09 Apr Evolution Homework

In Mendelian genetics, you examined the probability of getting different genotypes among offspring of specific parents and noted the role that probability played in that process.  Now, we are looking at population genetics which involves following the fraction (frequency) of particular alleles in a specific population across generations.  When allele frequencies change across generations, this is considered to be evolution.

When we talk about the process of evolution which we can define as a “change in the genetic makeup of a population over generations” or a “change in allele frequencies across generations”, we note both that 1) evolution happens in populations, not individuals and 2) that the time scale for evolution is generations, not years.  There are 4 processes that produce evolutionary change in populations:

• Natural Selection

• Genetic Drift

• Migration (aka gene flow)

• Mutation

For this assignment, write out answers to the questions below IN YOUR OWN WORDS.

1. Assume for a moment that there are 2 different dominant genetic disorders in humans:

Disorder A has no negative effects for most of one’s life but kills those with it between ages 55 and 60 so it shortens lifespan.

Disorder B causes infertility (an inability to have children) but, otherwise, does not affect overall health or survival.

The mutations causing these are on different genes and chromosomes.

Assuming that the mutations that produce these 2 disorders are equally likely and have been occurring as mutations for hundreds of generations, which of the two alleles, the one causing Disorder A or the one causing Disorder B, is expected to be most common in the human population (based only on the information given)?  Explain your answer.

2.  The evolution of resistance to antibiotics by bacteria is a major concern for the medical field as we rely on antibiotics to treat bacterial infections.  When humans take antibiotics, if a few individual bacteria are NOT killed by the antibiotic, they will be the parents of the next generation and this ability to NOT be killed (resistance) will become more common over time.  

Bacteria tend to evolve faster than human beings.  What is the MAIN reason for this?  Think about the time scale on which evolution operates.  

3.  Two individuals are arguing.  One says that genetic drift ONLY occurs in small populations.  The other insists that genetic drift occurs in populations of all sizes.  

a)  Which one is correct?  Explain

b)  What is the source of this confusion?  Why is population size an issue for genetic drift?

4.  In the case of complete dominance, we discussed how dominant alleles are NOT more likely to be “favorable” than recessive alleles and that dominant alleles are not destined to increase in frequency in a population.  Whether an allele is “good” or “bad” compared to another allele depends on the particular population and particular environment.  

However, a dominant allele will be “seen” by natural selection even if an individual only has one copy (since natural selection acts on phenotype) whereas a recessive allele will only be “seen” if an individual has 2 copies of that allele.  

a) Given that, will natural selection tend to increase the frequency of a favorable dominant allele faster or slower, on average, than a favorable recessive allele?  Explain.

b) Consider an allele that is unfavorable (so selection acts to reduce its frequency in the population over time).  Once an allele becomes rare, which type of allele (dominant or recessive) will tend to persist at a higher frequency over time?  Explain. 

c) Describe a hypothetical (or real) example of how which allele (of 2) is most favorable can change in 2 different environments.  (Allele G1 is more favorable in environment A while allele G2 is more favorable in environment B. Provide some detail about why (what causes the change in which is more favorable)!  

5.  When people discuss natural selection, they often focus on survival.  

Assume all individuals in a population have the same probability of survival, could natural selection still act in that population?  Explain.