Genetics 3E
Principles of Natural Selection
Natural selection is a driving force which causes an organism to change over time. A single organism does not control its genes which are inherited from its parents and no organism is perfectly adapted to its environment. But the organism is subjected to external forces through changes to its environment.
There are any number of these forces, they include
species competition, sexual selection, climate change, predation, and disease.
If the organism is to survive then its genes must change to ensure the organism adapts to any change.
Nature selects for those genes which are best fitted for the organism to survive. Thus, natural selection is constantly influencing the evolution of the species.

By contrast it has been seen that genetic drift is a random process the results of which can be advantageous or detrimental to the organism. Natural selection by contrast is not random but is driven by changes to the environment.
Likewise gene flow tends to keep the alleles in a population in an homogenized state. Natural selection always increases genetic variation.

There is genetic variability within each gene pool which causes each individual to be sightly different from the rest. These slight variations can lead to an improved performance by which the individual reproduces. This in turn allows the individual to create more of the genetic variations which helped it to survive - a positive feedback. Thus the offspring will further benefit from the genetic variations which allowed its parents to succeed. Over time, the frequency of the more successful organisms will increase.

Thus natural selection can be defined as the reproductive success of classes of genetic variants in the gene pool.

Any of the organisms without these genetic variants will not reproduce as much and so in the long run will be less successful and will go extinct. Nature then is constantly exerting a selective force on the different genetic combinations that try to reproduce, and in this way, natural selection is the major driving force of evolution.

How is it that a particular organism within the gene pool is more successful than its neighbours ?
DNA does not always replicate perfectly in the replication process; mistakes occur which result in mutations. The new variety of gene so produced - the allele - can be beneficial to the organism making it fitter to survive any change to its environment. Because each single organism is genetically unique there is no way of predicting when or where on the genome a mutation will take place. Nature only selects those mutations which are beneficial.

Types of Natural Selection
Natural Selection can act on any trait which is heritable. The trait can normally be observed in the phenotype: examples are a colour, length of leg or wing, height etc. Natural Selection tends to do one of three things within the population regardless of the trait: it can:
keep the trait the same - stabilizing selection
move the trait in one direction - directional selection
select for extreme values of the trait - disruptive selection.
trait disribution
The distribution of a trait within a population can be shown as a standard bell curve.
Here it is shown that about 68% of the population lies within a ⅓ of the width, and 95% of the population within ⅔ of the width.
The area underneath the curve represents the total number of individuals carrying the said trait.
Stabilizing Selection
is experienced within a population when pressure is exerted on the extreme phenotypes (dark colours). That is it selects against those individuals showing the outmost value of the trait. This means that the most common phenotype in the population is selected which is then reproduced in future generations. As will be seen below Stabilizing Selection is the opposite to Disruptive Selection;
Stabilizing Selection causes a narrowing of the phenotype which in time means there will be fewer phenotypes in the population and therefore less genetic diversity. More individuals possess the mean value of the trait
stabilizing selection
directional selection
Directional Selection
in a population flavours an extreme phenotype over the others; the phenotype at the other end of the distribution (dark colour) curve is selected against. Thus the mean of the population graph shifts to the right.
This is often seen when a population moves to a new area and experiences new environmental pressures.
disruptive selection
Disruptive Selection
occurs when selection pressure acts against those individuals in the center of the distribution trait (dark colour). The population is then pushed towards the two extremes of the curve. The result is that the curve becomes bi-modal and in time two separate species might be produced. Note that in this form of selection the genetic variation within a population is increased.

In addition to the three forms of Natural Selection described above, two further examples should be considered.
Sexual Selection
is a kind of natural selection in which the different genders in a species exert forces on each other. This results in changes in appearance or traits in their offspring. These traits might appear to be somewhat arbitrary in that they do not appear to serve a function in reproduction. For example in birds the following examples might evolve: brightly colored feathers, the ability to do a ritualized dance, or certain nesting traits such as decorating. Some birds have adopted complex mating rituals to choose potential mates. Sexual selection can then produce some bizarre results as seen in birds of paradise.

Predator-Prey Selection
A predator will always try to catch and consume the easiest food source. As a result this causes the prey to evolve to be harder to catch. In turn, the predator becomes faster and more agile. This cycle is continuous and predators and prey are constantly causing each to change.

How does evolution lead to speciation?

It has been mentioned that each species is contained in its own gene pool. By definition a gene pool is the sum total of all the genes and their allelic variations. However, within any gene pool there are populations within which there are genetic variations. These arise simply because of the distribution of the number of different alleles and their frequencies because of the influence of the environment and time. Thus the composition of the populations can change over time through evolution. This occurs through various mechanisms as decribed above and on the previous page. The result is that each population within the gene pool is changed so that it is attuned to its own environment. Thus each gene pool determines which phenotypes will prevail in the population at any given time.

Speciation is the formation of new species. This occurs when the gene pool splits into two distinct lineages. The gene pools then become separated and isolated so that in time there is no interbreeding between the two populations. There are two basic ideas how the gene pools become separated. Firstly through geographic isolation (Allopatric Speciation), and secondly where reproductive isolation occurs within a single population without geographic isolation (Sympatric Speciation)

Geographic isolation most often occurs with populations that become completely separated by a physical barrier, such as a mountain range, river, or desert. The separated populations adapt to their own unique environments, becoming so genetically different from one another that members of one population cannot breed with members of the other. Perhaps the best example are the Darwin’s finches on the Galapagos Islands. Each island has its own species of finch which is locally adapted.

Sympatric speciation is thought to result from a combination of sexual selection and ecological factors. One of the best examples are the cichlid fishes in Lake Nyasain East Africa. Here it is seen that fishes of the same species form their own assemblages. It is thought that females within each assemblage develop a strong affinity for males having specific phenotypic traits like colour or size.
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