In Coyne's chapter "The Origin of Species" he discusses the idea of sympatric speciation, or speciation that occurs without geographic barriers. Coyne says that Darwin repeatedly enforced the idea that a species could diverge within a small unisolated area. It seems that Coyne diasagrees with Darwin and thinks that speciation would not occur with individuals that are constantly mixing. He says that "mathematical theories show tht sympatric speciation is possible, but only under restrictive conditions that may be uncommon in nature" (184). Do you agree with either scientist? Why? To what extent, do you feel, is either scientist correct? What are ways other than geographical barriers that could cause speciation to occur? Relate the idea of speciation to biological themes and/or things we have studied in class.
Monday, April 5, 2010
Sympatric Speciation
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I more agree with Darwin in that speciation can occur in an unisolated area. I think Coyne has a point in that if the individuals are constantly mixing, it may be difficult for speciation to occur, but the combination of the species mixing may be a factor that determines the new formed species.
ReplyDeleteOther than geographic barriers, an environmental stress could also cause speciation. Let's say that in an area, suddenly an environmental stress confronts a population such as an introduced predator that preys on the population. There could be two abnormal organisms of the species that can escape/survive the predator to reproduce. One abnormal organism maybe has cryptic coloration and blends in with the surroundings while the other just has an amazing running ability and is able to run away from the predator. Although these "mutants" were once part of the same population, they developed unusual traits and were able to survive and reproduce. Because of the difference in the way the two mutants escape the predator, they may each evolve into a separate species, but they do not necessarily need to be geographically isolated.
Frank states that he believes Darwin's theory to be more plausible because Coyne's argument about the intermixing of genes is flawed.
ReplyDeleteI, on the other hand, believe that Coyne's argument makes more sense. In Campbell, sympatric speciation is desribed as "less common than allopatric speciation...and can only occur if gene flow is reduced by such factors as polyploidy, habitat differentiation, and sexual selection" (Campbell 495). This leads me to believe that Coyne is correct in saying that mathematically, sympatric speciation is not very likely.
When we look at geographic barriers, it's clear that they are very influential to the development of a species. For a species to exist together in a given enclosed area, it is very unlikely that speciation would occur, simply because the organisms are exposed to the same geographic barriers. Thus, natural selection would dictate that the species evolves together into forms that are selectively advantageous. Therefore, it is unlikely that, as Frank says, "the combination of the species forming may be a factor that determines the new formed species". More likely is the hypothesis proposed by Coyne: the members of one gene pool will interbreed and so diverging forms will be constantly pulled back to a single species (184). That isn't to say the species won't evolve - rather, they will evolve as a whole, not branching off into different species because there is no selective advantage to doing so.
Of course, Darwin and Frank were both correct in saying that sympatric speciation does occur, however, while its easy to find evidence for geographic speciation, its much harder for sympatric speciation because there are fewer examples of sympatric speciation in nature. Only under certain narrow conditions can sympatric speciation occur.
Speciation can occur due to the environment as Frank mentioned, as well as due to various factors during the reproductive process. For example, sexual dimorphisms play a role in influencing mating and thus, gene intermixing, between individuals. As the species continues to reproduce, focusing on certain factors, the species may branch off into more selectively advantageous forms ultimately forming a new species.
Speciation is related to the theme of continuity and change because basically, speciation is describing the continued change in a group of animals. We are looking at why a species arises and evolves and why new species can emerge. We can find the answer in natural selection - this idea that species will continute to evolve and adapt and change based on the drive to survive and reproduce. There will be more and more change until ultimately, a new species emerges.
I agree with Frank that I also agree with Darwin. Coyne’s argument is that Darwin didn’t explain thoroughly how “one species splits into two” (170); however, I believe Coyne is wording the topic too bluntly. It is clearly implied from On the Origins of Species that evolution is a gradual process. One species does not immediately split into two. Which is where other factors come into play. I like what Frank said about environmental factors playing a key role. I agree that specific environments cause membranes of species to adapt, migrate, or die. Geographical barriers are an example of migrating. Those that decide to adapt may find a niche different to the one from before, simply because there are too many individuals (even within the same species) occupying that niche. This is how individual differences within a species arise without geographical barriers. These individual differences may be subtle, which is touched on by Coyne, who cites the fruit flies as an example. There are nine different species of fruit flies, but even under microscopic conditions the differences between the species are subtle. I believe that continuity and change is a major theme relating to sympatric speciation. Changing environments (not just ones that are geographically isolated) cause changes in niches occupied by individuals in the same species. As a result, individuals of the species adapt and form a new species, which may only be slightly different than the origin species. Through these subtle changes within species and changing environments, more and more species arise.
ReplyDeleteSorry Frank, but I have to disagree. I believe Coyne's argument against sympatric speciation is valid. As Coyne explains, "...it's hard to split one gene pool in two while its members remain in the same area, because interbreeding between the diverging forms will constantly be pulling them back into a single species" (Coyne 184). If all of the members are in the same area, then variations in the gene pool may occur. However, those variations will be just that, small variations in the genome of one single species. The website of Berkely's "Evolution 101" describes a species as, "...a group of individuals that actually or potentially interbreed in nature" (http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml). Two different species would not interbreed. It is pretty difficult for this situation to arise to form a divergent species if all of the organisms of the original species are constantly mixing.
ReplyDeleteI believe that ultimately, while the occurence of sympatric speciation is conceivably possible, sympatric speciation is highly unlikely and is a minor, if present at all, mechanism of evolution. Therefore, I agree more with Coyne's viewpoint.
This debate over sympatric speciation does embody the theme of continuity and change. Speciation requires changes in the genome of an organism. However, many of the same genes must be passed down to the next generation.