Download DNA & RNA

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.
2.
3.
4.
5.
No natural selection
No mutations
No gene flow
No genetic drift
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 = .4
Equilibrium
Actual
p2 =
25/50
2pq =
10/50
q2 =
15/50
• Since the numbers are not identical, we
know this population is not in HardyWeinberg equilibrium