Saturday, March 20, 2010

Geographic vs. Sympatric Speciation

It is widely accepted that geographically isolated populations can develop into separate species However, on page 184, Coyne mentions that Darwin believed that "new species, espectially plants, could arise from within a very small, circumscribed area." This type of speciation is called sympatric speciation. Describe geographic speciation and sympatric speciation, and why some biologists argue over the practicality of sympatric speciation.


  1. Geographic speciation is when one organism gets spread into two geogrpahic areas that differ in environmntal conditions. Due to the differences in surroundings, the two populations evolve into two new species due to natural selection chosing the individuals with the best adaptions to their surroundings. Geographic speciation is the more commonly accepted form of speciation because it is clear to see how two different environments could lead to two different species evolving.
    Sympatric speciation is when two species evolve in the same geographic area due to natural selection changing the populations into species that are different from their ancestor. This theory seems difficult to accept because it would be hard for two species to evolve from the same ancestor without a geographic barrier preventing them from interbreeding. Sympatric speciation ia a rare occurence that is not commonly seen because it is hard to identify and doesn't happen very often. Some scientist argue sympatric speciation is not practicle because it so rarely seen and hard to prove that it exists. Coyne talkes about it being hard to conclude that two related species that live in an area arose in the same place(184). Other see it as an opportunity to disprove creationism because the rarity of it is what correlates with evolution since it does not seem probable that an organism could evolve into two species within a confined area. It does not make sense that a creator would have sympatric speciation occur on continents but not on certain islands, helping to prove that evolution must be true.

  2. As Annie mentioned, Geographic Speciation, or Allopatric Speciation, is the divergence of two groups of the same species into two different species as a result of natural selection resulting in individuals with gradual adaptations to each respective environment, over time becoming two separate species. This theory of speciation is widely accepted as it can be so readily proven with the evolutionary paths of many species today; the divergence of bonobos and common chimps, for example, or the African and Asian elephants, resulted from environmental differences separated by geographic boundaries.
    Sympatric speciation, or the evolution of two separate species from a single species in the same environment is not as readily provable, as it is such a rare occurance mathematically, as Jerry Coyne points out on page 184; speciation could only occur of the two populations did not reproduce with one another again as natural selection slowly resulted in the formation of individuals more and more adapted to their particular niche.
    The idea that two groups could adapt themselves to separate niches so thoroughly that species to result while remaining so geographically close is a controversial idea due to the extremely few species examples in the real world; as Coyne points out on the same page, one of the only ways to do it is to take a look at 'habitat islands' where there is far less possibility of species leaving for another geographic location, undergoing evolution, and returning, a problem that Annie mentioned about finding examples on the less isolated patches of land or water. Even among these habitat islands where the probability of sympatric speciation should be relatively high, there are only a few examples. One that is mentioned in the book is the radiation of species in isolated volcanic lakes, but even with this, Coyne admits that "we don't know why or how it happened"(185). Thus, while some new findings may indicate a high probability that such speciation did not result from geographic differences, there are few ways we can detirmine for sure since the necessary conditions for it are so much more stringent than geographic speciation and thus could potentially cause controversy among biologists over whether an example can conclusively constitute sympatric speciation.

  3. Geographic speciation, also known as allopatric speciation, is the formation of new species in populations that are geographically isolated from one another. This occurs when “gene flow is interrupted when a population is divided into geographically isolated sub-populations” (Campbell 492). The reason for their divergence is due to the fact that different geographic regions are likely to have different selective pressures. Temperature, rainfall, food supply, predators, and water availability is very likely to differ between two isolated areas and thus the two populations will have to adapt to their environment and undergo specific physical changes that help it survive in its surroundings (Global Change). The lack of connectedness between the two species allows the two groups to further diverge and eventually after an extended period of time, the two populations will no longer be reproductively compatible. Like Coyne and Annie stated, most evolutionists accept the idea that geographical isolation of populations is the most common way that speciation occurs (Coyne 184). However, Darwin introduced the concept that speciation does not require large-scale geographic distance to reduce gene flow and lead to new populations. This type of speciation is referred to as sympatric speciation, from the Greek word “syn”, meaning together (Campbell 495). This theory of evolution is very controversial because some evolutionists don’t believe it is possible for it to occur because they feel that interbreeding would soon eliminate any genetic differences that might appear (Speciation). Although such contact and the ongoing gene flow that results makes sympatric speciation less common, it can still occur if gene flow is reduced among the two diverging populations. Factors that can heighten this speciation include polyploidy (chromosomal alteration in which the organism possesses more than two complete chromosomal sets), habitat differentiation, and sexual selection. For instance, in a lake with only one current population, disruptive selection-such as competition favoring fish of a specific appearance (ex red bellys)- coupled with assortative mating- mates selecting those with phenotypes like theirs- can lead to a divergence in a population and the start of a new species. Ecological factors play a huge role in sympatric speciation. The reason sympatric speciation does not seem practical is because it appears as though “interbreeding between the diverging forms will constantly be pulling them back into a single species” (Coyne 184). This type of speciation is mathematically possible, but the probability of it actually happening is very low, and hence scientists are skeptical towards it.