Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 20 (p. 438)

1. Natural selection is a process in which members of a population inherit traits that enable them to better survive and produce offspring than others in the population. Evolution, as Darwin defined it well, is "descent with modification."

2. Adaptation results from the possession of features that promote the chance of an organism’s survival and reproduction. The natural selection of Darwin's concept of evolution is the process, and adaptation is the result.

3. Genetic polymorphism refers to genes that have more variation that can be explained by mutation. The variation provided by genetic polymorphism is a significant source of raw material upon which natural selection can act.

4. p=54/100=0.54 q=1-p=0.46. Therefore, heterozygous: 2pq=2(0.54 x 0.46)=0.4968=5/10; homozygous dominant: p²=0.54 x 0.54=0.29=3/10; homozygous recessive: q²=0.46 x 0.46=0.21=2/10; Check: 5/10 + 3/10 + 2/10=10/10.

5. The only alleles the subsequent population can build on are those few present in the population originators (the "founder"). If a population should become catastrophically reduced in size (through earthquakes, etc.), subsequent alleles in the population will only be those of the survivors, a phenomenon called the bottleneck effect.

6. Inbreeding has no effect on allele frequencies. Inbreeding tends to throw together deleterious recessive alleles, which is typically not favored by natural selection. Marriage between close relatives contributes to inbreeding.

7. Selection is the ultimate success of an organism so that it survives to leave more offspring and hence pass on more of its genetic material. The "successful" organisms will leave more offspring and hence more of their own genes, skewing the allele frequencies, even if only slightly. Directional selection eliminates one extreme from a range of phenotypes, such as the elimination of Drosophila individuals in a population that move toward light. Stabilizing selection eliminates both extremes from a range of phenotypes, such as reduced survival values in duck eggs at the higher and lower extremes of weight. Disruptive selection actually eliminates some of the intermediate forms instead of extremes, as seen in increased predation on nonmimic butterflies in Africa.

8. Few traits are completely independent of others, and their interactions increase with successive matings; selection can only act on traits in which the homozygotes and heterozygotes are clearly phenotypically distinguishable.