Answers to Mastering Concepts Questions
Chapter 13
13.1
1. Evolution is genetic change in a population over time.
2. Before Darwin’s theory of evolution by natural selection was published, people’s ideas on the origin and diversification of species included:
- there is no change in species;
- organisms originated from special creation and they do not change;
- species change as they move into new environments;
- special creation produced species, and catastrophes cause species to go extinct;
- species change through interactions with the environment when they use some body parts and processes extensively.
3. As Darwin journeyed in HMS Beagle, he observed the uniformity of geological processes such as volcanism, earthquakes, and erosion. He collected fossils and observed that each continent had animals that were characteristic and different from those on other continents. On the Galapagos, he observed and collected finches from different islands and noted differences in giant tortoises on different Galapagos islands. These observations led Darwin to the idea of “descent with modification.”
4. In artificial selection, humans select for the traits they desire in plants or animals. In natural selection, environmental factors favor some traits over others.
5. Darwin’s ideas challenged prevailing beliefs about life’s diversity by using natural processes to explain how species arise. Darwin saw humans as just another species and rejected the idea that humans were specially created. He thereby demoted humans from an exalted position, putting them on an equal footing with other species competing for resources.
13.2
1. An adaptation is a characteristic that helps an individual survive and reproduce in its environment. Adaptations become more common within a population when they are heritable and when they increase the odds of survival and reproduction.
2. Genetic variation arises randomly as a result of sexual reproduction, crossing over, and mutation. If a certain allele combination allows an individual to survive and reproduce more abundantly than other individuals, over many generations it will be become more common in the population. If an allele combination is harmful, it is likely to disappear from the population.
3. Natural selection can favor different phenotypes at different times because environments are always shifting in dramatic or subtle ways.
4. Natural selection does not produce perfectly adapted organisms because each population’s evolution is constrained by its existing gene pool, which limits the possible allele combinations and may not contain every allele necessary to confront every challenge. Chance events may also wipe out adaptive allele combinations. Genetic illnesses that produce symptoms only after reproductive age also can maintain harmful alleles in a population.
5. Evolutionary fitness is measured by reproductive success.
6. Directional selection selects for one extreme of a trait that varies in a continuum. For example, in a population of white, grey, and black butterflies, directional selection would favor either black or white. Disruptive selection selects against the average value of a trait and favors the extreme ends of a continuum of variation of a trait (e.g. white and black, but not grey). Stabilizing selection selects for the average value of a trait (e.g. grey).
7. Being a carrier for an inherited disease could be beneficial if carrier status confers a survival or reproductive advantage. For example, people who are heterozygous for sickle cell anemia have a survival advantage in an environment where a fatal form of malaria occurs.
13.3
1. The five conditions required for Hardy-Weinberg equilibrium are:
- no mutations;
- mating is random;
- no emigration or immigration;
- the population is large enough to eliminate random changes in allele frequencies (genetic drift);
- natural selection does not occur.
2. The concept of Hardy-Weinberg equilibrium is important because it shows that evolution has the potential to occur at all times; in fact, evolution is probably occurring at all times in all populations.
3. In the Hardy-Weinberg equation, p= the frequency of the dominant allele in a population, q= the frequency of the recessive allele, and pq= the product of p and q. The equation p2 + 2pq + q2 = 1 says that all individuals in a population are homozygous dominant, heterozygous, or homozygous recessive (for a gene with only two alleles). By applying the Hardy-Weinberg equation at different times, it is possible to observe changes in gene frequency and determine whether evolution is occurring.
4. Hardy-Weinberg equilibrium does not occur in real populations because the conditions for it are never met in real populations.
13.4
1. Mutations introduce new alleles, which may or may not alter the phenotype of an organism.
2. A mutation in one organism passes to subsequent generations if it is a mutation that can be inherited. In sexually reproducing organisms, the mutation must appear in a gamete-producing cell. Mutations in somatic cells will not be passed to the next generation. By contrast, a mutation in an organism that produces asexually will be transmitted to all of its offspring.
3. Sexual selection can yield traits that make individuals of one sex more obvious to predators or otherwise expose them to extra risk. These “dangerous” traits persist in the population if members of the opposite sex prefer individuals that possess the traits. Additional mating opportunities mean greater reproductive success, and the result is additional offspring with the same traits.
4. Gene flow is the movement of genes between populations. Gene flow introduces new alleles into a population, disrupting Hardy-Weinberg equilibrium.
5. In the founder effect, a new population is started by a small group of individuals that colonize a new area. In a population bottleneck, a large and genetically variable population experiences a massive die-off, and only a few individuals survive to continue the population. Both effects result in populations with restricted variety in their gene pools and are therefore prone to genetic drift.