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Biology 413 (Zoogeography) Mid-term Exam PRACTICE QUESTIONS
Winter Term 2 – 2015
PART I
Answer Questions 1 – 2, using 1-2 sentences or less in the space provided (point-form is OK).
Questions 1-3 should take no more than ~ 5 minutes each.
Question 1 [2 points]:
Explain why we often see the driest deserts on the western side of continents at the Horse Latitudes.
Over land, Horse Latitude belts (30° N and S latitude) represent the world’s deserts and Mediterranean
climates. In summer, land is warmer than ocean water, and cool westerly winds over oceans absorb
water as the air warms and expands while passing over warmer land. In winter, land is cooler than the
ocean and moisture-laden westerly winds can bring precipitation to these regions, but this is minimal.
The world’s driest deserts occur where very cold currents/winds encounter warm land (e.g., Atacama
desert in South America, western Australia, Namib Desert in Africa).
Question 2 [2 points]:
Define the following terms as they relate to mechanisms that explain disjunct distributions of species:
1. Dispersal: In the case of dispersal, disjunct distributions form due to a barrier that was previously
established prior to the separation of populations, such that a subpopulation of individuals actively
crosses that barrier to colonize new unoccupied areas or habitats.
2. Vicariance: In the case of vicariance, a physical barrier is established that separates two populations
that were already present. After long periods of geographic isolation without gene flow, the populations
can diverge to become new species.
Question 3 [4 points]:
In the space below, draw two phylogenies for species A, B and C, where species A is the outgroup and
species B and C are sister species. Give the two phylogenies an x-axis of time (2 points).
Explanation: Patterns of disjunct populations, subspecies and species can occur
due to dispersal across a preexisting geographical barrier, or through
vicariance, the fragmentation of a widespread ancestral population with the
appearance of a new barrier. In the dispersal scenario, the barrier is older than
the geographic disjunction and divergence. With vicariance, the appearance of
the geographic barrier is responsible for the pattern of disjunction, so it cannot
be older than the resulting speciation event. Consider the establishment of a
large river or mountain chain. In phylogeny 1, with dispersal, a river or
mountain was present, and subsequently, some individuals were able to
establish populations on the other side of the barrier and undergo divergence in
allopatry (assuming gene flow was limited to a small number of events and
thereafter restricted). In phylogeny 2, with vicariance, populations may require
varying amounts of time to eventually become separate species, but the
establishment of a geographic barrier in this case would occur at some point at
or following the timing of speciation.
Suppose that species B and C have disjunct geographical distributions due to an event that created a
barrier (e.g., establishment of a larger river). In phylogeny 1, draw a vertical bar on the phylogeny for
the timing of that event that would support the formation of species B and C via dispersal. In phylogeny
2, draw a vertical bar for the timing of that event that would support the formation of species B and C
via vicariance. (2 points).
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PART II
For Questions 4b and 5, use complete sentences for your answers (point-form is not OK).
Question 4 [10 points total for parts a and b]:
Figure 1.1 and 1.2 show the relationship proposed by Santiago Claramunt and colleagues to support
their hypothesis for an “intermediate dispersal model” in birds.
Figure 1.1 Relationship between wing-aspect ratio and dispersal ability
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A
geographic barrier:
strong
B
C
intermediate
weak
distribution:
Figure 1.2 Relationship between dispersal ability (x-axis) and speciation rates (y-axis).
a) In Figure 1.2 above, A, B and C correspond to a location along the x-axis. The gray ovals depict the
typical size and pattern of a species’ distribution under A, B and C. On the lines provided directly
under A, B and C, use one word to describe the relative strength of a geographic barrier for species
that represent these locations along the x-axis. [3 points]
b) Provide an explanation for the proposed “intermediate dispersal model,” based on what you have
learned about mechanisms of speciation and dispersal. Use Figures 1.1 and 1.2 to support your
answer. [7 points]
Figure 1.1 shows a positive relationship between a bird’s dispersal ability and its wing-aspect ratio.
Thus, birds with higher wing-aspect ratios should be better at colonizing areas beyond geographic
barriers compared to birds with low wing-aspect ratios. In Figure 1.2, we see a hump-shaped
relationship between dispersal ability and speciation rate. For birds with low dispersal capacity (A), the
strength of a given geographic barrier should be large. As such, the geographic range of this species
should be spatially restricted. Because the species is a poor disperser, the likelihood of establishing
populations outside the range is low, offering little or no opportunity for allopatry and consequently, low
speciation rates (we assume sympatric speciation is unlikely). For birds with high dispersal capacity (C),
a given geographic barrier will not be effective in preventing dispersal. Species with high dispersal
capacity would be expected to exhibit a larger geographic distribution with high gene flow, preventing
any divergence of populations in allopatry. In contrast, an intermediate dispersal ability (B) allows some
individuals to overcome geographic barriers, resulting in the establishment of allopatric (isolated)
populations (as depicted in the figure). However, dispersal (gene flow) is not high enough to prevent
divergence in allopatry, resulting in relatively high speciation rates. Once allopatric populations are
established, they will likely diverge as the result of either differing ecological selection pressures or
genetic drift until reproductive isolation is attained.
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Question 5 (8 points):
Sympatric speciation is one of several models or modes of speciation based on geography. Define
sympatric speciation and provide three criteria that must be met to conclude that a speciation event
occurred under the sympatric model.
Sympatric speciation is the mode of speciation that occurs within spatially overlapping populations. In
this case, two populations overlap extensively when population differentiation begins and populations
maintain contact throughout the speciation process until two new species are recognized. As such,
sympatric speciation requires divergent selection in the face of ongoing gene flow. We discussed four
criteria that must be met to recognize a case of sympatric speciation, but you can provide three for full
credit: (1) Populations must be sympatric or overlapping in distribution; (2) The species that form must
have substantial (genetically-based) reproductive isolation; (3) Sympatric species must be sister species,
or must be the most closely related taxonomic groups in a given clade; (4) the biogeographic history of
the species that are formed should make any allopatric phase in the speciation process highly unlikely
(i.e., there is no evidence to suggest that populations were at one time isolated to allow divergence in
allopatry).
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