Chapter Study Outline
1. Demes, Reproductive Isolation, and Species
a. Evolution is about groups of potentially reproducing organisms.
b. Deme refers to members of a species that produce offspring.
c. All the genetic material within a population is referred to as the gene pool.
d. The term species refers to the populations and their members that are capable of breeding with each other and producing viable, fertile offspring.
i. Species are defined on the basis of reproductive isolation.
e. Population genetics studies change over time (or the lack of it) in gene pools.
2. Hardy-Weinberg Law: Testing the Conditions of Genetic Equilibrium
a. It is a method for studying genetic change in populations.
b. If no change is occurring within the population, gene frequencies at a locus remain the same.
c. If change is occurring, evolution is happening within the population.
3. Mutation: The Only Source of New Alleles
a. Mutation is the only source of new genetic information.
b. Mutation can be any heritable change in the structure or amount of genetic material.
c. Different types
i. Point, frameshift, transposable elements
d. Spontaneous mutations have no known cause.
e. Induced mutations are caused by environmental agents (mutagens).
f. Most mutations are harmless.
4. Natural Selection: Advantageous Characteristics, Survival, and Reproduction
a. Based on Darwin’s principle that individuals with advantageous characteristics will survive and reproduce in higher numbers (reproductive success)
b. Patterns of Natural Selection
i. Directional selection favors an extreme form of a trait.
ii. Stabilizing selection favors the average form of a trait.
iii. Disruptive selection favors individuals at both extremes.
c. Natural Selection in Animals: The Case of the Peppered Moth and Industrial Melanism
i. Numerous examples exist for natural selection in animals, such as the sea dragon and the much studied peppered moth.
ii. The peppered moth is the best evidence of natural selection documented.
iii. The peppered moth was found in England.
iv. The peppered moth had two forms: light and dark.
v. As pollution covered trees in the moths’ habitat, the lighter peppered moths were more easily preyed on by birds and the darker form became more prevalent.
vi. In the 1970s, stricter pollution laws again changed the moths’ habitat, and the darker form became the easier prey; the lighter form became more common.
d. Natural Selection in Humans: Abnormal Hemoglobins and Resistance to Malaria
i. The Hemoglobin S gene causes sickle-cell anemia in humans.
ii. Individuals with the SS genotype suffer from sickle-cell anemia, an illness fatal without medical intervention.
iii. The Geography of Sickle-Cell Anemia and a Possible Association with Malaria
(1) 20–30% of people living in equatorial Africa have the S gene.
(2) High frequencies of the S gene overlap areas where malaria is endemic (constantly present).
(3) A relationship has been documented between possession of one S gene and higher survival when exposed to malaria.
iv. The Biology of Sickle-Cell Anemia and Malarial Infection
(1) Heterozygotic individuals (those with the AS genotype) do not suffer from sickling crises that the SS genotype causes.
(2) They do possess somewhat lower oxygen levels in their hemoglobin, which is where the malaria parasite finishes its life cycle.
(3) The parasite generally cannot survive and reproduce.
v. History of Sickle-Cell Anemia and Malaria
(1) Sickle-cell tied to spread of Bantu people, who carried the S mutation into equatorial Africa
(2) The Bantu introduced agriculture into the region; large, cleared areas were ideal environments for mosquitoes carrying malaria.
vi. Other Hemoglobins and Enzyme Abnormalities
(1) Thalessemia, G6pd
(2) These show distributions and relationships to malaria similar to those of the sickle-cell gene.
5. Genetic Drift: Genetic Change Due to Chance
a. Random change in allele frequency over time
b. Can lead to one allele being lost and the other fixed in a population
c. May occur in a group that is endogamous (reproducing only within the group)
d. Founder Effect: A Special Kind of Genetic Drift
i. Occurs when a small segment of a population becomes reproductively isolated from the larger population
ii. Causes the smaller population to diverge genetically from the parent population
6. Gene Flow: Spread of Genes across Population Boundaries
a. Gene flow often refers to migration, though it is influenced by culture and social structure.
b. Effects of gene flow (exchange of genes between populations) have increased over time.
c. Genetic Markers Document Gene Flow: Agriculture and Origins of Modern Europeans
i. Differing theories of how agriculture was introduced to Europe can be studied through genetics.
ii. Genetic evidence demonstrates some gene flow but not a uniform westward expansion, leading to new hypotheses.