Chapter 17 Emergence of Evolutionary Thought
I. Fire, Brimstone, and Human History
A. About 3,000 years ago, tremendous volcanic activity in the Mediterranean Sea area caused widespread destruction.
B. Ignorant of the geologic forces responsible for these events, people in the region interpreted them as punishment.

II. Growing Awareness of Change
A. The Great Chain of Being
1. Many of the ancient Greeks, including Aristotle, attempted to explain the natural world by making direct observations.
2. By the fourteenth century, the ancient view of gradual levels of organization from lifeless matter to the most complex organisms had been formalized into the Great Chain of Being.
a. The chain extended from lowest forms to spiritual beings.
b. Each being (species) had its fixed place in the divine order—unchanged and unchanging since creation.
B. Questions from Biogeography
1. When global voyages of the sixteenth century revealed unusual species not known in Europe, the students of biogeography began to question, “Where do all these species ‘fit’ in the Great Chain?”
2. Furthermore, if all species had been created at the same time and place, “Why were certain species found in only some parts of the world but not others?”
C. Questions from Comparative Anatomy
1. Studies of comparative anatomy of seemingly unrelated animals led to questions of why certain structures should be so similar.
2. One explanation: Some body parts were so perfect at the time of creation there was no need for any variation; but what about bones still present but without function (pelvic girdle in snakes, tail bones in humans)?
D. Questions About Fossils
1. Studies of stratification (layering of Earth’s rocks) revealed that deposits had been laid down slowly, one above the other.
a. The layers held recognizable remains or impressions of organisms—fossils.
b. The arrangement of the layers suggested that different organisms had lived at different times.
2. de Buffon’s explanation: Perhaps species originated in more than one place, and perhaps species became modified over time—evolution!

III. Attempts to Reconcile the Evidence With Prevailing Beliefs
A. Cuvier’s Theory of Catastrophism
1. Georges Cuvier believed in an original creation of all species.
2. Cuvier further suggested that the abrupt changes in the fossil record in different rock strata reflected the concept of catastrophism.
a. After each catastrophe, fewer species remained.
b. The survivors were not new species; it was just that their ancestors’ fossils had not been found.
B. Lamarck’s Theory of Desired Evolution
1. Lamarck believed that simple forms had changed into more complex ones by a built-in drive for perfection up the Chain of Being.
a. The drive was by “fluida” substances in the nerves.
b. For instance, a giraffe stretching its neck to reach higher branches would result in longer necks in the offspring.
2. His theory of inheritance of acquired characteristics expounded the idea that changes are brought about by environmental pressure and internal “desire” but there was no proof.

IV. Darwin’s Journey
A. Naturalist Inclinations of the Young Darwin
1. As a child (early 1800s), Darwin was curious about nature, but in college he first pursued premedicine and finally a degree in theology.
2. Botanist John Henslow arranged for Darwin (at age 20) to sail around the world as a ship’s naturalist.
B. Voyage of the Beagle
1. Throughout the trip, Darwin studied and collected a variety of plants and animals.
2. He was also reading Lyell’s Principles of Geology, which proposed uniformitarianism—the notion of a gradual, lengthy molding of the earth’s geologic structure.
3. Thus, the earth was possibly millions, not thousands, of years old—enough time for evolution.
C. Evolution by Natural Selection: The Theory Takes Form
1. Darwin returned after five years at sea and began pondering the “species problem”—what could explain the remarkable diversity among organisms?
2. Field observations provided two clues:
a. In Argentina, Darwin observed extinct glyptodonts that bore suspicious resemblance to living armadillos; Darwin wondered if the present species had evolved from the extinct one.
b. On the Galapagos Islands, the dozen or so species of finches all varied from one another to some extent but resembled the mainland finches to some degree also; perhaps they had descended from common ancestors.
3. How could these modifications occur?
a. Thomas Malthus had suggested that as a population outgrows its resources, its members must compete for what is available.
b. Darwin felt that if some normally variant members of a population bore traits that increased their survival, then nature would select those same individuals to survive, reproduce, and possibly change future populations’ traits.
4. The Theory of Natural Selection
a. First Correlation
1) All populations tend to reproduce beyond the resources available to sustain them.
2) Population sizes remain quite stable.
3) Natural resources also are remarkably stable.
4) Limited resources put limits on population growth.
5) Therefore: When resources become limited, competition dictates that not all individuals will survive.
b. Second Correlation
1) Members of a population show inheritable variation of traits.
2) Some heritable traits are more adaptive than others.
3) Over time, natural selection (a measurable difference in survival and reproduction of individuals) occurs.
4) Therefore: Over time the character of the population changes (evolves) as traits increase and decrease in frequency.
5. Alfred Wallace reached much the same conclusion independently, but encouraged Darwin to publish (alone) his ideas in book form in 1859.
6. Fossil evidence of transitional forms first appeared in Archaeopteryx (1861) and subsequently in genetic studies.



Chapter 18 Microevolution
I. Designer Dogs
A. The many varieties, or breeds, of dogs have been produced through artificial selection by humans.
B. Some breeds would have been selected against by nature had it not been for human protection.

