Chapter 24:

Population

GeneticsLesson 24.4: Genetic Drift

Learning Objectives

• Describe the phenomena of the bottleneck effect and founder effect and explain how they contribute to changes in allelic frequencies.

• Explain what is sometimes referred to as "survival of the luckiest."

Genetic Drift

• So far, natural selection was the explanation for genetic change

• What is genetic drift?• Changes in allele frequencies due to

random chance

• Observation that by chance, allele frequencies “drift” from one generation to the next.

Genetic Drift

• Allele frequency changes occur regardless of those allele-carrying individuals and their fitness.

• For example:• How is it known which alleles will be

in the gametes that fuse together during fertilization?

• It’s influenced by random chance!

Genetic Drift

• Has a greater impact in small populations

• Effects of Genetic Drift• Elimination or fixation of an allele• Depends on if allele frequency reaches

0 or 100%

• Number of generations needed for this depends on population size

Genetic Drift and Population Size

Question

• The relative effect of random chance (a.k.a. random sampling error) is much smaller/bigger when the sample size is large/small.

• The relative effect of random chance (a.k.a. random sampling error) is much smaller when the sample size is large.

Genetic Drift Has a Greater

Impact in Small Populations• In the end, regardless of population

size, genetic drift = allele loss or fixation• More generations are needed in large

populations than small ones

• Decrease in population size could allow genetic drift to alter allele frequencies.

Bottleneck Effect

• Different events can get rid of population members w/out regard to genetic composition• Events: earthquakes, floods, drought,

human destruction of habitat

• Bottleneck effect: allele frequencies in a population change due to genetic drift

Bottleneck Effect

• 2 reasons for change:1.Surviving population members have

different allele frequencies than original population

2.When population is small, genetic drift reduces genetic variation fastera.Alleles could even get eliminated

Examples of the Bottleneck Effect

Founder Effect

• Small group of individuals separates from large population, and establishes a colony in a new place.

• Ex.) few people leave a population and become founders of a separate island.

Founder Effect

Founder Effect

• 2 major consequences:

1. The smaller new population won’t have as much genetic variation as the original, larger population.

2. The allele frequencies in the new population will be different than the original population (by chance).

• Ex. of the Founder effect: Old Order Amish population in Pennsylvania.

• Group of 8,000 people left the original group.• That newer population had Ellis-van Creveld

syndrome at 7% frequency rate (much higher compared to original population).

The Neutral Theory of Evolution

• Majority of genetic variation is due to genetic drift instead of natural selection.• Reason: genetic drift can affect

frequencies of helpful and detrimental alleles.

• Genetic drift promotes neutral variation, which does not affect reproductive success.

The Neutral Theory of Evolution

• Neutral theory of Evolution: most genetic variation is caused by the accumulation of neutral mutations that got high frequencies through genetic drift.

• Ex.) a mutation in a gene that changes one codon won't affect the entire amino acid sequence of the encoded protein, so both genotypes are still equal in terms of fitness.

The Neutral Theory of Evolution

• Non-Darwinian evolution: new mutations can spread throughout a population because of genetic drift.• a.k.a. "survival of the luckiest".

• Darwin's idea: natural selection is responsible for adaptive changes during evolution is accepted • Ex.) giraffes with their long necks

• Kimura's idea: most of DNA sequence variation is due to neutral variation.

• At the genomic and molecular level, genome sequencing from different species is consistent with the neutral theory of evolution.

• Ex.) changes of the coding sequences in structural genes