Module 8 Lesson 1 - 4

Conditions Necessary to Maintain Hardy-Weinberg Equilibrium

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Hardy-Weinberg equilibrium describes an ideal population that maintains its allele frequencies. Using the Hardy-Weinberg equilibrium, we can determine the effect of evolutionary influences on a given population. Five conditions are required to maintain the Hardy-Weinberg equilibrium in a population.

1. The population is closed. In other words, no immigration of individuals (alleles) in or emigration out occurs. No gene flow happens.
   
   
2. The population is large enough that chance events do not alter the frequencies. Some individuals may not produce offspring, and others may have large numbers of offspring. If the population is large enough, this chance event will not alter the allele frequencies. In other words, no genetic drift occurs.
   
   
3. Mating must be random. Individuals cannot mate based on favourable phenotypes or genotypes.
   
   
4. No net mutations occur. Mutation always occur in a given population, but to stay in equilibrium, the mutation rate from B to b must equal the mutation rate from b to B.
   
   
5. No natural selection occurs. The environment cannot favour the survival of individuals with one allele over the other.

Which populations are in Hardy-Weinberg equilibrium? If you look carefully at this list of conditions, you can see that no natural population likely can meet all these conditions. Hardy-Weinberg equilibrium is a hypothetical construct, and populations in equilibrium are very rare.

If Hardy-Weinberg equilibrium is a hypothetical situation, then why are we interested in these five conditions that lead to Hardy-Weinberg equilibrium? The five conditions keep populations in equilibrium and prevent evolution from happening. If any of the five conditions are not met, then the population is evolving.  

To review and summarize the information of this lesson, watch this video:

Hardy-Weinberg:  Mind Your P's and Q's

Self-Check

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1. Identify two kinds of populations that could exhibit the five conditions of Hardy-Weinberg equilibrium. 
   
   
2. About 11% of the Canadian population is lactose intolerant. This means some people have difficulty digesting milk products. Is this likely the same in other cultures? Explain.
   
   
3. A population of 20 deer was introduced to an island where no deer lived nor had lived previously. Although the herd included several bucks (males of breeding age), one who was much larger and stronger was able to fight the other bucks and breed with the 10 or so females. This same scenario repeated three years in a row. Is this population in Hardy-Weinberg equilibrium? Explain.
   
   
4. State a reason each of 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-beak and straight-beak birds inhabited an island. With falling sea levels, a small group of birds was able to establish a colony on a nearby island. All those birds were straight-beaked.
   
   
3. Poplar trees typically are 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 nightfall. 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 will pollinate local trees.


Check your work.

Self-Check Answers


1. Populations in game reserves or national parks might exhibit Hardy-Weinberg equilibrium as long as the population is not too small. The goal of these preserves is to maintain the genetic diversity that exists.

Agricultural livestock populations are kept in Hardy-Weinberg equilibrium to ensure uniform quality of meat and handling properties. (However, remember that considerable artificial selection and genetic drift occurred to obtain the level of uniformity within the livestock population.)


2. The percentage of lactose intolerant people in other cultures is probably not equal to that in Canada. The allele for lactose intolerance began as a mutation somewhere on the planet. Populations that are derived from the region where the mutation occurred will have a higher frequency of that allele. Furthermore, Canada does not have a geographic or cultural gene pool, but Canada's gene pool consists of genes from many geographical areas. 
3. This population of deer is not in Hardy-Weinberg equilibrium. 

• The population is small.
  
  
• Mate selection occurs and mating is not random.
  
  
• To a degree, natural selection occurs in that the strongest, largest deer (the most adapted) breeds and passes his genes to the next generation.
  
4.
1. change in mutation rate
   
   
2. small population  (Chance events of genetic drift can occur.)
   
   
3. non-random mating
   
   
4. natural selection
   
   
5. open population (gene flow)