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Gene pools

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Natural selection

In the wild, each species may exist as one population or multiple populations. Different populations correspond to defined areas - habitats.

The sum of all present alleles for a given gene in a given population is known as the gene pool.

This is essentially a way of thinking about all the individuals in a population contributing their alleles towards the overall allele frequency. The extent of different alleles present gives the genetic diversity of a population.




The allele frequency in a population's gene pool can change as a result of selection. The effectors of selection can be varied, yet the outcome is similar: advantageous or preferred alleles and the traits associated with them increase in frequency, while detrimental or disfavoured alleles and the traits associated with them decrease in frequency.

Selection can tend towards a "happy medium" and avoid either extreme. This is stabilising selection. If really small lions don't survive long, but really large lions can't supply themselves enough food, then the average lions are selected for and achieve the highest frequency.




There is another type called disruptive selection. Instead of shifting the traits towards a middle ground, disruptive selection splits the pool down the middle, where both extremes of a trait are favourable, but not a middle value.


An example of this is an original population of purple individuals which stand out quite a lot amongst red and blue flowers in a field. They will end up shifting towards either red or blue, but not staying purple as this attracts predators.


Genetic drift

The background fluctuation of allele frequency in a population that is not a result of any selection pressure, just chance, is called genetic drift. This can be due to deaths and sampling variables. For example, if offspring are the basis of the allele assessment, that does not provide the information regarding any alleles their parents might have had. Data on many generations would need to be obtained in order to draw conclusions regarding changes in allele frequency due to selection pressures. 





The founder effect

Suppose a boat travelled from one island to another. In the process, several lizards were transferred from the first island to the other. The lizards breed and settle down to form a new lizard population on their new island. This is called the founder effect. The small number of founding lizards formed the genetic base on which the whole population was built. This genetic base is significantly smaller than that of the original lizard population on the first island.

Therefore, the genetic diversity of the new population is lower than that of the original population.


Genetic bottlenecks

The only difference between the founder effect and genetic bottlenecks is the way in which the new genetic pool is formed. In the founder effect the new pool is formed when a few individuals from a population become geographically isolated, while in genetic bottlenecks the new gene pool is formed when only a few individuals from a population survive a mass disaster, or are the only ones to breed.

The effect is the same: the genetic variation of the new population is decreased compared to the original population.





The Hardy-Weinberg principle

How could we keep track of the frequency of each allele for a given trait when we have a dominant-recessive interaction? More specifically, how could we account for the visible dominant traits as homozygous or heterozygous, since both look the same?

This is where the Hardy-Weinberg principle comes in. Firstly, there are criteria for when this principle may be applied to a population:

1. Random mating must take place.

2. No migration must occur either inwards or outwards of the population.

3. No mutations must arise in the population.

4. No natural selection must take place due to one trait being better or worse adapted to the environment.

It's apparent that this is simply rarely, if ever, the case in a real wild population. However, the Hardy-Weinberg principle is useful at predicting allele frequencies in a reliable mathematical model.

The frequency of the dominant allele is noted while that of the recessive allele is noted q. Both must necessarily account for the whole population, therefore:

p + q = 1

The values are frequencies, so they are noted as percentages. 1 is 100% while 0.5 is 50% and 0.05 is 5%, etc.


Worked exercise

If we know that the frequency of the allele for dark fur in a population of koala bears is 0.2, and this allele is dominant over the one for light fur, work out the frequency of the allele for light fur in the population.

p = 0.2

p + q = 1

Therefore, 0.2 + q = 1 so q = 1 - 0.2

q = 0.8 or 80%.



Now the allele frequency has been worked out, how could we work out the actual phenotype of the koala bears in the population. How many are actually dark-furred? How many of the dark-furred ones are homozygous?

For this we use the same equation as before, but squared: (p + q)2 

This is equivalent to p2 + 2pq + q2 = 1

Where 2pq is the frequency of heterozygotes, and p2 and q2 the frequencies of homozygous dominant and homozygous recessive respectively.

We want to know how many koala bears have dark fur. We know that the allele frequency for dark fur is 0.2, so 0.22 is the percentage of homozygous dark fur individuals; = 0.04 (4%).

This trait being dominant, the heterozygotes must also have dark fur. The frequency of heterozygous dark fur is 2pq = 2*0.2*0.8 = 0.32 (32%).

So overall, there are (0.4 + 0.32) 0.36 or 36% dark-furred koala bears in the population.

This leaves the remaining 64% with light fur. Note the contrast between the light phenotype only being 64% while the allele frequency for light fur is 80%. If the allele were dominant over dark fur, the frequency would be higher rather than lower.

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