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Evidence for Natural Selection on Mating System in Mimulus (Scrophulariaceae)

Charles B. Fenster and Kermit Ritland
International Journal of Plant Sciences
Vol. 155, No. 5 (Sep., 1994), pp. 588-596
Stable URL: http://www.jstor.org/stable/2475032
Page Count: 9
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Evidence for Natural Selection on Mating System in Mimulus (Scrophulariaceae)
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Abstract

We used two approaches to examine the role of natural selection in the evolution of selfing in Mimulus. First, using cpDNA and isozyme variation we investigated the phylogenetic relationships among a number of populations of two selfing (M. micranthus and M laciniatus) and two outcrossing (M guttatus and M. nasutus) species to determine the frequency of evolution of selfing within the genus. We hypothesized that the independent evolution of selfing taxa from outcrossing taxa would be strong evidence that the mating system is under the influence of natural selection. Second, we used phenotypic selection analysis to measure the presence and intensity of selection on floral characters associated with the mating system in extant populations of the outcrossing species M. guttatus. An unrooted Wagner tree constructed from cpDNA variation and a dendrogram constructed from Nei's genetic distances at isozyme loci both indicated the independent origin of the two selfing taxa, M. micranthus and M. laciniatus, from different ancestral M. guttatus populations. Phenotypic selection analysis demonstrated that both direct and indirect directional selection intensities were variable between populations but could be high. Decreased stigmaanther separation was favored in the one population that was likely pollinator-limited. Our results indicated that natural selection was strong enough to cause the repeated evolution of selfing in Mimulus and that the selective agent may be inadequate pollinator service.

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