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Vanessa702 Vanessa702
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9 years ago
Explain the three main types of selection as the well as the isolating mechanisms. Describe how how all of these can contribute to changes in a population and the evolution of a species.
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9 years ago
Stabilizing selection -

When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection.
This is a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time. Stabilizing selection commonly uses negative selection (a.k.a. purifying selection) to select against extreme values of the character.
A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.

Directional selection

In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. Directional selection can quickly lead to vast changes in allele frequencies in a population. Because the main cause for directional selection is different and changing environmental pressures, rapidly changing environments, such as climate change, can cause drastic changes within populations. Typically directional selection acts strongly for short bursts and is not sustained over long periods of time


Disruptive Selection

In disruptive selection, selection pressures act against individuals in the middle of the trait distribution.It is believed that disruptive selection is one of the main forces that drive sympatric speciation in natural populations.The pathways which lead from disruptive selection to sympatric speciation are not often prone to deviation; it is a domino effect which depends on the consistency of each factor. These pathways consist of disruptive selection being based on intraspecific competition, which often leads to reproductive isolation, and finally sympatric speciation. It is important to keep in mind that disruptive selection does not always have to be based on intraspecific competition, so in some cases that step of the pathway can be eliminated. What can happen instead is that disruptive selection supports polymorphisms, which can lead to reproductive isolation, and lastly speciation.

When disruptive selection is based on intraspecific competition, it in turn promotes ecological niche diversification and polymorphisms. If two morphs, or forms, of a phenotype occupy different niches, it would promote less competition for resource use. Disruptive selection is seen in high density populations rather than in low density populations because intraspecific competition is more common when accompanied by a higher density population. This is due to the fact that higher density populations often have more competition for resources which drives polymorphisms or changes in niches in order to create less competition. If one morph has no need for resources used by the other morph, then it is likely that neither would feel the need to compete or interact, therefore supporting these two morphs to continue occurring in the population. This theory does not necessarily have a lot of supporting evidence in natural populations, but it has been seen many times in experimental situations using existing populations. These experiments further support that, under the right situations (as described above), this theory could prove to be true in nature.
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