Wednesday, April 7, 2010

Divergent Selection

On pages 175-176, Coyne describes what he calls divergent selection, based on geographic barriers in flowering plants, hummingbirds, and bees. He suggests that in one isolated region containing more hummingbirds than bees, flowers may evolve to become better suited to pollination by hummingbirds than bees- and the other way around in a region characterized by more bees. Then he speculates, that over time, should these two types of flowers that have evolved distinctly from one another suddenly be placed in the same location, the geographic barrier having disappeared, and the area containing equal amounts of bees and hummingbirds, then these flowers would each have their own type of pollinator and would not cross pollinate as a result, for they are now two different species. How difficult is it to maintain the ability to mix genes with closely related species (defining species by the definition given on page 172) or at least with those species derived from a common ancestor? Why would different species adapt their reproductive organs to become better suited for their own species rather than a wider range of potential mates? Wouldn't it be to the advantage of the flower placed back in the location described above to be able to be pollinated by both bees and hummingbirds? To what extent would it not? Why don't we see greater potential to mix genes and reproduce across species if sex has evolved as a means for genetic diversity?


  1. Well, by definition a species are a “reproductively isolated from each other” (172) so technically it is nearly impossible, barring artificial insemination and accidents, to have offspring of two different species. If talking about a species that is currently diverging, then a certain percentage of the differentiating species, probably not too much, would have to interbreed as to ‘recycle’ the common genes back into the gene pool, thus maintaining the ability to reproduce. On page 173 Coyne states an example about how closely related corals don’t breed because their spawning times are only a few hours apart, even a tiny difference in reproduction led to the creation, and maintenance of spate species.
    While the genetic diversity and change of the new species may be great, the practicality of hybrid is highly questionable. On 176, Coyne says that two highly different species “just don’t work well together” and can cause them to “die prematurely or turn out sterile.” The unique hybrid must overcome insurmountable obstacles to pass along its genes. It has to first survive with its new appendages that it probably cannot use as well as the parent species, it has to find a mate, overcoming sexual selection pressures and finally have offspring that are not sterile and do not die. This is probably why we don’t see flying bears or snakes with legs, but that would be pretty cool.
    Each species has its own set of selective advantages that helps it survive. Again, the practicality of a hybrid is low, and there are many obstacles that prevent two completely different species from mating, even similar species have different reproduction practices. Having organs that work best on your own species raises the amount of reproduction, increasing the diversity of the gene pool and fitness for the species. If natural selection favored ‘common’ reproductive organs, offspring would be more likely to die with traits that are less developed than their parents’ traits. For example, the flying bear may not be able to use its wings fully and waste energy trying to do so.
    A flower that was pollinated by the bee will be significantly different from one that is pollinated by a hummingbird. First of all, the bees are more specially adapted to their flower, and the hummingbird to its flower because of co-evolution. This would make each organism more efficient at pollinating their respective flowers. Also, the characteristics of the flower make the other organism less likely to attempt to pollinate; “Hummingbirds are strongly attracted to red flowers, although they also frequent pink, orange or other colored flowers” (eHow) “while bees are especially attracted to white, yellow, blue, and purple shades of flowers.” (Greenthumb) Even differences in flower color will influence the decision of the organism of whether to visit the flower or not. In addition, flowers not adapted to different pollinators may be damaged; the hummingbird has a long beak used to extract nectar from its preferred “tube or trumpet“ shaped flowers may damage a flower that the bee pollinates. The result is that the flower becomes injured, the hummingbird receives little nectar, and neither the damaged or regular flower is pollinated. Pollination by both organisms would only beneficial in terms of numbers of organisms helping pollination; otherwise it is ill advised for bees and hummingbirds to stray from their niches.

  2. Divergent selection does not result in flowers that can be pollinated by multiple species because it is not advantageous. As previously stated, flowers in an isolated region will become better suited for pollination by either bees or hummingbirds, depending on which is more prevalent. It would not be advantageous for a flower to have reproductive organs that allow it to be pollinated by both. For instance, if there are less bees in the area, it would be a waste of energy to produce organs that allow bee pollination. The odds of reproduction would be higher with specialized organs to reproduce with hummingbirds.

    On the contrary, hybrids are an instance in nature in which mixing genes is possible. Hybridization occurs when two closely related species create offspring. This can actually be evolutionarily significant since it creates rapid change. Hybrids can arise from breeders or in nature. For example, the common mule is a bred mix between a horse and donkey. In addition, oak tree species naturally hybridize very frequently. A common misconception is that all hybrids are sterile. According to the University of Indiana in Bloomington, weak, hybrid offspring can attain new combinations of strong genes that may just barely be able to procreate enough to pass on genes. This would create more genetic variation that is highly successful in evolution. At the university, researcher Jeffery Palmer and his colleagues compared species of ancient hybrid sunflowers with hybrids produced in the lab. The results showed that "synthetic hybrids quickly acquired the traits necessary to colonize the extreme habitats of their naturally evolved hybrid counterparts." Although divergent selection dictates the separation of genes in evolution, hybrids seem to quickly advance evolution.