Sunday, April 5, 2009
FLOWERS
Carroll states on pg 133-134 that flower color is determined by both “evolutionary change brought by natural selection” and by “a by-product of natural selection for other features”. What exactly does Carroll mean by this, and what are the reasons for changes in flower color? In your response also include how flower color is determined and how different enzymes produce different colored flowers, give examples. Also explain why is the evolution of the red flower more adaptive? In your response include the theories of evolution and natural selection.
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Flower color, in most cases is determined through inheritance. With a dominant allele (lets say P) and sometimes possessing a recessive allele (p). Inherited from the Parental generation, the parents can either be homozygous dominant/recessive (PP/pp) or heterozygous (Pp). In any combination, a general probability can be calculated to create a ratio for the first generation flowers. Through natural selection, the “best” most “attractive” colored flowers will survive, simply because they will attract the appropriate pollinators that can then fertilize the flower. Floral coloration and color vision was tested using the flower Hymenoptera. The analysis of 180 spectral reflection angiosperm blossoms revealed that there is a large difference in the wavelengths where the pollinators are most sensitive to spectral difference (basically, some colors are more attractive). The optimal set of three spectral photoreceptors for discrimination of floral color creates the basis of perception amongst pollinators. Some evidence suggests that hummingbirds, a pollinator of many flowers and plants, are attracted more to red flowers than any other color. This is due to the fact that most insects’ vision lies within the UV spectrum (outside of our red, orange, yellow, blue, indigo, and violet spectrum). And insects find the color red to be rather difficult to see. So the red, visible to many birds (including the hummingbird), and insects that do not see in UV light are therefore attracted to the red flower. This would then cause a selective advantage to a flower that is in a high population of hummingbirds (or non UV seeing insects), causing that flower to be pollinated and its genes to be passed on.
ReplyDeletehttp://www.entomology.cornell.edu/public/IthacaCampus/ExtOutreach/Outreach/Students/pageBodySections/0116/text_files/file/Wang_teacher_resource_pollination.pdf
http://www.springerlink.com/content/g328727727p7161j/
In studying natural selection and evolution, flower color is one of the most interesting examples to look at. Flower color is very important for plants because it is often used to attract pollinators. Birds or bees are attracted to specific colors more than others and this will increase the chances of reproduction and survival for the flowers because their pollen will be spread more abundantly. But changes in flower color can also can occur from mutations and in this case the degeneration of certain enzymatic pathways. In the novel, however, Caroll says that fossilization, not flower color is determined by both “evolutionary change brought by natural selection” and by “a by-product of natural selection for other features”. Flower color is just one of the examples where fossilization takes place. In this case the relaxed selection of the blue/purple color in plants promoted change in the flower to a red flower color. Fossilization and loss of genes (relaxed selection) is what occurs in the “absence of natural selection” (136) and this causes other evolutionary changes to take place.
ReplyDeleteIpomoea quamoclit is a species of Ipomoea that has red flowers rather than the usual blue or purple and is pollinated by hummingbirds rather than bees. The evolution of the red flower has adapted more because through natural selection, it has acquired this red color later that for some reason is an adaption for attracting hummingbirds. The production of color in the Ipomoea is determined by different sets of enzymes in enzymatic pathways. The pathway for the blue and purple pigments has actually become a fossil gene and “degenerated in the red I. quamoclit”. The ancestral form of the plant got its blue/purple color from the chemical cyanidin. But mutations in these flowers blocked the production of cyaniding which changed the color of the plant to red. The evolution of the red flower color came from the blue/purple pigments but has been more adaptive. Caroll says that “it appears that gene inactivation in this instance is a cause of evolution” (134). So the enzyme that promotes blue/purple color in the flowers has been deactivated by a mutation sometime along its history. And when this change actually proved beneficial, natural selection took its course and the red flowers began surviving and reproducing better so selection on the genes promoting blue/purple color became relaxed because it wasn’t necessary for the plant anymore to have only blue or purple colors.
http://www.nature.com/nature/journal/v428/n6985/full/nature02489.html
http://www.botany.unibe.ch/deve/publications/reprint/CurrOpPlantBiol_9_78.pdf
http://creation.com/morning-glorys-designer-label-clothing
The flower itself is by far the most important evolutionary development in plants and its success in angiosperms is unprecedented compared to any previous lineage of plants. As a main adaptation to terrestrial existence, plants had to acquire better ways to fertilize their eggs. After the development of the seeds and pollination in gymnosperms, the main evolutionary of these plants became to increase chance of pollination because pollination is obviously the determining factor in reproduction. The development of the flower was obviously not a one mutation development as Carroll explained does not occur. It was rather an accumulation of mutations. Nevertheless, the flower was such a powerful development in plants because it sped up their evolution with the development of coevolution between the animal pollinators and the plants. Coevolution can cause rapid evolution between both species because their evolutions encourage further evolution in the complimentary species and this cycle proliferates evolution.
ReplyDeleteOne aspect of this coevolution is flower color, an important factor in the reproduction of plants. One of the most important ideas to take away from the inheritance unit was that the chance of inheritance of a trait does NOT determine the percent of individuals who have that trait in a population. The determining factor in many cases is instead the selective advantage that a trait has. It does not matter how likely an individual is to inherit a certain trait. If this trait puts the individual at a disadvantage compared to others who do not possess the trait, then the long running trend that this species will see is an increased number of those without the trait. If a plant produces an attractive color for pollinators, it increases its chance of pollination and reproduction. Because of this coevolution relationship between angiosperms and animals, a vast variety of flower types have arose. Since you asked for the enzymes involved, I will explain the effect of natural selection on plant color beginning at the molecular level in the Azalea flower.
The pinkish-red color of the Azalea flower is determined by two pigments, anthocyanins and flavonols. The anthocyanin is a general pigment that determines whether the flower has color or not. Depending on the specific pH of the petals, the color is difference. The main pigment that gives the bright red color is flavonol. This pigment is created by the enzyme dihydroflavonol-4-reductase (dfr). If a flower contains a gene that creates a lot of the dfr enzyme, this flower will have a lot of flavonol pigment and attract many pollinators, ensuring its reproduction. If its gene is incompetent in creating sufficient flavonol pigment to attract pollinators, it will most likely fail to reproduce. Notice that in this process, the inheritance chances of the different alleles does not matter; it is the power of the inherited advantage that matters. As we can see, natural selection is acting on the positive selective coefficient for having a gene that creates a lot of dfr. This process is likely the long process that created the bright pinkish-red colors of the Azaleas.
http://www.actahort.org/members/showpdf?booknrarnr=743_15
http://en.wikipedia.org/wiki/Anthocyanin