Terms to Know: natural selection where the dominant has the highest fitness, natural selection where the recessive has highest fitness, fitness codominance, heterosis, underdominance, fixation, loss, genetic variation.

Study Questions:

1. Draw graphs of allele frequency versus time (measured in generations) for both alternate alleles for each of the following situations.  For each, draw a graph starting with the allele in the homozygote that has the highest fitness of the two homozygotes rare, AND draw a graph starting with the allele in the homozygote that has the higher fitness of the two homozygotes common.  (a) Heterosis at the A locus in which the fitness of aa is greater than the fitness of AA.  (b) Natural selection where the dominant at the B locus has higher fitness than the bb recessive.  (c) Fitness codominance at the C locus with alleles C and c; cc has the highest fitness.  (d) Natural selection where the recessive (dd) at the D locus has the highest fitness.  (e) Underdominance at the E locus; the fitness of ee is slightly higher than the fitness of EE.
2. Which forms of natural selection result in an allele being fixed over time? Which maintain both alleles but only with one allele so rare that homozygotes for that allele are almost never produced?  Which maintain both alleles in frequencies where both homozygotes are likely to be produced each generation?

1.  Under what situation of natural selection (dominant has highest fitness, recessive has highest fitness, fitness codominance, heterosis, or underdominance) is most genetic variation maintained over time? (Note: genetic variation is high when both alleles are present in moderate to high frequencies, it is low when one allele is very common and one very rare, and it is entirely absent when one allele is fixed and one is lost.)  Explain why genetic variation is maintained in this situation and explain why it is either low or absent in the other situations.

2.  In situations with complete dominance, such as the peppered moth, how does the initial evolution of an allele that starts out in very low frequency compare between a situation in which the recessive has the highest fitness and a situation in which the dominant has the highest fitness?  Give a plausible explanation for the difference.  How does the genetic variation present in the population after natural selection has been occurring for a long time (e.g. for 100 generations) compare between a situation in which the dominant has the highest fitness and a situation in which the recessive has the highest fitness?  Give a plausible explanation for the difference.

3. What, if anything, is the effect of population size on each of the following for a trait evolving through only genetic drift: (a) the probability that an allele is fixed within 100 generations (b) which allele gets fixed (if either does) (c) how long it takes for an allele to become fixed?  Clearly explain why population size has the effect it does on each of these; if it does NOT have an effect on any of these, explain why it has no effect.

4.  In which situation of natural selection (dominant has highest fitness, recessive has highest fitness, fitness codominance, heterosis, or underdominance) does the result of natural selection over many (e.g. 100) generations depend on the initial allele frequencies as well as on the fitnesses?  Which of the following depends on initial allele frequency in this case: the amount of genetic variation present after 100 generations or the specific allele that became fixed?  Give a plausible explanation for why the outcome of natural selection depends on initial allele frequencies in this case.

5. Consider three different loci, each with two alternate alleles.  At the G locus, the homozygous recessive genotype has the highest relative fitness at a locus and there is complete dominance.  At the R locus, the heterozygote has the highest relative fitness.  At the E locus, the heterozygote has the lowest relative fitness.  At equilibrium, how will these three loci differ in how much genetic variation is present?  Why? At equilibrium, how will the average fitness of the population (that is, will all individuals be of high fitness, giving the population a high average fitness, or will some be of lower fitness, lowering the average population fitness) differ among these three loci? Why?

6.  Suppose you observe that in a population, one homozygote for a genetic locus results in a genetic deformity so that these homozygotes have very low fitness.  Each generation, 5% of the individuals born have this deformity; this percentage remains constant over many generations.  Which form of natural selection (dominant has highest fitness, recessive has highest fitness, fitness codominance, heterosis, or underdominance) is most likely to be occurring at this locus?  Clearly explain why this form of evolution would result in this situation by describing the expected result, after many (e.g. 100) generations of this form of evolution, explaining why this result is expected for this form of evolution, AND explaining why this result is NOT predicted by the other forms of evolution.

7.  For which TWO of the forms of evolution you modeled in the computer assignment (genetic drift, natural selection where the dominant has highest fitness, natural selection where the recessive has highest fitness, fitness codominance, heterosis, underdominance) could the allele that becomes fixed differ in different populations for which the only difference between the populations was allele frequency.  Explain why it is possible for populations to differ this way for each of the two forms of evolution (the explanations are different; explain each one.)  For each case, state whether the allele that is most likely to be fixed is initially rare or common; explain why, in each case, the probability for an allele to be fixed depends on whether it is rare or common.