Chapter 15: Processes of Evolution
Modified from Principles of Life: Hillis, Sadava, Hill and Price.
15.1 Evolution is Both Factual and the Basis of Broader Theory
1. We call the mechanisms of evolution the “theory” of evolution. Strictly speaking, however, the term “theory” is incorrect. Why?
In science, a theory typically refers to an untested hypothesis. The concept of evolutionary theory is not limited to a single hypothesis, but refers to many different concepts. Additionally, there are vast amounts of data that support evolutionary theory.
2. What contributions did the work of Mendel, T.H. Morgan, Watson and Crick, and E.O. Wilson make to our understanding of evolutionary theory?
Each of these scientists further contributed to our understanding of evolutionary theory. The rediscovery of Mendel’s work led to an understanding of the basic process of genetic inheritance. Morgan’s studies on fruit flies lead to the discovery of the role of chromosomes in inheritance. Watson and Crick’s discovery of the structure of DNA led to a detailed understanding of molecular evolutionary processes. E.O. Wilson’s work spurred research in the evolution of behavior.
15.2 Mutation, Selection, Gene Flow, Genetic Drift and Nonrandom Mating Result in Evolution
3. Genetic drift, population bottlenecks and the founder effect are all means by which allele frequencies can change. Are these processes random or nonrandom? Explain.
These processes are random and typically occur in small populations. Genetic drift refers to random changes in allele frequencies. A population bottle neck occurs after an environmental event in which a large percentage of a population has died. In this case, the survival of an individual is due solely to chance, as are the resulting allele frequencies among these survivors. In the founder effect, the small population is unlikely to have the same allele frequencies as the source population. As with the other two mechanisms, the alleles present in the founding population is going to be random.
4. Describe what is meant by sexual selection and discuss what behavior ecologists discovered about sexual selection of tail length from studies of male widowbirds.
Sexual selection is the spread of a trait that improves the reproductive success of an individual. The trait may improve the ability of its bearer to compete with other members of its sex for access to mates, or it may make its bearer more attractive to members of the opposite sex. By artificially lengthening or shortening the tails of male widowbirds, behavioral ecologists were able to show that increased tail length in males made them more attractive to females but did not give the long-tailed males an advantage in their interactions with other males.
15.3 Evolution Can Be Measured by Changes in Allele Frequencies
5. Discuss the main application of the Hardy-Weinberg rule in evolutionary biology.
No population meets H-W conditions for genetic equilibrium. However, by determining how a population deviates from the expectations of H-W, evolutionary biologists can identify which evolutionary agents are affecting the population.
6. In a population with 600 members, the numbers of individuals of three different genotypes are AA = 350, Aa = 100, aa = 150. Answer the following questions about this population:
a. What are the genotype frequencies of AA, Aa and aa?
- AA = 350/600 = 0.58
- Aa = 100/600 = 0.17
- Aa = 150/600 = 0.25
b. What are the frequencies of the A and a alleles?
- A = (700 + 100) / 1200 = 0.67
- A = (300 + 100) / 1200 = 0.33
c. What would be the expected genotype frequencies of AA, Aa and aa if this population were in genetic equilibrium?
- AA = p2 = (0.67)2 = 0.45
- Aa = 2pq = 2 x 0.67 x 0.33 = 0.44
- aa = q2 = (0.33)2 = 0.11
d. Is this population in genetic equilibrium? Explain.
The population is not in genetic equilibrium. Using the observed allele frequencies for p and q, the observed genotypic frequencies differ from those predicted by H-W.
15.4 Selection Can Be Stabilizing, Directional or Disruptive
7. Explain how stabilizing selection in human birth weights could be considered purifying selection.
Stabilizing selection reduces variation in a population, but does not change the mean. Since human babies that are born lighter or heavier than normal die at higher rates than babies whose weights are closer to the mean birth weight, there is a stabilizing selection for birth weight. If high or low birth weight were linked to a deleterious mutation in a gene, then this gene would be eliminated from the population, resulting in purifying selection.
15.6 Recombination, Lateral Gene Transfer and Gene Duplication Can Result in New Features
8. Describe the advantages and disadvantages of sexual and asexual reproduction.
In asexual reproduction, an organism’s offspring is genetically identical to the parent. One advantage is the rapid rate of reproduction possible in asexual species. The major disadvantage is that deleterious mutations cannot be easily eliminated from the population. Sexual reproduction has the disadvantage of being slower and potentially breaking up advantageous combinations of genes. It has the advantage of creating greater genetic variation and a means to eliminate deleterious alleles from the population.
Additional Resources: use the web addresses listed
- Animated Tutorial 15.1 Natural Selection PoL2e.com/at15.1
- Animated Tutorial 15.2 Genetic Drift Simulation PoL2e.com/at15.2
- Animated Tutorial 15.3 Hardy Weinberg PoL2e.com/at15.3
- Activity 15.1 Darwin’s Voyage PoL2e.com/ac15.1
- Activity 15.2 Gene Tree Construction PoL2e.com/ac15.2