Download HOMEWORK 02: ANSWER KEY

BIO 295
Islands & Evolution
- from the Galapagos to New York City
HOMEWORK 02: ANSWER KEY
The Beak of the Finch: Chapters 4-6
Answer each question in at least one paragraph (ca. 150-200 words). Refer to the syllabus (section “Written
assignments”) for formatting instructions.
1. Why could small variability in beak size make a big difference? What did the Grant team measure
to illustrate the crucial importance of beak size? What role does seasonality play in the pattern
they found?
different sized beaks open different sized seeds, Grants measured beak size and seed availability and
hardness, in wet season finches eat all kinds of seeds (independent of their beaks) but in the dry season they
stick to the seed sizes their beaks are specialized on
p. 56-60
2. Why can’t two finches co-exist on the same island if they eat the same food in the same way? Who
is David Lack and how did he figure this out? There are two other experiments mentioned in the
book that illustrate this point. Briefly describe these experiments. Do you know the technical term
for the principle? (term not mentioned in book, extra points for this)
2 species are thrown into competition, one species will drive other to extinction, David Lack studied finches
in great detail (observations, measurements, distribution of beaks), published monograph “Darwin’s
finches”; Paramecium and barnacles colonize different areas, minimize competition by making themselves
as different as possible; competitive exclusion
p. 54-56
3. Peter Boag was planning a straight-forward experiment as part of his thesis. What was he
interested in testing? How did he plan on implementing his experiment? Describe the methods he
was planning to use. Did he succeed? If not, why?
Peter Boag was interested in heritability, wanted to measure how parent beak size related to offspring beak
size, in order to resolve nature vs. nurture he wanted to conduct an egg-switching experiment, didn’t
succeed, no data to collect due to drought (breeding season lost and birds were dying)
p. 67-68
p. 71-76
4. What effect did the 1977 drought have on Daphne Major and how did this affect the finches? Why
were dead finches important? What was the beak size difference that made the difference
between a finch that survived and one that died?
food (seeds) became scarce, struggle for existence among finches grew intense, finches mostly didn’t breed
because no rain, overall number of seeds went down, average hardness and size of remaining seeds went
up, birds were dying (population reduced by 60-85 %); dead finches were important data points for
measuring selection; 0.5 mm size difference
p. 70-82 (Chapter 5)
p. 77 (top)
p. 79 (top)
5. Natural selection is not by itself evolution. Why not? What is evolution? Explain this by using the
finches and the event of the drought as an example.
natural selection is only the mechanism, takes place within a generation, evolution takes place across
generations; birds are variable in beaks, variations affect fitness (“sensitive to these variations”), variations
heritable  prerequisites for evolution by natural selection; drought selected for larger beak sizes, in the
aftermath of drought population shifted to larger beaks, natural selection lead to evolution, heritability is
key
p. 69
p. 79, 81
6. John Endler did a series of experiments to show evolution in action using a different study system
(=study organism). Describe his experiments: what he was testing and what he found.
“streams as islands”
guppies in northeastern South America, male guppies wear colorful spots, spots are heritable (like finches’
beaks), Endler found pattern: in streams with more guppy enemies spots smaller & fainter, in streams with
fewer enemies spots larger & brighter; experiment: collected guppies from streams with different numbers
of enemies, let them mix & breed, then randomly assigned them to tanks mimicking their natural habitat
(each with different number of enemies), guppies in each tank ended up looking like guppies that live with
same predators in wild, he “shuffled and reshuffled the deck”
p. 89-96
Add. note: Guppies look the way they look because of two forces pushing in opposite directions. Natural
selection camouflages males better and better as protection against predators while sexual selection
makes males colorful and as different from gravel bottom as possible in order to attract more females.
“without natural selection all fish would have gotten gaudier”, “without sexual selection none of them
would have gotten gaudier” (p. 94-95)
7. How did the drought affect the sex ratio of finches on the islands? Why might this be of
importance? Explain Trevor Price’s findings on the biology and behavior of the finches (plumage,
territory, etc.).
skewed sex ratio because of size differences between males and females, females smaller than males, only
largest finches survived; small subsample of male survivors had chance to mate but every female mated,
females picked largest males, sexual selection magnified results of natural selection because of skewed sex
ratio; Trevor Price found that males with bigger territories attract more females, black plumage indicates
age and experience, females prefer jet-black males, variation in plumage in males suggests that there are
hidden costs to wearing black and hidden benefits to wearing brown
p. 78
p. 80 (bottom)-81 (top)
p. 85-88