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E9. Natural selection and geographical isolation can lead to speciation.


Student Outcome: E9.1

Explain how the geographical separation of populations may result in the divergence of each subgroup, so that interbreeding is no longer possible.


The most common way for species to split, especially in animal species, is when the population becomes geographically isolated into two populations. This is referred to as allopatric (geographic) speciation.


One model of allopatric speciation:

  • A single population is fragmented by a barrier;
  • geographical isolation leads to genetic divergence;
  • when the barrier is removed, the two populations come back into contact with each other, and there is selection for increased reproductive isolation;
  • if reproductive isolation is effective, speciation is complete.


Geographic isolation leads to reproductive isolation. Once two populations are reproductively isolated, they are free to follow different evolutionary paths. They are likely to differentiate for two reasons:


  1. Different geographic regions are likely to have different selective pressures. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate.
  2. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.


In short, physical isolation turns a single population into two, which, because of their lack of connectedness, may follow different evolutionary paths. What happens next? The fate of the populations depends upon time and factors related to their different environments. If the two populations are soon rejoined, they may not differ very much, and likely will become a single population again.


Differentiation also depends upon the strength of selective pressures. Strong selection can cause rapid change.


Given time and selection, the two populations become two species. They may, at some later time, spread back into contact. Then we can ask, are these two "good biological species"?


The real test of the biological species concept is when two populations, on the threshold of becoming two species, come back into contact. They may simply merge. They may be so different that they do not even recognize one another as species.


Often, though, species may come into contact when not yet fully reproductively isolated. In that event, natural selection should reinforce the reproductive barriers. Why? Because individuals that waste their reproductive effort -- their gametes -- on individuals with whom they will produce inferior offspring are less likely to pass on their genes to the next generation.


Source: http://www.globalchange.umich.edu/globalchange1/current/lectures/speciation/speciation.html


An introduction to evolution

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This simulation show how new species can develop.


This interactive shows the evolution of mammals - great looking with lots of information - well, I think so!


This simulation (which takes a while to load) shows a timeline since the big bang. Great if you have time and by the great John Kyrk.


This web site is by the Australian Museum on the evolution of humans. Really simple to get through and worth reading if you have the time.


This site here is the HHMI site containing a series of lectures on Evolution. I haven't seen them all but if you have the time and are interested in pursuing the topic further, they could be worthwhile.


Here is a video about Evolution in Action in the Lower Congo. American Museum scientists are on a quest to understand why so many species have evolved here.

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