Friday, April 2, 2010
Biological Species Concept
Ernst Mayr defines a species as a "group of interbreeding natural populations that are reproductively isolated from other such groups." (172) There are many different reasons for reproductive isolation; for example, Coyne brings up the fact that sometimes the factor is timing, and some species can't interbreed because their mating/flowering seasons don't overlap to allow that (172). In the case of an organism like the mosquito Anopheles gambiae, it is a selective advantage to look like other species of mosquito but not interbreed with them, because these species provide for camouflage and allows Anopheles to infect people with malaria without humans being able to properly combat these organisms. Name another reason for reproductive isolation between similar species. Why does this barrier exist? Is it simply a technical issue like with mating seasons, or does isolation provide an advantage of some sort?
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Species may be reproductively isolated because they simply do not share a niche. In the mosquito example, some mosquitoes are parasites while others are not. Because the species have different niches, they may not come across one another to mate. Also, according to the book, "species often have different mating displays or pheromones, and don't find one another sexually attractive" (173). I think this barrier is more of a technical issue because much of the ways mating between species is prevented is by logistical and preference differences such as their niche and sexual preferences.
ReplyDeleteI agree with the above comment that niche is a key factor in reproductive isolation. However, I think that the reason why reproductive isolation exists is because species don't find it advantageous to exchange genes. Mating seasons may be a contributing factor, but, as we learned earlier in the year, the competitive exclusion principle states that two species cannot occupy the same niche. Genes code for specific traits, and certain traits are more favorable to have in certain niches. Also, sometimes exchanging genetic material could prove to be unfavorable to both species. On page 173, the example is given of two species of fruit fly that live on opposite sides of a volcano. When the two species do meet and breed, their offspring are sterile. Thus, it does not provide a selective advantage to either species to breed since their offspring cannot reproduce.
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ReplyDeleteAs pages 172-173, Frank, Priyanka and Michael said, factors such as niche and pheromones play crucial roles in reproductive isolation.
ReplyDeleteIn a "fruit fly on fruits after a hurricane" example from the source below, it was mentioned that the fruit flies only mated with the fruit flies who prefered their fruit. For example, a fruit fly on a banana would not mate with a fruit fly on a mango. Once these flies go back to the mainland they won't mate with the mainland flies because of their food preference.Gene flow would be greatly reduced which would result in large genetic differences between the species. Coyne said, " The members of one species simply don't hybridize with members of another.....they [the groups] constitute distinct gene pools that don't intermingle" (172).
This example falls under the continuity and change them because these fuit flies change their gentic make up and mate with only flies that prefer their food. So as Frank, Priyanka and Michael said, Niche does play an important role.
This could be a good thing as it creates variation and "variety is a spice of life".
Source: [http://evolution.berkeley.edu/evolibrary/article/_0_0/evo_44]
Both Priyanka and Frank present equally logical and acceptable answers to your question. Coyne discusses numerous reasons for reproductive isolation on pages 172-173, depending on the species in question. Reproductive isolation can occur because mating seasons don't overlap. Coyne writes "Some corals, for example, reproduce only one night a year" (172). If corals of another species aren't at their reproductive stage at this part of the year, then cross-fertilization is impossible. This mechanism, however, makes it such that only organisms that have the same mating season can reproduce. Since organisms of the same species have the same mating seasons, reproductive isolation ensures that an organism of a certain species mates with another organism of the same species. Another cause of reproductive isolation is the pheromones that certain species use to attract mates. We learned in the behavior and ecology unit that pheromones are chemical substances that are "especially common among mammals and insects and often relate to reproductive behavior" (Campbell 1125). Coyne cites Drosophila as his example, saying "Females in my Drosophila species have chemicals on their abdomens that males of other species find unappealing" (Coyne 173). Because of the specific pheromones that a female Drosophila uses, male Drosophila are attracted, while males of other species are essentially 'scared away.' Coyne also writes that "isolating barriers can also act after mating" (Coyne 173). He goes on to explain that if crossbreeding does occur, the fetus may not form, but if a fetus is formed then they could die before birth or be sterile after birth. An example of a sterile crossbreed is the mule, which is created when a female horse and a male donkey reproduce. The barriers that prevent the proliferation of crossbred species essentially force species to mate with suitable mates of the same species. There is no single factor that causes reproductive isolation; different species have different factors, whether it be non-overlapping mating seasons or pheromones that target only a specific species. Reproductive isolation relates to the theme of evolution because reproductive isolation ensures that organisms of the same species mate together. If organisms of different species try reproducing together, they can either no produce offspring or produce sterile offspring.
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