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Natural Selection

Natural selection is the process of naturally 'screening' traits (characteristics) within individuals within a species for or against a certain outcome.

Natural selection can affect morphological, physiological, biochemical and anatomical characters, however natural selection can only act on traits which can be genetically inherited and which are expressed. In summary, natural selection acts on the phenotype of an organism. The theory states that if an organism has a trait that is of benefit, then it will have a greater chance of reproducing and passing on that gene. However if an organism has a gene which is causing it disadvantage, then the organism will die and not pass on that gene. Hence it can be loosely termed 'survival of the fittest'.

To understand natural selection, it is important to understand the concept of 'biological success'.

Biological success is measured by the number of offspring the organism leaves in the next generation, and the number of genes that that organism contributes to the gene pool of the next generation.

For example: An organism through a mutation has a 'super' gene, which makes it the best, most suited organism in its environment. If it is killed before it can reproduce, then it has a biological success of zero, as no offspring could inherit the 'super' gene. However, if that same organism were spared, and had lots of offspring carrying the 'super' gene that makes this organism so successful, then it would have had a high level of biological success, as the gene would have been passed on to improve the species.

Biological success is what contributes to 'natural selection', based on the premise that every trait or character is either an advantage or disadvantage. Natural selection does not have to be gradual - in fact if there are sudden and major environmental changes then evolution (through natural selection) can occur relatively quickly.

So, natural selection is a complex process in which the total environment determines which members of a species survive to reproduce and so carry on their genes to the next generation.

Natural selection does not necessarily involve a struggle between species.

There are other selective mechanisms as well as natural selection, and these are outlined below...

1. Stabilising Selection

Stabilising selection is where the extremes of a population are selected against, and the normal is selected for. This situation can be pictorially demonstrated with a bell curve representing any given population...
Graph: Natural distribution versus Stabilising Selection
Figure 1: These schemeatic graphs demonstrate the frequency of individuals with a certain set of traits/phenotypes/genotypes within a population. The "natural distribution" graph (left) shows the population 'as is', whereas on the right selection pressures have selected against the extremes of a population, so that the range of variance within that population is focussed towards the centre (normal).
The graph on the left of Figure 1 shows a normal population, with the greatest number of individuals in the middle; they are normal. On the edges are the extremes; these are a minority. The total area under the curve equals 100% of the population. Under selective selection, the individuals on the extremes are selected against, and the number in the middle (the normal) rises.

2. Directional Selection

Directional selection is where one extreme of a population which has a trait is targeted against, and the normal and other extreme is targeted for. This forces the shifting of the mean within a population.
Graph: Natural distribution versus selection against one extreme and shifts in the normal
Figure 2: These schemeatic graphs demonstrate the frequency of individuals with a certain set of traits/phenotypes/genotypes within a population. The "natural distribution" graph (left) shows the population 'as is'. The middle "selection against one extreme" graph shows the application of a selection pressure agains one extreme of a population but in favour of the other exteme and the normal. The graph on the right shows the application of an overall selection pressure which shifts the mean in one direction so that the normal is altered.
The normal is pushed away from the extreme that is being selected against, because that extreme no longer exists, and so those that were not-so-extreme become the new extremes.

3. Disruptive Selection

Disruptive selection is where the normal is selected against, and the two extremes are selected for. This may ultimately lead to speciation (the evolution of a new species).
Graph: Normal distribution versus selection against the mean versus speciation
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