BIO 325: Introductory Ecology, First Exam, Summer 4-Week 2010
Name:___________________________
ID:______________________________
Some potentially useful formulas:
lx 
l x 1 n x 1

lx
nx
s x , x 1 
1
Rx 
lx

l m
tx
t
nx
n0
t
 mx  s x , x 1 Rx 1

R0   lt mt
t 0
True/False and Short-Answer Questions (50 points: 5 points each)
1. True or False? In monkeyflowers, inserting the carotenoid Yup genotype of M. cardinalis into the
genetic background of M. lewisii resulted in an increase in pollinator visits by bees and a reduction
in pollinator visits by hummingbirds.
2. True or False? Narrow sense heritability, h2, is a measure of the degree of phenotypic
resemblance between parents and offspring.
3. True or False? When the whole is other than the sum of its parts, the effects of the parts combine
multiplicatively in determining the whole.
4. True or False? Narrow sense heritability, h2, can be estimated by dividing the selection response,
R, by the selection differential, S.
5. True or False? Regulatory genes evolved more quickly in the Tarweeds than in the Silverswords.
6. True or False? Under the hypothesis of peripatric speciation, the populations in question are
geographically isolated from one another.
7. True or False? Bill width in African finches is an example of disruptive selection.
8. True or False? Rx includes reproductive success for age classes before age x.
9. True or False? The slope of the regression line of fitness versus phenotype is called the selection
differential, S.
10. Why (algebraically) does the effect of a single offspring on its mother’s R0 decrease as the
mother’s age, x, increases?
R0 = Σlxmx . lx decreases as age, x, increases, while the number of offspring at stake in the question
(±1) remains constant. Thus the later in life one adds or deletes 1 offspring, the smaller its effect on
R0.
Essay Questions: (50 pts, 25 pts each)
Consider the phenomenon that stickleback spine length is correlated among lakes with predation
regime, that is, whether the major predators are invertebrates (aquatic insects) or vertebrates (fishes
and birds). In lakes dominated by invertebrate predators, sticklebacks have short spines; whereas, in
lakes dominated by vertebrate predators, sticklebacks have long spines. Your task is to construct an
hypothesis to explain this phenomenon, and to outline the design of an experiment to test your
hypothesis. Your experiment should differentiate what variable(s) you manipulate experimentally from
what variable(s) you measure in response to your manipulation.
1. Outline your experiment in terms of the scientific method, from Phenomenon to Tests.
Scientific Method
1. Phenomenon: something in nature that we wish to explain. The phenomenon that we wish to
explain is the among-population (among-lake) correlation between stickleback spine length and
the dominant predators in the population (lake), i.e. vertebrates (birds, fishes) versus invertebrates
(aquatic insects)
2. Hypotheses: educated guesses of cause and effect. Past selection on spine length by predators
resulted in the pattern we see today. Invertebrate predators are agents of selection against longer
spines and for shorter spines; whereas, vertebrate predators are agents of selection against
shorter spines and for longer spines.
3. Theory: the conceptual framework within which our hypotheses are framed. Darwin’s theory of
natural selection. (see below)
4. Predictions: logical deductions of our hypotheses (predictions must be independent of the data or
knowledge that went into formulating our hypotheses). In selection experiments where predation
regime is the agent of selection, the selection response will be longer spines in the presence of
vertebrate predators and shorter spines in the presence of invertebrate predators.
5. Tests: empirical falsification or verification of our predictions. This is the part where we design an
experiment and execute it. The treatment variable is predation regime: vertebrate versus
invertebrate, and the response variable is spine length. We place sticklebacks into two treatments:
1. an environment with vertebrate predators; 2. an environment with invertebrate predators. These
could be in the lab or the field, as long as we can control the predation regime. The trick here is to
choose a source population for the experiment. Marines would be one possible choice, because
they are the putative ancestor of all freshwater populations. Another option would be the F 2
hybrids of crosses between fish from a short-spine/invert-predation lake with fish from longspine/vert-predation lake. Although this source may appear more artificial than Marines, these F2
hybrids would have sufficient genetic variation already present to be able to respond to a selection
experiment (you wouldn’t have to wait for mutation).
2. Outline how your hypothesis works in terms of Darwin’s 5 steps of natural selection. Apply the
your content knowledge of how Darwin’s 5 steps work to this particular phenomenon.
Darwin’s Natural Selection
1. Individuals within a population vary in phenotype. V P > 0, where VP is phenotypic variance.
You’ve been given the observation of considerable variation among populations in spine length.
2. Some of this phenotypic variation is heritable, or due to genes, and passed on from parent to
offspring. VA> 0, where VA is additive genetic variance. Heritability is given by, h2 =VA/VP. You
were given data in class on the PitX-1 gene that effects the sizes of the pelvis and the spines. If
you forgot that, you could propose to test the assumption of heritability. Based on what you’ve
been given in this problem (and learned in class), the easiest way to determine heritability of spine
length would be to cross fish from short-spine/invert-predation lakes with fish from long-spine/vertpredation lakes. If spine length is heritable, you should see intermediate spine lengths in the F1
hybrids (assuming spine length genes don’t express dominance), and the full range of spine
lengths (long, intermediate, short) in the F2s. (The PitX-1 gene behaved as if full pelvises and
spines lengths was dominant over the reduced phenotype, so you might have to go to F2s in this
case).
3. Individuals produce excess offspring, which leads to a struggle for existence. In stationary
populations, each female replaces herself on average with one daughter. Birth rates that are
higher than this lead to the 'struggle for existence.' This idea comes from economist Thomas
Malthus, and is a key component of Darwin's theory. You can basically assume this to be true if
populations are near their carrying capacities, yet have birth rates capable of exponential growth
were it not for death rates canceling them out.
4. Some individuals, because of their particular phenotypic characteristics have higher rates of
survival or reproduction than others. This is selection within the parental generation. In
quantitative genetics, this is measured by the selection differential, S, which is the difference in
average phenotype before versus after selection, within the parental generation. Here we want to
quantify the effects of invertebrate vs vertebrate predation on spine length, after selection minus
before selection, within the parental generation. This can be done by comparing spine lengths of
the starting population with the spine lengths of the survivors after an episode of predation (S), or
by regressing survival on spine length (β).
5. Those favored phenotypes will increase in frequency in future generations. This phenomenon is
realized in the offspring of the parental generation. In quantitative genetics, this is known as the
response to selection, R, which is the change in average phenotype between the parental
generation before selection and offspring generation after selection. Here we want to quantify the
evolution of spine length in terms of selection responses over several generations, in the two
predation treatments. This can be done by comparing (and analyzing) mean spine lengths of the
parental generation with those of each succeeding generation for the duration of the experiment,
e.g. 5-10 generations.