Posted: Jun 25, 2010 8:47 pm
by hotshoe
jaredennisclark wrote:There are also well documented examples of laboratory recreated Macro evolution in Monkey Flowers (Mimulus). In this example, two species of flowers known to have shared a common ancestor both reproduce via pollination by insects animal pollinators. One species using Bees, the other Hummingbirds. These two distantly related species show some striking differences in flower shape, flower color, sexual organs, etc. all having co-evolved with their respected insect pollinators. From what I understand Schemske and Bradshaw used some modern molecular methods to to recreate this evolution in a controlled laboratory experiment. (Schemske and Bradshaw 1999; Bradshaw and Schemske 2003).

Sorry, not being pedantic; hummingbirds aren't insects - which is exactly why Mimulus is significant in having evolved two completely different pollinator-attracting strategies within such closely related species of flowers.

Your point about the power of evolution is supported by the fact that the species changes in Mimulus must be recent (recent, biologically speaking, although not as in recent human history) since they are native to the very young geological area of the Sierras near Yosemite. So, evolution can produce species-level change within a few thousand or tens of thousands of years. Wouldn't we reasonably calculate that it could produce ten times as much change in 100,000 years ? And 100 times as much change in 1,000,000 years ? Perhaps more importantly, the Mimulus studies provide experimental support that not all evolutionary change has to be imagined as a slow accumulation of minute effects, but can occur by large effect of a single gene change (or the large effects of a few gene changes).

Following up on this, I found their 1999 paper available in full, free:
Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus)

Douglas W. Schemske and H. D. Bradshaw, Jr. wrote:... Two experiments are required to elucidate the genetic architecture of reproductive isolation by pollinator-mediated selection. First, the genetic basis of traits such as flower color, size, shape, and nectar reward must be determined for plant species with different pollinators. Second, the response of wild pollinators to each floral trait must be evaluated in a geographic region where the plant species co-occur. We have completed the first experiment, using linkage mapping with molecular markers to identify quantitative trait loci (QTL) that control complex floral traits in M. lewisii and M. cardinalis. We found that most floral traits had at least one QTL of large effect (explaining >25% of the F2 phenotypic variance), suggesting that pollinator-mediated selection in this system could involve “major” genes (21, 22). Here, we report results from the second experiment, identifying the ecological significance of floral traits and the effect of simple genetic changes on pollinator visitation in nature.

... We tested the hypothesis that adaptation to different pollinators may involve genes with large phenotypic effects by comparing visitation rates as a function of QTL marker genotype for petal carotenoid concentration and nectar volume, the two traits with the greatest impact on bee and hummingbird visitation, respectively (Fig. 2 B). A single Mendelian locus controls the distribution of carotenoid pigments in the petals (20). F2 plants homozygous for the recessive M. cardinalis allele at the yup locus (yellow upper; ref. 20) have carotenoids distributed throughout the petals, and are orange- or red-flowered (Fig. 1 D, E, K, and L), whereas F2s carrying the dominant M. lewisii allele are pink-flowered (Fig. 1 F–J). There was no effect of yup genotype on hummingbird visitation rate (Fig. 3 A), but bee visitation was 80% lower in plants homozygous for the M. cardinalis allele (Fig. 3 B). This clearly shows that genetic variation for petal carotenoid concentration affects bee visitation and supports earlier findings that bees visiting Mimulus species in the section Erythranthe strongly prefer pink over red flowers (26).

... Taken together, our results provide evidence of striking differences in the floral preferences of bees and hummingbirds, and considerable opportunity for the adaptive divergence of floral traits through pollinator-mediated selection. This stands in contrast to recent suggestions that pollinators typically have broad preferences, and are therefore unlikely to contribute to floral evolution or the reproductive isolation of sympatric taxa (25, 34, 35). Floral traits associated with bumblebee and hummingbird pollination, such as petal carotenoid pigments and nectar volume, appear to be under relatively simple genetic control, with major QTLs responsible for pollinator discrimination and reproductive isolation in nature. This work contributes to the growing body of evidence that adaptation may often involve genes of large effect (3, 5, 36–39).


Science - it works !