Wednesday, September 5

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24.1 1 Two bird species in a forest are not
known to interbreed. One species feeds and mates
in the treetops and the other on the ground. But
in captivity, the two species can interbreed and
produce viable, fertile offspring. What type of
reproductive barrier most likely keeps these
species separate? Explain.
2
24.1 1 Since the birds are known to breed
successfully in captivity, the reproductive
barrier in nature must be prezygotic. Given the
species differences in habitat preference, the
reproductive barrier is most likely to be habitat
isolation.
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• 24.1 2
• Which species concept can be used for both sexual
  and asexual species?
• Which can only be applied to sexual species?
• Which would be most useful for identifying
  species in the field?

4
24.1 2 a. All species concepts except the
biological species concept can be applied to both
asexually and sexually reproducing species
because they define species based on
characteristics other than the ability to
reproduce.
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24.1 2 b. The biological species concept can be
applied only to extant sexual species.
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24.1 2 c. The easiest species concept to apply
in the field would be the morphological species
concept because it is based only on the
appearance of the organism. Additional
information about its ecological habits,
evolutionary history, and reproduction are not
required.
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24.2 1 Explain why allopatric speciation would
be less likely to occur on an island close to a
mainland than on a more isolated island of the
same size.
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24.2 1 Continual gene flow between mainland
populations and those on a nearby island reduces
the chance that enough genetic divergence will
take pace for allopatric speciation to occur.
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24.2 2 Normal watermelon plants are diploid (2n
22), but breeders have produced tetraploid (4n
44) watermelons. If tetraploid plants are
hybridized with their diploid relatives, they
produce triploid (3n 33) seeds. These
offspring can produce triploid seedless
watermelons and can be further propagated by
cuttings. Are the diploid and tetraploid
watermelon plants different species? Explain.
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24.2 2 The diploid and tetraploid watermelons
are separate species. Their hybrids are triploid
and as a result are sterile because of problems
carrying out meiosis.
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24.2 3 In the fossil record, transitional
fossils linking newer species to older ones are
relatively rare. Suggest an explanation for this
observation.
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24.2 3 According to the model of punctuated
equilibrium, in most cases the time during which
speciation (that is, the distinguishing
evolutionary changes) occurs is relatively short
compared with the overall duration of the
species existence. Thus, on the vast geologic
time scale of the fossil record, the transition
of one species to another seems abrupt, and
instances of gradual change in the fossil are
rare. Furthermore, some of the changes that
transitional species underwent may not be
apparent in fossils.
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24.3 1 How can the Darwinian concept of descent
with modification explain the evolution of such
complex structures as the vertebrate eye or heart?
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24.3 1 Such complex structures do not evolve
all at once, but in increments, with natural
selection selecting for adaptive variants of the
earlier versions.
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24.3 2 Explain why the concept of exaptation
does not mean that a structure evolves in
anticipation of some future environmental change.
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24.3 2 Although an exaptation is co-opted for
new or additional functions in a new environment,
it existed in the first place because it worked
as an adaptation to the original environment.
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24.3 3 How can heterochrony cause the evolution
of different body forms?
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24.3 3 The timing of different developmental
pathways in organisms can change in different
ways (heterochrony). This can result in
differential growth patterns, such as those
producing different patterns of webbing in
salamander feet.