Clark Larsen Answers Students FAQs
1. Question: What is the Hardy-Weinberg law of equilibrium and how is it used?
Response: Godfrey Hardy and Wilhelm Weinberg created a mathematical equation used to test whether genes in a population are being affected by one of the four forces of evolution. The Hardy-Weinberg equilibrium is a model for an ideal set of conditions in which a gene is in a state of equilibrium. There is no mutation, natural selection, gene flow, or genetic drift occurring. If these conditions are met, then the allele frequencies and genotype frequencies will remain constant from one generation to the next. If these conditions are not met, and a gene is not in equilibrium, the allele frequencies will change from one generation to the next. A gene that is not in equilibrium is likely being impacted by mutation, natural selection, gene flow, or genetic drift.
See Table 4.1 Punnett Square for H-W law of equilibrium
2. Question: What is the difference between spontaneous mutations and induced mutations?
Response: Spontaneous mutations are those that occur randomly in DNA during cell division and have no known cause. Examples of spontaneous mutations include point mutations and frameshift mutations. Point mutations are the result of incorrect base pairings during cell division. If the amino acid code (triplet) is not altered by the change in bases, the point mutation is said to be synonymous. The resulting protein is not altered. If the incorrect base pairing does change the amino acid code (triplet), the point mutation is nonsynonymous. The resulting protein can be drastically affected. In some cases, the protein is incomplete and cannot function in its intended capacity. A frameshift mutation results from the insertion or deletion of a nitrogen base into the DNA sequence. Frequently, this affects the amino acid code (triplet) and the protein. Induced mutations are those changes in the DNA that result from exposure to toxins, chemicals, or radiation. These agents are called mutagens, because of their ability to cause a DNA mutation.
See Figure 4.8 spontaneous mutations
3. Question: What are the three major patterns of natural selection and what are their effects on the population?
Response: Directional selection is a pattern of natural selection in which one extreme of a trait is favored over other phenotypic traits. This selection causes allele frequencies to shift in one direction. The favored trait and its allele increase in frequency over time, while the non-favored traits and their alleles decrease in frequency. Stabilizing selection is selection against the extremes of a trait, favoring the average values. Over time, the genetic diversity is reduced for the population, as the frequency for the average traits increase and the frequencies for the extreme traits decrease. Disruptive selection is selection for the extremes of a trait. This leads to a discontinuous pattern of variation in which the frequencies of the extremes of a trait are substantially higher than the frequency for the average values. If maintained, this type of selection could lead to a speciation event.
See Figure 4.9 Types of Selection
4. Question: Why are there high levels of sickle cell allele in areas with high rates of malaria?
Response: Research has shown a relationship between sickle cell allele and malaria. In regions of endemic malaria, such as equatorial Africa, people who have normal hemoglobin are highly susceptible to infection and death from malaria. Those individuals who are homozygous for the sickle cell allele suffer the disabling and deadly consequences of sickle cell anemia. However, because they have very low oxygen content in their blood, the malarial parasite rarely infects them. Individuals heterozygous for sickle cell allele have the highest fitness. They do not have full-blown sickle cell anemia, or the high risk of death from the disorder. In addition, having one copy of the sickle cell allele affects the oxygen-carrying capacity of the red blood cells. This oxygen-diminished environment is not ideal for the malarial parasite, thus affording the heterozygotes with some protection from malaria. As a result, there is selection for the heterozygotes in malarial environments.
See Figure 4.16 Sickle-Cell Mutation
5. Question: What are the four forces (causes) of evolution?
Response: The four forces of evolution are: mutation, gene flow, genetic drift, and natural selection. Mutation is a random heritable change in a gene or chromosome, resulting from additions, deletions, or substitutions of nitrogen bases in the DNA sequence. Mutations may create advantageous, deleterious, or neutral traits for the organism. Gene flow is the exchange of genetic material between two populations. Also known as admixture, gene flow works to decrease the variation between the two populations. Genetic drift is the random change in allele frequency from one generation to the next. Genetic drift has much more effect in small populations, which may have an allele drift to fixation, in which all members have that allele. Natural selection is the process by which some organisms have a greater chance of surviving and reproducing than others due to features that are better adapted to the environment. As a result, those advantageous features are passed on at a higher frequency than less advantageous traits.