Chapter 11 Answers
Evolution and Natural Selection
Visual Understanding
Figure 11.12
Because of prolonged drought, the trees on an island are producing nuts that are much smaller
with thicker and harder shells. What will happen to the birds that depend on the nuts for food?
They will need thicker, sturdier bills. Birds with thinner, less sturdy beaks will not survive. This
is directional selection.
Table 11.2
A very heavy rainstorm floods a mountain river, changing its course and digging a deep canyon
through the soft soils of the meadow in the valley below. What happens to the mice on either
side of the valley?
They are now two separate populations, geographically isolated because of the canyon, and they
may very well, through genetic drift or some differential population pressure become two
different species over time.
Challenge Questions
The Theory of Evolution
How does microevolution differ from macroevolution?
Microevolution is the changing of individuals over time in a population. Microevolution results
in changes in gene frequencies in a population of organisms. Macroevolution is the formation of
new species. This is what is usually envisioned when someone talks about evolution, but it is
only through microevolution that macroevolutionary changes occur.
How Populations Evolve
How likely is it that any naturally occurring population on a continent (not an island) will meet
all the assumptions of the Hardy-Weinberg equations?
It is highly unlikely that any naturally occurring population, even a large population, will meet
all of the assumptions of Hardy-Weinberg. Almost every population of organisms faces at least
one of the conditions that limits the Hardy-Weinberg rule. It is for that reason that the HardyWeinberg rule is usually considered a "null" hypothesis.
Adaptation Within Populations
Use Hardy-Weinberg and the tenets of natural selection to explain why there are colorful, large
guppies in pools without pike cichlids, and drab, small guppies when pike cichlids are present.
The population is relatively large, there is probably little mutation, no migration, and natural
selection is not occurring above the waterfall (there is no external selection pressure).
Individuals do not mate completely randomly; females choose the larger, more colorful males.
Because of this nonrandom mating the population allele frequencies are changing toward larger,
brighter males. Nonrandom mating is the most important factor driving the allele frequencies of
the population above the waterfall.
Below the waterfall the population is still relatively large, with little mutation and no migration.
However there is an external selection pressure, predation by the pike cichlid. Avoidance of
predation by being smaller and less colorful is more important than the advantages gained by
being larger and brighter and possibly mating with more females. Selection for avoidance of
predation is the most important factor driving the allele frequencies of the population below the
waterfall.
How Species Form
A new volcanic island erupts from the ocean. Over millions of years, the black lava breaks down
into black sand, seeds drift to the island and it becomes populated with plants, some flying
insects, and a few birds, including a small hawk. In a storm, several palm trees are uprooted
from the white sands mainland and float out to the island, carrying several insectivorous lizards
of one species. Describe the subsequent changes that take place in the lizard population as they
survive and reproduce.
The insectivorous lizards have plenty of food and no competition from other species. They do,
however, have a selection pressure from the hawk. They must either be very fast, come out only
at night, or blend in with the black sand background by changing from the initial very light color
to a much darker skin color. The most likely change is that, over time, the lizards born with the
darkest skin are the ones that survive and reproduce, until they become a completely different,
very dark species, distinct from the light-skinned species on the mainland.