Changing Allele Frequency

Chapter 23

What you need to know!

• The conditions for Hardy-Weinberg Equilibrium

• How to use the Hardy-Weinberg equation to calculate allelic frequencies and to test whether a population is evolving

Sources of Microevolution

• Changes in the allele frequency of a single population

• Only populations can evolve (not individuals)• Natural Selection: differential reproductive

success of certain phenotypes lead to a(n) increase/decrease of certain alleles

• Mutation: introduces new alleles• Gene flow: add or remove alleles to a gene

pool based on migration

Sources of Microevolution

• Genetic Drift: Random change of allele frequency in small populations

• Founder Effect: spike in gene change due to genetic drift after a small population inhabits a new region

• Bottleneck effect: a small surviving group (near extinction) gives rise to a new population with a dramatically different gene pool

Sources of Microevolution

• Non-random mating:

• Sexual Selection

• Mating more often occurs between close neighbors than distant neighbors

• Inbreeding in small populations

Genetic Equilibrium

• In 1908, 2 mathematicians (Hardy & Weinberg) stated that the allelic frequency in a given population accounts for changes in populations

• They develop the concept of genetic equilibrium: how alleles in a population could stay constant from one generation to another (no evolution)

Equilibrium Requires

1. No natural selection

2. No mutations

3. No gene flow

4. No genetic drift

5. Random mating

Variables

• We have two copies (alleles) for each gene– Dominant alleles– Recessive alleles

• p = frequency of dominant alleles– p = (# of dominant alleles)/(total alleles)

• q = frequency of recessive alleles– q =(# of recessive alleles)/(total alleles)

• Check your work: p + q = 1

Example

• A rabbit population has two different alleles for fur color:

• B = brown and b = white• The rabbit population has 50 members• 25 rabbits are BB - brown• 10 rabbits are Bb - brown• 15 rabbits are bb – white• Find p and q

Genotypic Frequency in Equilibrium

• Homozygous dominant genotypes = p2

• Heterozygous genotypes = 2pq

• Homozygous recessive genotypes = q2

• The sum of all genotypes = 1

• p2 + 2pq + q2 = 1

Example

• Are our rabbits in genetic equilibrium?• p = .6, and q = .4Equilibrium Actualp2 = 25/502pq = 10/50q2 = 15/50• Since the numbers are not identical, we

know this population is not in Hardy-Weinberg equilibrium