Module 8 Population Genetics

4.8.1 page 4

Explore

It would seem logical that if each breeding couple removes two alleles from the gene pool in order to reproduce, and then returns them to the gene pool in the form of offspring, that the relative frequencies of each allele in a gene pool should not change from generation to generation – in other words the population should stay in equilibrium.

 

 

Hardy Weinberg Equilibrium (HWE)

Godfrey Hardy and Wilhelm Weinberg are famous for making just that observation: That as long as certain conditions are met, allele frequencies should stay the same, generation after generation - a situation they so modestly called “Hardy Weinberg Equilibrium.”

 

Conditions necessary to Maintain Hardy Weinberg Equilibrium

1. The population is closed. (In other words, no immigration of individuals (and alleles!) in, or emigration out. In other words, there must be no gene flow.
2. The population is large enough that chance events (e.g. individuals who don’t mate) will not alter the frequencies. (Missing one penny out of 5 is a much bigger event statistically than missing one penny out of a thousand). No genetic drift.
3. There must be random mating. (No picking favorite phenotypes or genotypes as mates!)
4. There are no net mutations (There are always mutations, but to stay in equilibrium, the mutation rate from B to b has to equal the mutation rate from b to B.)
5. There is no natural selection. (The environment can not be favoring the survival of individuals with one allele over the other.)

 

So, which populations are in HWE? If you look carefully at this list, you can see it would be difficult if not impossible to think of a natural population that meets all these conditions – which is exactly the point. Hardy Weinberg Equilibrium is a hypothetical construct, and we rarely see populations in equilibrium.

 

If HWE is largely a hypothetical situation, then why are we interested in these 5 conditions that lead to HWE? The five conditions listed above keep populations in HWE and prevent evolution from happening. If any of the HWE conditions are not met, then the population is by definition, evolving.  

 

Read

To review these concepts read p. 680-82 of your textbook: Introducing the Hardy Weinberg Principle. You may wish to make summary notes, or include some example problems with their solutions in your course folder.

 

Watch and Listen

To review and summarize the information of this lesson, you may view the following video entitled The Hardy-Weinberg Principle: "Minding Your p's and q's.

 

Try This

TR 1. Identify two kinds of populations that would exhibit the five conditions of HWE. 

TR 2. Look at p. 700 of the text, question 4. Note that 11% of the Canadian population is lactose intolerant. Would this be the same in other cultures? Why or why not?

 

Self-Check

SC 3. A population of 20 deer was introduced to an island where no deer had previously lived. Although there were several bucks (males of breeding age), one who was much larger and stronger was able to fight off the other bucks, and was thus able to breed with the 10 or so females in the breeding population. This same scenario repeated itself for three years in a row. Is this population in Hardy Weinberg equilibrium? Why or why not?

 

SC 4. State a reason why the following might NOT be in Hardy Weinberg Equilibrium: 

1. Populations growing within range of radiation from the Chernobyl nuclear power plant in the Ukraine in 1986.
2. A mixed population of hooked and straight beaked birds inhabited an island. With falling sea levels, a small group of birds were able to establish a colony on a nearby island. All of those birds were straight beaked.
3. Poplar plants are typically wind pollinated. In the absence of wind during flowering, many of the ‘flowers’ are fertilized by pollen from the same flower.
4. Evening-scented stocks are flowers that give off a beautiful fragrance towards night-fall. Some flowers are more fragrant than others and attract more bees.
5. In very strong winds, poplar pollen from distant populations can be brought in and end up pollinating local plants.

Module 8: Lesson 1 Assignment

Submit your completed Module 8: Lesson 1 Assignment to your teacher for assessment.