Monday, April 12, 2010
Sister Species
In the chapter The origin of species, Mr. Coyne talks about sister species. Sister species are "species that are each others closets relatives [but] were ofter separated in nature by geographical barriers" (175). For example sister species of sea urchins were found on opposite sides of the isthmus of Panama. In your response explain how the geographical separation of sister species could have something to do with how the species came from a common ancestor. Also explain the natural selection that arises from having sister species. Provide an example of sister species in your explanation.
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When a two populations of a species are geographically separated, the gene flow between the two populations stops, and evolutions differs in the two populations. According to Campbell, this is called allopatric speciation, "when a population is divided into geographically isolated subpopulations" (Campbell 492).
ReplyDeleteSister species could be separated in ways such as water levels in a lake subsiding resulting in two or more smaller lakes or by a canyon/fault emerging through an area separating a population. Sister species are geographically isolated when a "formidable barrier" (493) is formed that prevent the two subpopulations from interacting and reproducing.
Between the the two subpopulations, natural selection will be different because the environmental pressures will different in the subpopulations and different mutations will occur. Environmental pressures may be different between the two subpopulations because different predators or prey may be present in the two subpopulations, or an introduced species was introduced in one of the subpopulations. Mutations arise when their is an error during cell division such as polyploidy or when their is an error in DNA processing/copying. Mutations will be different between the two populations because accidents during cell division/DNA processing are rare, so it is very unlikely for a mutations to appear in both subpopulations. Natural selection will act differently upon the two subpopulations because the environmental stresses and mutations may increase or decrease the fitness of certain species which will change whether an organisms dies or survives to reproduce.
An example of a sister species is the monkey flower Minulus guttatus. A certain populations of M. guttatus lives in an area with such high soil copper levels to be toxic to nontolerant organisms. When this special populations of M. guttatus are bred with regular non-copper tolerant M. guttatus, "the offspring survive poorly" (493). Also, genetic analyses have shown that the copper-tolerant gene is responsible for the poor survival of hybrid offspring. Thus, although the populations of M. guttatus are closely related, they are unable to interbreed successfully.
As Anika stated, sister species are "species that are each other's closest relatives" (Coyne 175). Often these species were separated by a geographical barrier such as an ocean or a mountain, like in the example Coyne provides in the book of a plant species that evolves due to different selective pressures on opposite sides of a mountain.
ReplyDeleteErnst Mayr proposed that the geographic isolation of populations along with the different natural and sexual selection occuring in each separated population causespeciation. The species would have started in one location, but the geographic barrier would have separated it into two populations.
Another example of sister species besides the sea urchins in Pananma is the worm Lumbriculus variegatus.Only recently did researchers at the University of Gothenburg discover that this species of worm is actually two distinct species. They appear very similar, but their genetic makeup differs enough that the two can't mate. It is possible that these two species of worms were originally one that was seperated by some sort of geographic barrier.
The idea of sister species relates to the theme of interdependence in nature because species develop due to selective pressures from other organisms and the environment. These conditions are different in populations separated by geographic barriers.
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ReplyDeleteAccording to Coyne, "if you can explain how reproductive barriers evolves, you've explained the origin of species" (174). Formation of a reproductive barrier implies speciation; a common ancestor is reasonable when there are reasons behind the splitting of the species into two groups unable to interbreed.
ReplyDeleteMost often, reproductive barriers between sister species result from geographic barriers. Coyne gives the example of a flowering plant which, by traveling in the stomachs of birds, is spread to the other side of a mountain (175). The species then, through natural selection, adapts to its new environment. Whereas the group on the old side encounter many hummingbirds, the new group is around many more bees. The new population evolves to better fit pollination by bees by changing the size of its nectar tubes and the amount of nectar held inside. If the two groups were to occupy the same environment again, the flowers' genes could not mix through cross-pollination, and the two groups would be "reproductively isolated" (176).
As Brandon pointed out, this type of speciation clearly relates to an interdependence in nature. We see that the divergence of flowering plants in this case was directed by the size of bee and hummingbird populations that they relied on for pollination. In many cases like this, the evolutionary path of one species involves and depends on the surrounding species.
Another example of sister species is found in Darwin's finches. There are 13 species of finches that diverged from a common ancestor after allopatric speciation. On certain individual islands, no divergence occured (the environmental pressures faced by the finches was the same) but in all the islands together 12 cases of speciation must have occured (different environments causing adaptations that lead to reproductive barriers).
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html