm in height, with variously shaped, opposite leaves and bilabiate flowers that have four stamens, one style with a bilobed sensitive stigma, and five corolla lobes that range in color from orange to red rarely yellow and from lavender-pink to magenta-pink rarely white. See Grant (1924) for further details. When considered species by species, corollas of M. cardinalis Douglas vary from orange to red rarely yellow and are sharply and fully reflexed, hummingbirdpollinated flowers. Corollas of M. verbenaceus Greene are partially reflexed; that is, the upper two corolla lobes are reflexed, whereas the lower three are gently recurved. Flowers are orange -red to red rarely yellow and also are hummingbird-pollinated. Corollas of M. nelsonii Grant are partially re-flexed also and have orange-red to red flowers, which are longer than those of M. verbenaceus (6-7 cm versus 4-5 cm). Corollas of M. eastwoodiae Rydberg and M. rupestris Greene, the two cliffdwelling species, are partially reflexed, red, and typically hummingbird-pollinated also. And last, flowers of the Rocky Mountain variety of M. lewisii Pursh are magenta-pink with all five corolla lobes gently recurved rather than reflexed, thus forming a bee-landing platform; flowers of the Sierra Nevada variety of M. lewisii are lavender-pink rarely white with corolla lobes thrust forward. Both varieties of M. lewisii are bee-pollinated. Mimulus lewisii flowers and those of M. eastwoodiae and M. rupestris produce only modest amounts of nectar, whereas the other species produce abundant nectar (Table 1). Thus, the species differ markedly in flower shape, flower color, nectar production, and, consequently, in pollinators servicing the flowers. In the formation of the six species, evolution appears to have responded to selection imposed by pollinator preferences and ecological opportunities. The result is that members of the complex have radiated into a wide variety of different habitats and niches. A bright yellow-flowered mutant has appeared on the scene in this setting of pollinator-driven, ecologically opportunistic evolution. In two populations of M. cardinalis bright yellow-flowered morphs have become well established. One population is in the Siskiyou Mountains of Oregon, which is the northern limit of the range of M. cardinalis (Grant 1924). The other population is on Cedros Island, Baja California, and is at the southern limit of the species range. As Mayr (1976) suggests, new forms often evolve from isolated populations such as these on the periphery of a species