Lesson Quiz Review
17.3 Darwin’s Theory: Natural Selection
Use the information below to review the correct answers on the Lesson Quiz.
1. Natural selection is the mechanism by which species evolve over time. Nature, or the
environment, determines fitness. Individuals with high fitness are more likely to survive,
reproduce, and pass their traits to offspring. Over time, this mechanism develops
adaptations in species, which are traits that help an organism survive in its environment.
2. Natural selection depends on several factors. A population must have a variety of
inheritable traits, some of which provide greater fitness, or a greater advantage in the
environment. More organisms must be born than can survive, and the individuals with
greater fitness are more likely to survive than those with lower fitness.
3. A fruit fly (an insect) and a fruit bat (a mammal) are distant relatives, and their shared trait
of eating fruit reveals very little about their evolutionary history. Nevertheless, they must
have had an ancient common ancestor, as do all pairs of living things.
4. Adaptations include any inheritable body structure or behavior that helps an individual
survive or reproduce in its environment. Many animals have adaptations for defending
themselves from enemies, and these behaviors are at least partially instinctive, meaning
they are inherited from generation to generation.
5. When environmental conditions remain constant, a species may remain about the same
from generation to generation. Natural selection occurs when the environment of an
organism changes. If the desert should become cooler and wetter, then the kangaroo rats
that could tolerate or thrive in the cool, wet climate would have greater fitness than the
others of their kind. The kangaroo rats with greater fitness would become more likely to
survive and reproduce, and their traits would become more common in the population.
6. The diagram shows that an ancestral bear gave rise to all 8 modern bear species, and did
so through several evolutionary lineages. As Darwin explained, the species developed
because different populations became isolated in different environments. Each population
developed adaptations that allowed it to thrive and reproduce in its environment.