WELCOMETOPIC:FACTORS THAT ALTER ALLELIC FREQUENCIES

SIJI SKARIAHM.Sc BOTANY

ST.THOMAS COLLEGE KOZHENCHERRY

ALLELEAn allele is an alternative form of a gene (one

member of a pair) that is located at a specific position on a specific chromosome.

Organisms have two alleles for each trait.

(RR,rr)

The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

ALLELE FREQUENCYTHE FREQUENCY OF ANY GIVEN ALLELE IN A POPULATION,

RELATIVE TO ALL THE OTHER ALLELES AT THE SAME LOCUS, IS KNOWN AS ALLELE FREQUENCY.

Allele frequency =Number of copies of a particular

allele in a population

Total number of all alleles for that gene

in a population.

FACTORS THAT ALTER ALLELIC FREQUENCIESMUTATIONGENETIC DRIFT – BOTTLE NECK EFFECT

AND FOUNDER EFFECT MIGRATIONSELECTIONNONRANDOM MATING INBREEDING COEFFICIENT

1.MUTATIONAre the source of new alleles in the gene pool.Mutation are essential for evolution.Once a mutation occurs, the allele frequency is

changed.Alleles resulting from unfavourable mutation are

selected against and only remain in the gene pool if they are recessive(remain ‘hidden 'in heterozygotes).

Neutral or silent mutations are not acted upon by selection.

Eg: A change in a base code(GGG to GGC) that codes for the same amino acid. The same protein is made-no change result from this mutation.

In this way, mutations increase the opportunity for evolution of adaptations different from characteristics of the ancestral population.

These mutations will affect the frequency of alleles.

2.GENETIC DRIFTIt was explained by Sewall Wright(1931)hence called Sewall

wright effect.

Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters.

Genetic drift can happen when a natural disaster or similar event randomly kills a large portion of the population.

The remaining survivors may have allele frequencies that were very different from the previous population, a phenomenon known as the bottleneck effect.

BOTTLE NECK EFFECTPopulation may be suddenly reduced in numbers.Usually from a catastrophic environmental

event(fire,flood,drought etc)Or by sudden, severe selection pressure(often

human activities eg:rapid habitat destruction,introduction of predators).

After the event the population may recover to grow again to return to normal levels.

As population numbers drop rapidly, it is likely that the range of alleles decreases and the frequency of alleles changes.

When small population subject to genetic drift.

FOUNDER EFFECT Another way genetic drift can occur is if a portion of

the population separates from the old population to start a new population(Island).

The alleles in the new group will be found at higher frequencies than in the original population, a phenomenon known as the founder effect.

In extreme cases, a founder population may be a single individual(wind blown seed).

3.MIGRATION

Migration is the movement of individuals from one population to another or the movement of breeding individuals into or out of isolated populations.

Results in evolutionary change because alleles move with the individuals. We call this movement gene flow.

Immigration: Individuals migrate into a population.Emigration: Individuals migrate out of a population.Both processes allow for gene flow between

populations.Gene flow may change the frequency and or the range

of alleles in the populations

If population are large, migration may have little or no effect on allele frequency.

However, if population are small, migration may have a big impact on allele frequency.

4. SELECTIONThe force of natural selection tends to reduce the

genetic variability of populations in a way that increases adaptation.

It does so by removing or reducing the frequency of some phenotypes and increasing the frequency of others.

Environmental factors(biotic&abiotic)act as a selecting agents of phenotypes.

When environmental factors change, different phenotypes will be selected for.

As phenotype is largely determined by genotype, successful genotype alleles will increase in frequency in the gene pool.

Favourable alleles increase in frequency in a gene pool, while unfavourable alleles decrease.

After a certain number of generations, the frequency of alleles and phenotypes might change so that the population becomes reproductively isolated from others of that species.

It is now a new species.

Three kinds of selection cause changes in the normal distribution of phenotypes in a population.

1. Stabilizing selection- eliminates those phenotypes most different from the norm, thus reducing the frequency of phenotypic extremes.

•Stabilizing selection using phenotype variation within a population of butterflies as an example.

•Imagine a butterfly population that shows continous variation in wing colour.

•Suppose there is a shift in the environment, so that forms at either end of the curve are selected against.

•Overtime, if the environmental pressure remains constant the more extreme forms will be eliminated.

•Eventually, intermediate phenotypes will become predominant in the population.

2.Directional selection-This selection is always associated with environmental change.

Examples: Evolution of horse is a good example of directional selection in which a small forest dwelling animal, Hyracotherium, had to undergo successive changes in its body when the environment changed from forest to grassland, giving rise to a tall, fast-running, grazing horse.

3.Disruptive selection- eliminates average phenotypes and encourages the extremes. This tends to result in distinct phenotypes in the same population.

For example, animals that can avoid predation by being a certain color.

There are cases of non-poisonous butterflies evolving directional color changes that make them look like poisonous butterflies, because their predators tend to avoid the color of the poisonous butterflies.

5.NONRANDOM MATINGNon-random mating means that individuals of

many species have a choice about which partners to mate with.

The result of nonrandom mating is that some individuals have more opportunity to mate than others and thus produce more offspring (and more copies of their genes) than others.

INBREEDING COEFFICIENTThe mating of closely related individuals such as

cousins, self-fertilized plants, which tends to increase the number of individuals that are homozygous for a trait.

Inbreeding coefficient (F) F measures the probability that two genes at any locus in

an individual are identical from the common ancestor(s) of the two parents.

This means the degree to which two alles are more likely to be homozygous (AA or aa) rather than heterozygous (Aa) in an individual, because the parents are related.

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