Homework questions are due at the START of lab. See the course syllabus for information on format. One question is due during each lab period as noted on the syllabus. This page lists all the homework questions for the semester (you only have to do one each week.)
Homework Question For Week #1:
Hummingbirds have long bills that are considered to be an adaptation for obtaining nectar from flowers with a long, tubular shape. Explain what it means for a trait to be an adaptation (in the evolutionary sense). Assuming long bills really are an adaptation for obtaining nectar, as proposed, describe how hummingbirds evolved from having short bills to having a long bills; in your explanation, consider where the first long bill would have come from and then how all hummingbirds would have come to have them and apply Darwin's four postulates clearly to this situation.
Homework Question For Week #2:
Suppose that as long bills were evolving in hummingbirds, bill length was controlled by a single gene with two alleles, S and L, such that SS individuals had short bills and LL individuals had long bills. Further, suppose that S was genetically dominant to L so that SL individuals had short bills just like SS individuals. Since long bills allowed hummingbirds to obtain more food, hummingbirds with long bills survived best and produced, on average, 2 surviving offspring during their lives, while individuals with short bills produced, on average, 0.75 surviving offspring during their lives. Use this information as needed for parts a-d of this question.
(b) Suppose that in a virtually infinite population of hummingbirds that mate randomly with regard to bill length, do not mutate, and do not move among populations, the frequency of the L allele in the gametes at the start of a generation is 0.3. What will be the frequency of the LL genotype in the zygotes produced by union of these gametes?
(c) Continuing from question (b), determine the frequency of the LL genotype in the surviving adults that develop from the zygote population.
(d) Continuing from question (c), determine the frequency of the L allele that will start the next generation when the adults surviving from the zygote population reproduce. Has it increased in frequency, decreased in frequency, or stayed the same over the generation? Is this what you expected? Why/why not?
(e) Another trait in this hummingbird population is the presence or absence of a white spot on the belly of the hummingbirds; this is also controlled by a single gene with two alleles such that SS individuals have a spot, NN individuals do not have a spot, and S is dominant to N so SN individuals have a spot and look just like SS individuals for this trait. Presence or absence of the white spot has no impact on survival or reproduction; individuals mate at random with regard to whether or not there is a spot, there is no mutation, and it's the same population as in the previous part of the question so it's virtually infinite in size and individuals do not move among populations. 3/4 of the individuals in the population have a white spot. What is the frequency of the N allele?
Consider three species of hummingbirds: Singing Hummingbirds, Chanting Hummingbirds, and Yodelling Hummingbirds. Yodelling Hummingbirds have almost no genetic variation within populations; different populations have fixed genetic differences from each other. In contrast, Singing Hummingbirds have high levels of genetic variation within populations and different populations have the same alleles as each other in very similar frequencies. Chanting Hummingbirds have low levels of genetic variation within populations (more than Yodelling Hummingbirds) and populations do not have fixed differences from each other, but the frequencies of alleles are very different in different populations.
(a) What do the described levels of within and among population variation suggest about the level of gene flow among populations in each of the three hummingbird species? Clearly explain why these levels of within and among population variation occur with the gene flow regimes you have stated; refer to the impacts of other forms of evolution as necessary in your explanation.
(b) Assuming you are correct in what you have said about levels of gene flow, in which species of hummingbird (Singing, Chanting, or Yodelling) do you expect the highest level of local adaptation of populations to their specific environmental conditions? Clearly explain why.
You see a nature show about hummingbirds in which the narrator says "hummingbirds are the most evolutionarily advanced of the birds. Since their ability to obtain nectar evolved to promote species survival, they have speciated into a wide range of diverse and beautiful species." Identify two evolutionarily incorrect points from the narrators statement, and clearly explain why each is evolutionarily incorrect.
You study hummingbird body shape and size and find that, as they develop, overall body size and bill thickness develop together, so that hummingbirds with larger bodies also necessarily have thicker bills. What is this phenomenon called? In a particular environment where hummingbirds occur, the climate is sometimes cold, and large bodied birds survive the cold better because they store more heat in their body mass. Nectar producing flowers vary in size; in some, the nectar can only be reached by hummingbirds with thin bills, so thin-billed birds can get nectar from all the local nectar producing flowers but thick-billed birds can get nectar from only some of these flowers. Given all this information, explain what a perfectly adapted hummingbird would look like (in terms of bill thickness and body size) if you could design one, and why such a perfectly designed hummingbird will not actually evolve in this situation. Suppose that, in this environment, food is limited and ability to get enough food has a much bigger impact on survival to reproduction than does being well adapted to the cold. What bill thickness and body size do you predict will evolve? Why?
