1. The Hardy-Weinberg Equilibrium
Allele Frequencies in a Population
G.H. Hardy
English Mathematician
Dr. Wilhelm Weinberg
German Physician

2. KEY CONCEPT Hardy-Weinberg equilibrium provides a framework for understanding how populations evolve.

3. Hardy-Weinberg equilibrium describes populations that are not evolving.
Biologists use models to study populations.
Hardy-Weinberg equilibrium is a type of model.

4. Hardy-Weinberg Equilibrium
“Allele and genotype frequencies in a population tend to remain constant in the absence of disturbing influences”
Disturbing influences:
non-random mating
mutations
selection
limited population size
random genetic drift
gene flow
migration

5. In Reality…
The conditions for Hardy-Weinberg equilibrium are never met in nature.
There are always some disturbing influences in nature
Hardy-Weinberg equilibrium can be approximated in the lab
It has usefulness as a model for studying real populations

6. The Hardy-Weinberg equation is used to predict genotype frequencies in a population.
Predicted genotype frequencies are compared with actual frequencies.
used for traits in simple dominant-recessive systems
must know frequency of recessive homozygotes
p2 + 2pq + q2 = 1
"The Hardy-Weinberg equation is based on Mendelian genetics. It is derived from a simple Punnett square in which p is the frequency of the dominant allele and q is the frequency of the recessive allele."

7. p + q = 1 p2 + 2pq + q2 = 1 The Equations
A gene has two alleles, A and a
The frequency of allele A is represented by p
The frequency of allele a is represented by q
The frequency of genotype AA = p2
The frequency of genotype aa = q2
The frequency of genotype Aa = 2pq

8. An Example…
Assume a population in which 36% of the population are homozygous for a certain recessive allele, a. Assume the population is at equilibrium.
Question #1: What is the frequency of the recessive allele, a in this population?

9. An Example…
Assume a population in which 36% of the population are homozygous for a certain recessive allele, a. Assume the population is at equilibrium.
Question #2: What is the frequency of the dominant allele, A in this population?

10. An Example…
Assume a population in which 36% of the population are homozygous for a certain recessive allele, a. Assume the population is at equilibrium.
Question #3: What percentage of the population are homozygous for the dominant allele, A?

11. An Example…
Assume a population in which 36% of the population are homozygous for a certain recessive allele, a. Assume the population is at equilibrium.
Question #4: What percentage of the population are heterozygous for this trait?

12. An Example…
Assume a population in which 36% of the population are homozygous for a certain recessive allele, a. Assume the population is at equilibrium.
Question #5: Why do we have to start the problem with the percentage of the homozygous recessive in the population?
Answer: It is not possible to tell the homozygous dominant (AA) from the heterozygous (Aa) by examining the phenotype!

13. Genetic drift changes allele frequencies due to chance alone.

14. Gene flow moves alleles from one population to another.

15. Mutations produce the genetic variation needed for evolution.

16. Sexual selection selects for traits that improve mating success.

17. Natural selection selects for traits advantageous for survival.

18. In nature, populations evolve.
expected in all populations most of the time
respond to changing environments

19. SPECIATION KEY CONCEPT New species can arise when populations are isolated.

20. The isolation of populations can lead to speciation.
Populations become isolated when there is no gene flow.
Isolated populations adapt to their own environments.
Genetic differences can add up over generations.

21. Reproductive isolation can occur between isolated populations.
members of different populations cannot mate successfully
final step to becoming separate species
Speciation is the rise of two or more species from one existing species.

22. Populations can become isolated in several ways.
Behavioral barriers can cause isolation.
called behavioral isolation
includes differences in courtship or mating behaviors

23. Geographic barriers can cause isolation.
called geographic isolation
physical barriers divide population
Temporal barriers can cause isolation.
called temporal isolation
timing of reproductive periods prevents mating