Y12 genetic variation 3 Multiple alleles to natural selection

Multiple alleles

When genes can have more than 2 alleles. 2 are still selected to give the genotype.

Lethal allele

An allele that produces a phenotypic effect that causes the death of the organism.

Gene pool

This is all the alleles of all the genes that exist in a population of living things at any one time.

natural selection

A natural process where organisms with favourable alleles will be able to adapt to the changing environment (selection pressure). Those able to adapt will survive and reproduce successfully, those that can't adapt will die out and not reproduce.

Allele frequency

Number of times that an allele occurs in a gene pool compared with the number of alleles in that pool for the same gene

Stabilising selection

A type of natural selection which favours the mean of the distribution because the extremes are at a selective disadvantage. Occurs when environment is stable.

Directional selection

Is where individuals of one extreme type are more likely to survive and reproduce.

This happens when the environment changes, and causes corresponding genetic change to the population

Disruptive selection

Is where two different extreme types are selected for perhaps because they live in two different habitats.

This leads to two distinct types developing, and eventually these may become different species.

Migration

Individuals moving into (immigration) or away (emigration) from an area.

This can bring new alleles into a gene pool or reduce allele frequency. This can lead to more variation.

Genetic drift

This is the change in allele frequencies in a population due to chance (or random).

Some alleles can be lost from the small population. This can make a big difference in the frequency of particular alleles.

Founder effect

This is when a small group move away from the larger population and begin a new population elsewhere.

They need to adapt to the different environment and end up evolving differently from the original 'founder' species. They have different genes.

Bottleneck effect

This is when there is a big reduction in the size of a population and the frequency of some alleles as a result of a natural disaster, e.g. fire, flood, earthquake.

The survivors will be a random small number, which may not carry all of the alleles that were first present. Some alleles will be in higher amounts, where others could be lost.

This reduces variation and causes extinction.

Genetic biodiversity

different genes contained within a population

Evolution

The gradual change in a species over time as living things adapt to their changing environment. Those with beneficial alleles will adapt and reproduce successfully passing beneficial alleles on to the next generation.

Genetic drift

This is the change in allele frequencies in a population due to chance (or random).

Some alleles can be lost from the small population. This can make a big difference in the frequency of particular alleles.

Sex linked genes (alleles)

A sex-linked gene is located on one of the chromosomes that is responsible for the sex of the individual. Like humans, XX = female and XY = male