9C Gene Flow & Genetic Drift

U4 AO2

Importance of Genetic Variation

Mutations introduce new alleles (variations) into a population.

This genetic diversity produces a variety of phenotypes.

Having a large amount of variation or genetic diversity within a species allows species to adapt to changes in the environment, increasing chances of survival.

Gene Flow

Gene flow is the movement of alleles into or out of a population due to migration or interbreeding.

 

Migration can include emigration from (exiting) or immigration to (entering) a population.

Gene Flow: Effect on Allele Frequencies

A population may gain or lose alleles through gene flow:

• Emigration results in a loss of alleles

• Immigration results in the gain of alleles

 

When individuals temporarily enter a population and interbreed with local individuals before leaving again, they contribute to the gene pool of that population.

Genetic Drift

Genetic drift is the change in a population's allele frequencies (gene pool) due to random/chance event

The effects of genetic drift are strongest in small populations.

 

Reduces the genetic diversity of a population through random removal of alleles. This means one phenotype which does not necessarily provide a selective advantage for survival can, by chance, become the predominant allele in the population.

Genetic Drift occurs through

• The bottleneck effect

• The founder effect

The Bottleneck Effect

The bottleneck effect is a severe reduction in genetic diversity due to a catastrophic event that may change allele frequency.

 Bottleneck events can dramatically decrease population size quickly, impacting allele frequencies.

The remaining individuals have less genetic, and therefore phenotypic, variation. The remaining population is not genetically representative of the original gene pool and is often affected by inbreeding and genetic drift.

Bottleneck Effect Formulaic Answer

1. Catastrophic event severely reduces the population size and the surviving group of individuals are genetically unrepresentative of the original population

2. This causes a change in allele frequency compared to the original population

3. Remaining population is likely to have reduced genetic diversity compared to original population.

The Founder Effect

The founder effect occurs when a small number of individuals, that are genetically unrepresentative of the original population, colonize a new location.

 

VCAA: The small number of individuals will not have all the variations of alleles as seen in the original population therefore have less genetic diversity.

 

Allele frequencies change as a result of the migration of a small subgroup of a population.

Founder Effect Formulaic Answer

1. A group of individuals who are genetically unrepresentative of the original population break away from and colonize (found) a new population.

2. Populations may experience different selection pressures and so allele frequencies change.

3. Overtime, the two populations become so different that they cannot interbreed to produce fertile offspring, becoming different species.

Genetic Drift: Effect on Allele Frequencies

Genetic drift reduces genetic variation in a population as shown by the effects of the bottleneck and founder effect variation has two major risks:

 

1. Inbreeding: keeps harmful alleles in the population.

2. Lower adaptive potential: entire population is vulnerable to new selection pressures due to the absence of advantageous alleles.

 

As genetic diversity decreases, the chance of extinction increases; no members of a particular species exists.

Founder vs Bottleneck Effect

The founder effect is similar in concept to the bottleneck effect, but it occurs via colonization rather than catastrophe.