bild 3 lecture 11

bild 3 lecture 11 - The genetics of speciation As we learn...

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The genetics of speciation As we learn more about genes, it is possible to follow the genetic changes that are important in the speciation process. One remarkable example is found in two species of flowering plant that live in the Sierras. The ranges of Mimulus lewisii and M. cardinalis overlap in the Sierra Nevada mountains of California. M. lewisii is generally found at higher elevations than M. cardinalis . It is pollinated by native bumblebees, while M. cardinalis is pollinated chiefly by hummingbirds. Because these species have different pollinators, hybrids are rarely found in nature. The two species can, however, be hybridized readily in the greenhouse, and the F 1 and the F 2 can reproduce readily with no obvious loss of fitness. The figure below shows the parental and F 1 flowers, and a sampling of the many colors and shapes of flowers that are seen when the F1 plants are crossed to yield the F 2 .
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M . c a r d i n a l i s F 1 F 2
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Parental (top), F 1 (middle) and some of the F 2 flowers (below) of crosses between Mimulus lewisii and M. cardinalis . Doug Schemske and Toby Bradshaw grew large numbers of F 2 plants in a greenhouse, and then transplanted these plants to an experimental field when they were about to flower. When the plants bloomed, their flowers showed a wide variety of morphologies that were various mixtures of the characters of the two parental species. They then recorded visits by bees and hummingbirds to the plants, and found that bees preferred to visit some flowers and hummingbirds preferred to visit others. In addition, some types of flower were visited by both bees and hummingbirds and others were visited by neither. Armed with this information, they examined the plants for the presence or absence of particular alleles at a number of genetic loci that were known to be responsible for the differences between flower color, morphology and other properties that they suspected would be important in determining pollinator preference. They then looked to see whether these genetic differences between the flowers were correlated with the numbers of bee or hummingbird visits (figure below). Pink flowers Red or orange flowers Small amounts of nectar Medium amounts of nectar Large amounts of nectar ! ! !
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Correlations of genotypes of the F 2 Mimulus flowers with number of pollinator visits. This experiment could not have been done on the original species, because their flowers were only visited by one type of pollinator. But when the F 2 flowers were used, with their wide variety of phenotypic characters, it was possible to see which of these characters was most important in determining which type of pollinator would visit. Two polymorphic loci in the F
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This note was uploaded on 11/05/2008 for the course BILD BILD 3 taught by Professor Woodruff during the Spring '08 term at UCSD.

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bild 3 lecture 11 - The genetics of speciation As we learn...

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