Chapter 8 in Freeman and Herron (2001) describes three main methods for studying adaptation: the experimental method, the observational method, and the comparative method. Which would be the most powerful method for studying the adaptive significance of a trait within a population? Why? Which would be the most appropriate method for determining how an apparently adaptive structure had evolved from some other structure? Why?
You are studying the relationships between the following insects: a hummingbird, a swift, a pigeon, and a sparrow. You find that humminbirds and swifts have narrow wings and pigeons and sparrows have broad wings. Outgroup comparison indicates that broad wings are primitive, narrow derived. Which character state (broad wings or narrow wings) provides evidence for relationship? Clearly explain why based on a general explanation of whether primitive or derived states provide evidence for phylogeny and a clear explanation of why this is the case.
Homework for Week #8
Someone asks you how many species of hummingbirds exist. Explain why the answer to this question depends on whether you are using the biological or phylogenetic concept of what a species is. According to which concept will there be more species? Clearly explain why based on the criteria used by each concept for defining species and on the kind of situation for which the two concepts disagree about whether groups are separate species or the same species.
Homework for Week #9
A small number of male and female ruby-throated hummingbirds get blown out to an island during a storm and establish a new ruby-throated hummingbird population on the island (which has no other hummingbirds). Why might this newly established population be especially likely to evolve to be a new hummingbird species -- even more likely than it would be for large populations of ruby-throated hummingbirds that became geographically isolated to become different species? Now present a counter argument that this newly established population is unlikely to evolve to be a new hummingbird species.
Homework for Week #10
There are over 300 species of hummingbirds and only about 70 species of the most closely related group of birds, the swifts. Give a clear, complete, plausible explanation for why there are so many more hummingbird species than swift species (note: you don't need to know anything about hummingbirds and swifts to answer this) that is based on the concept of key innovations. Explain what additional phylogenetic information you would need to test this hypothesis.
Homework for Week # 11 (based on reading in Ch. 14 of your textbook):
What is the RNA World hypothesis? What does it mean to say that RNA has both a genotype and a phenotype (what gives it its genotype and what gives it its phenotype) and why does this make the RNA World hypothesis likely? What is chirality, and what problem(s) does it pose for the RNA World hypothesis. (Note that the answer to this later part of the question is later in the chapter than the information needed to answer the first parts of the question.)
Homework for Week # 12:
Hummingbirds make up one family of birds, the Trochilidae; this level of classification indicates that they are moderately closely related. You plan on studying the phylogeny of the major groups within the Trochilidae. You have the molecular techniques available that would allow you to use one of the following forms of DNA for your study (all these areas of DNA occur in all hummingbirds): DNA that codes for part of the ribosomal RNA that makes up the parts of the ribosome that are crucial to its function, DNA from a pseudogene, or DNA that codes for an enzyme. Clearly explain which you would use; include an explanation of why a certain rate of evolution is appropriate for your study based on the problems would you have if you chose DNA that evolved at the wrong rate, an explanation of why the DNA you selected should evolve at the appropriate rate, and an explanation of why the DNA forms you did not select would evolve at other rates.
Homework for Week #13:
You have identified two genes present within a group of species of hummingbirds; both apparently affect feather coloration. The hummingbirds in this group all have green bodies, but their throat colors differ (some are red, some orange, some purple, some blue). You hypothesize that one gene affects body feather coloration and the other throat feather coloration. What would you predict about rates of replacement and silent substitution in each of these two genes if your hypothesis is correct? Explain why, in terms of whether colors have apparently been subject to positive or negative (purifying) selection, AND why these different forms of selection lead to different predictions about whether replacement or silent substitutions will be more common.
Homework for Week # 14 (based on reading in Ch. 17 of your textbook):
What is "deep homology"? Use this concept to explain the apparent convergent evolution of legs between arthropods and vertebrates.
Bonus Homework Question: If you have missed any homework points,
you can make up a point by e-mailing me an answer to this question before
lecture on 28 April. It's multiple choice, so just e-mail me an e-mail
that has "Bonus Homework" as the subject and has the letter for what you
think is the correct answer as the message.
Why have so many questions this semester been about hummingbirds?
a. they are the most diverse group of birds, which are the most
diverse group of animals, so Dr. Irwin thinks you need to know more about
them
b. they're pretty and Dr. Irwin likes them a lot so she decided to
write questions about something she liked
c. Dr. Irwin always focuses most of her homework questions on one group
of organisms to show that all these evolutionary principles can be applied
to pretty much any group of species
d. both a and b
e. both b and c
f. all of the above