Notes to Instructors
Chapter 24 The Origin of Species
What is the focus of this activity?
As noted earlier, certain key words used in discussions of evolution may have very
What is the particular activity designed to do?
Activity 24.1 What factors affect speciation?
This activity is designed to help students understand how evolutionary changes in a
Activity 24.2 How does hybridization affect speciation?
This activity is designed to help students understand how environmental differences and
Answers
Activity 24.1 What factors affect speciation?
1. The Galápagos Archipelago consists of a dozen islands all within 64 km of their
nearest neighbor. From 1 to 11 of the 13 species of Darwin’s finches live on each
island. Many evolutionary biologists believe that if there had been only one island,
there would be only one species of finch. This view is supported by the fact that
Cocos Island is isolated (by several hundred kilometers of open ocean) from the
other islands in the archipelago and only one species of finch is found there.
a. How does the existence of an archipelago promote speciation? Explain and
provide an example.
Notes to Instructors 173
The distance between islands makes it very difficult for organisms on one island
to migrate to another island. For example, it is likely that only a few birds made it
b. Is the mode of speciation that occurred on these islands more likely to have been
allopatric or sympatric? Explain.
This mode of speciation is more likely allopatric. For genetic changes to
c. Is the type of speciation seen on the Galápagos Archipelago more likely to be the
result of anagenesis or cladogenesis? Explain.
It is more likely the result of cladogenesis. Anagenesis is the gradual change of
of a species into two or more species.
2. Hybrids formed by mating two different species are often incapable of reproducing
successfully with each other or with the members of their parent populations.
Explain why this is the case. (Hint: Consider what you know about chromosome
numbers and meiosis.)
In diploid animals, meiosis produces gametes that each contain half of the
3. Because most hybrids can’t reproduce, their genes (and the genes of their parents)
are removed from the population. Only the genes of individuals who breed with
members of their own species remain in the population. This implies that there is a
strong selective advantage for genes that enable individual organisms to recognize
members of their own species. Today a wide range of reproductive isolating
mechanisms has been identified.
Each of the following scenarios describes a reproductive isolating mechanism.
Indicate whether each is a prezygotic or postzygotic isolating mechanism. Explain
your answers.
(Note: A prezygotic isolating mechanism by definition prevents the formation of the
a. Crickets use species-specific chirp patterns to identify a mate of their own species.
This is a prezygotic mechanism because it prevents matings between different
species.
b. Two species of butterfly mate where their ranges overlap and produce fertile
offspring, but the hybrids are less viable than the parental forms.
c. Two species of a plant cannot interbreed because their flowers differ in size and
shape and require pollination by different species of bee.
Activity 24.1 175
d. Two species of firefly occupy the same prairie and have similar flash patterns, but
one is active for a half-hour around sunset while the other doesn’t become active
until an hour after sunset.
This is another prezygotic isolating mechanism. The behavior of the two species
4. Many of our most successful grain crops arose as hybrids; most are also
allopolyploids. These crops can successfully reproduce. Explain.
An allopolyploid can be produced by hybridization between two species to form a
zygote. If the chromosomes of the zygote duplicate for the first mitotic division but
Activity 24.2 How does hybridization affect speciation?
As noted in the text, two species of toads, Bombina bombina and Bombina variegata,
share a hybrid zone that is 4,000 km in length and only 10 km wide. The frequency of
alleles specific to B. bombina decreases from close to 100% on one edge of the hybrid
zone to 0% on the opposite edge. Similarly, the frequency of B. variegata–specific
alleles decreases across the hybrid zone (beginning at the opposite edge) from close to
100% to 0%.
1. On the following graph, map the general percentage of each type of species-specific
genes across the hybrid zone. Use an X to indicate the frequency of
B. bombina–specific genes and an O to indicate the frequency of
B. variegata–specific genes.
176 Activity 24.2
2. How would you predict this gene distribution would change over time if:
a. reinforcement occurs?
Note: In the graph above, each X or O represents the percentage of B.b or B.v
Activity 24.2 177
O
X
100%
0%
B.b edge of range B.v edge of range
% of genes
in the
population
b. fusion occurs?
Note: In the graph above, each X or O represents the percentage of B.b or B.v
alleles in a sampled individual. If fusion or weakening of reproductive barriers
c. Hypothetically, which (a or b) is more likely to occur if all environmental
conditions across the two species ranges are similar? Explain.
Similar environmental conditions across the range imply no selective advantage
d. Hypothetically, which (a or b) is more likely to occur if the environmental
conditions vary gradually across the species ranges such that one end of the range
was much colder than the other for example? Explain.
Very different environmental conditions at the ends of the hybrid range imply
e. As noted in the text, what changes in allele frequencies have been recorded over
this hybrid range in the last 20 years? What does this indicate?
In reality, there has been little change observed in the hybrid ratio for these two
178 Activity 24.2
Read the following statement. Then, based on your knowledge of cell biology, genetics
and evolution, decide to agree or disagree with the statement. Whichever you decide,
write a short paragraph that provides solid evidence defending your choice.
“When you think about sexual reproduction, it makes no sense. After all, evolution
selects for organisms that are best fit. In a population of sexually reproducing organisms,
a mutant that reproduced asexually would increase its representation more quickly than
the wild type, that is, it would have a higher fitness. So we should expect asexual
reproduction to be much more widespread among eukaryotic species than it is.”
While there are many ways of addressing this, a reasonable student answer may be
similar to the following:
Activity 24.2 179
24.2 Test Your Understanding