STUDY QUESTIONS #4

Due in lab week of March 6

Write brief but complete answers to questions 1,6, and 10

1. Why is natural selection the only cause of evolution that can produce adaptation?

Other causes of evolution are genetic drift, mutation and migration. Genetic drift and mutation are random processes, producing changes in gene frequencies that have no relationship to what might be advantageous in a particular population. Migration makes populations more genetically similar, and can be disadvantageous if organisms migrate between environments where selection acts differently. Only natural selection leads to evolution that increases fitness, by NON-RANDOMLY increasing the representation in a population of genotypes that are better adapted to their environment. These are the individuals that survive and reproduce better than others.

2. Why is selection against a deleterious recessive allele so much less effective at ridding the population of the allele than is selection against a deleterious dominant allele?

When a recessive allele reaches low frequency, and most of the homozygotes have been eliminated by selection, there are still many individuals in the population that are heterozygous, thus bearing one recessive allele. The allele is invisible to natural selection when heterozygous, because heterozygotes don't show the recessive phenotype. Thus, it's very hard to get completely rid of a deleterious recessive, because heterozygotes serve as a reservoir.

3. Define directional, stabilizing and disruptive selection. Draw a picture and give an example of each.

Directional selection occurs when one end of the phenotype distribution has a higher fitness than individuals with other phenotypes (e.g., selection for larger beak depth in Galapagos finches after the drought). In stabilizing selection, individuals with an intermediate phenotype have the highest fitness, with individuals that are either greater or less than the mean at a disadvantage. A real example is human birth weight, but the one I gave in class was root length-- it's bad to have roots that are too short to reach the water table, but also bad to have roots that are too long because energy is used for them that could be used for reproduction.

4. Why does natural selection depend on the environment in which an organism is located?

Natural selection is defined as differential survival or reproduction of individuals with different phenotypes. In different environments, the phenotypes that work the best differ from those that work well in other environments. Thus, the individuals with best survival and reproduction in different environments are liable to have different phenotypes (extreme example, long ears are great for rabbits but bad for whales). This is what we mean when we say that selection may act differently depending on the environment.

5. Evolution by natural selection occurs within populations, and does not produce changes in an individual during its lifetime. Why is this the case?

An individual cannot change its genotype, and evolution is the change in gene frequencies or in the mean of genetically based traits. Though the genotype of an individual is fixed, the average genotype of a population can change. This is evolution.

6. What evidence did the scientists who studied the evolution of bill shape in finches on the Galapagos provide to document that the change toward deeper bills was actually an evolutionary change?

They showed (a) that the birds that survived the drought had deeper bills than did the average bird before the drought, (b) that beak depth was passed on from parents to offspring, and that (c) beak depth changed over time in the progeny of the surviving birds.

7. Why do human activities cause so many opportunities for natural selection?

Humans have a huge impact on the environment of many animals, and can cause very pronounced and rapid environmental changes. When the environment changes, different traits may be favored by natural selection, because it may take a different phenotype to be successful under the altered conditions.

8. Using the steps required for natural selection to occur, describe how cancer can be viewed as natural selection among cells (hint: reproductive excess is not really involved here, but the other 3 conditions are).

There is variation among cells, such that some cells escape regulation of mitosis (variation in the phenotype). The escapees start to divide rapidly (differential reproduction), and they garner lots of resources. Their progeny do the same (inheritance of the successful phenotype), and they can even spread throughout the body and outcompete cells in different organs over time.

9. Why is it so hard to get a drug to combat the HIV virus?

The retrovirus HIV is rapidly mutable due to mistakes made in reverse transcription and an absence of repair. Thus, many new genotypes are being generated during this infection, and if any of these new genotypes are resistant to the drug, they will have a survival advantage, and will replace other virus genotypes till they are all resistant.

10. Your best friend is feeling sick and goes to the doctor. Though he/she is told that the illness is probably just a cold virus, your friend badgers the doctor into prescribing an antibiotic. After 3 days, your friend feels totally fine and decides to hang onto the rest of the medicine in case he/she gets sick again later in the semester. What are three things you have learned about the evolution of antibiotic resistance that you can explain to your friend? Don't just say what he or she should have done, be sure you explain WHY.

a. Be sure to finish your medicine so that any resistant bacteria are kept at low levels till your body can wipe out every last one of them. This prevents resurgence of the disease and transfer of resistance to body bacteria.

b. Don't self-administer antibiotics, because you should only take them when you really need them, and a doctor should decide this. Otherwise, people are tempted to take antibiotics when they get a cold, and viruses aren't touched by antibiotics.

c. If you take left-over medicine, you won't have a full dose (see #a)

d. Don't pester your doctor for drugs-- taking antibiotics unnecessarily increases the chance of evolution of resistance, both in the population as a whole and in your own body! Save the drugs for when you really need them..