Brent Cornell

    
Genetic drift is the change in the composition of a gene pool as a result of chance or random events

• It will occur faster and be more significant in smaller populations, where chance events have a bigger impact on the gene pool
• Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift

Allele frequencies will change significantly when a large population is reduced to a small population

• Two mechanisms by which this population change may occur are via population bottlenecks and the founder effect

Population Bottlenecks

Population bottlenecks occur when an event reduces population size by an order of magnitude (~ >50%)

• These bottlenecks may result from natural occurrences (e.g. fires, floods, etc.) or be human induced (e.g. overhunting)
• The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift
• As the surviving members begin to repopulate, the newly developing gene pool will be divergent to the original
• Example:  Northern elephant seals have reduced genetic diversity compared with southern seals due to overhunting

Population Bottleneck – Gene Pool Comparison

Founder Effect

The founder effect occurs when a small group breaks away from a larger population to colonise a new territory

• As this population subset does not have the same degree of diversity as a larger population, it is subject to more genetic drift
• Consequently, as this new colony increases in size, its gene pool will no longer be representative of the original gene pool
• The founder effect differs from population bottlenecks in that the original population remains largely intact
• Example:  Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community

Founder Effect – Gene Pool Comparison

Allele Frequency Comparisons

Allele frequencies represent the prevalence of a particular allele in a population, as a proportion of all the alleles for that gene

• Consequently, allele frequencies are either represented as a percentage or as a value from 0 to 1.0

Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection)

• Population bottlenecks and the founder effect will exacerbate genetic differences between geographically isolated populations
  
  

Allele frequencies between populations can be compared using the Allele Frequency Database (ALFRED)

How to Compare Allele Frequencies:

1.  Go to the Allele Frequency Database (ALFRED)

2.  Type a gene name into the search parameter

3.  Choose a specific gene loci

4.  Select a polymorphism

5.  Choose a frequency display format

Examples:

• SCL45A2 – involved in human melanin production (allele frequency differs in Northern Europe)
• ABO – blood groups show regional variation (e.g. O blood more common in Africa)
• LCT – lactase is required for lactose tolerance (more common in Northern Europe / Africa)

Comparison of Frequency of Leu → Phe Mutation in the SCL45A2 Gene