There are two main types of speciation
Allopatric & Sympatric
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Allopatric speciation
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Sympatric speciation
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A little extra
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A geographic barrier separates the populations
Natural selection acts on each population (different selection pressures act on the different populations). Over time, different alleles will become more common in each population and others will become less common. Over time the populations will become quite different! Like in the example below!
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Genetic drift may also occur, with bottlenecks or founder populations creating populations with different allele frequencies compared to the original population. Mutations can also add new alleles to the mix. This helps to make populations more different.
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As a result of all the changes in allele frequencies that occur over time (years and years), the populations become more and more different. Gene flow must still not occur because if it did, alleles would be shared, making the populations more similar. Eventually, the populations will be so genetically different that they won't be able to interbreed successfully. If they were ever to come into contact they would not be able to produce viable offspring. This means speciation has occurred.
There is no geographic barrier present in sympatric speciation and the populations live in the same area/habitat.
However, they are separated by differences in their ecological niche (their way of life), which occur usually as a result of intraspecific competition, for example
However, they are separated by differences in their ecological niche (their way of life), which occur usually as a result of intraspecific competition, for example
Some of the population may move to a new place within that habitat, such as from in the trees to the forest floor. Now different selection pressures act on each population and select for/against different adaptations
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Some of the population may change their diet in order to reduce competition. Diet becomes a selection pressure and those best able to access a lot of nutrients will be selected for, those unable to get much food will be selected against
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Some of the population may adjust the time of their activity; perhaps they were diurnal but now become crepuscular. Those best adapted for activity at those times will be fitter than those not as well adapted
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Due to the lack of gene flow resulting from one or more of the above niche differentiation types, genetic differences accumulate over time. Natural selection, genetic drift and mutations all can act on the populations to make them more different.
As a result of all the changes in allele frequencies that occur over time (years and years), the populations become more and more different. Gene flow must still not occur because if it did, alleles would be shared, making the populations more similar. Eventually, the populations will be so genetically different that they won't be able to interbreed successfully. If they were ever to come into contact they would not be able to produce viable offspring. This means speciation has occurred.
As a result of all the changes in allele frequencies that occur over time (years and years), the populations become more and more different. Gene flow must still not occur because if it did, alleles would be shared, making the populations more similar. Eventually, the populations will be so genetically different that they won't be able to interbreed successfully. If they were ever to come into contact they would not be able to produce viable offspring. This means speciation has occurred.
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There are also two patterns of evolution you need to be familiar with
Divergent & Convergent
evolution
evolution
Speciation is an example of divergent evolution.
This is where a species is separated (either allo- or sym-) and become very different, due to different selection pressures. Species often share similar structures (due to evolving from a common ancestor), however they may have different functions. These structures are called homologous structures. In the above picture are forelimbs of four different animals that show a common structure, yet serve very different functions. This is an example of a homologous structure.
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Convergent evolution is where two unrelated species, from different ancestors become very similar in phenotype (over time) due to living in a similar environment and having common selection pressures. Species often have different structures that serve the same purpose / have the same function. These structures are called analogous structures.
In the above picture are the forelimbs of three different aquatic animals that show different structures, yet serve the same function. From the outside, the animals look very similar and could be easily mistaken for having a common ancestor, but with a closer look you can see they have evolved similarly due to sharing the same selection pressures.
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and here's two more that are also important to know
Co-evolution & Parallel evolution
Co-evolution
Also described as reciprocal evolutionary change. It is when two species are in a close relationship (i.e. competition, predation, parasitism etc) and an evolutionary change in one species acts as a selection pressure on the other species, causing it to change. Over time both species change as a result of the effect the other species has on it.
A cat, quick enough to snatch a bird successfully today, might not be speedy enough to catch the next generation of birds, because only the swiftest of yesterday's birds remain to provide offspring. The above quote shows how co-evolution occurs - the speed of the bird acts as a selection pressure on the cats - only the fastest cats will get food, survive and reproduce. Over time, cats become faster, which then becomes a selection pressure on the birds; only the fastest birds will survive and reproduce... over time both species become faster because of the effect each has on the other.
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Parallel evolution
Parallel evolution is when decedent populations from a common ancestor, although in different geographic locations, develop similar adaptations independently of one another.
So instead of becoming very different in different environments, they respond to the challenges in their environment in very similar ways. |