II. Microevolutionary Processes
A. Variation in Populations
1. Populations evolve, not individuals.
a. A population is a group of individuals belonging to the same species, occupying the same given area, and showing certain morphological, physiological, and behavioral traits in common.
b. A population exhibits immense variation in the individual members, which by definition are of the same species.
2. Sources of variation (caused by genes, manifested in phenotype) include:
a. Gene mutations create new alleles.
b. Abnormal changes in chromosome structure or number can occur.
c. Crossing over and genetic recombination are normal results of meiosis.
d. Independent assortment of chromosomes occurs in meiosis.
e. Fertilization between genetically varied gametes produces “new” combinations of genes.
3. Only mutation creates new gene forms; all others listed above shuffle existing genes.
4. Genetic Equilibrium
a. Allele frequencies change when a population is evolving.
b. The Hardy-Weinberg rule is used to establish allele frequencies at genetic equilibrium (no evolution), which is possible under these conditions:
1) No mutations are occurring.
2) The population is very, very large.
3) The population is isolated from other populations of the same species.
4) All members survive, mate, and reproduce (no selection).
5) Mating is random.
c. Because these five conditions are rarely fulfilled in natural populations, any deviation from the reference point established by the “rule” will indicate evolution.
d. Microevolution is the change in allele frequencies brought about by mutation, genetic drift, gene flow, and natural selection.
B. Mutation
1. Mutation is heritable change in DNA and provides the ultimate source for phenotypic variation.
2. Mutations are random and may be harmful, neutral, or beneficial to the individual depending on other interactions.
a. A lethal mutation is an expression of a gene that results in death.
b. Neutral mutations whether or not they are expressed in phenotype have no effect on survival and reproduction.
c. Beneficial mutations are those that bestow survival advantages.
C. Genetic Drift
1. Genetic drift is the random fluctuation in allele frequencies over time, due to chance occurrences alone; it is more rapid in small populations.
2. In the founder effect, a few individuals (carrying genes that may/may not be typical of the whole population) leave the original population to establish a new one.
3. In bottlenecks, some stressful situation greatly reduces the size of a population leaving a few (typical or atypical?) individuals to reestablish the population.
4. Genetic drift reduces genetic variation, which may make populations more susceptible to disease or environmental change.
D. Gene Flow
1. Genes move with the individuals when they move out of, or into, a population.
2. The physical flow (and resultant shuffling) tends to minimize genetic variation between populations.
E. Natural Selection
1. Natural selection is a major microevolutionary process that results in the differential survival and reproduction of individuals of a population that differ in one or more traits.
2. Correlations between inheritance and populations:
a. Natural populations have great reproductive potential, but food supplies are usually limited; therefore, competition for survival occurs.
b. Members of a population show heritable traits, some of which are more adaptive than others; therefore, bearers of adaptive traits will leave more offspring (differential reproduction). Over generations the character of the population will change—evolve—as traits change in frequency.

III. Evidence of Natural Selection
A. Stabilizing Selection
1. Stabilizing selection favors the most common phenotype in the population.
2. It counters the effects of mutation, genetic drift, and gene flow.
3. Example: Human birth weights of about 7 pounds have greatest survival chances.
B. Directional Selection
1. Directional selection shifts allele frequencies in a consistent direction in response to environmental pressures.
2. Peppered Moths
a. At first the light-gray form of the peppered moth enjoyed a survivorship advantage on the light-gray tree trunks, but when industrial pollution darkened the tree trunks, the numbers of dark-gray moths increased because they escaped notice by bird predators.
b. Mark-release-recapture methods showed that more dark moths were recaptured in the polluted (dark tree trunks) area.
3. When insecticides are first applied, susceptible insects (most of the population) die, but the few that have the adaptation that affords survival will live and pass the heritable character on; eventually most of the population will become resistant.
C. Disruptive Selection
1. Disruptive selection favors forms at the extremes of the phenotypic range and selects against the intermediate forms.
2. Finches on the Galapagos Islands belong to groups possessing either long beaks or deep, wide beaks; these adaptations allow exploitation of food resources.

IV. Selection and the Maintenance of Different Phenotypes
A. Balanced Polymorphism
1. This is a variation on the stabilizing theme in which two or more forms of a trait are maintained in fairly stable proportions depending on survival value in the environment.
2. For example: Humans with alleles for both normal hemoglobin (HbA) and sickle-cell hemoglobin (HbS) have the greatest chances of surviving malaria.
B. Sexual Dimorphism
1. Sexual selection is based on any trait that gives the individual a competitive edge in mating and producing offspring.
2. Usually it is the females that are the agents of selection when they pick their mates.

V. Speciation
A. Defining the Species
1. A species is one or more populations of individuals who can interbreed under natural conditions and produce fertile offspring, and who are reproductively isolated from other populations.
2. Speciation is the process whereby a group of formerly interbreeding individuals become reproductively isolated from the rest.
B. Divergence
1. Divergence is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between them.
a. Barriers will prevent gene flow.
b. When divergence is great enough, interbreeding will stop; speciation has occurred.
2. Geographic barriers can separate populations quickly, such as in an earthquake, or gradually as in climate changes.
C. Reproductive Isolating Mechanisms
1. These are defined as any aspect of structure, function, or behavior that prevents interbreeding.
2. Isolation of Gametes: Incompatibilities between egg and sperm prevent fertilization.
3. Mechanical Isolation: Two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding.
4. Hybrid Inviability and Infertility
a. Sometimes fertilization does occur between different species, but the hybrid embryo is weak and dies.
b. Sometimes hybrids are vigorous but sterile (example: mule produced by a male donkey and a female horse).
5. Behavioral Isolation: This works to prevent interbreeding by altering patterns of courtship to the extent that sexual union is not achieved.
6. Isolation in Time: Different groups may not be reproductively mature at the same season, or month, or year.
D. Speciation Routes
1. Allopatric Speciation
a. Geographic isolation by natural barriers may separate populations that then undergo genetic drift or selection.
b. Allopatric refers to the “different lands” the two species occupy.
2. Sympatric Speciation
a. Speciation within the “same native land” may arise by ecological, behavioral, or genetic barriers.
b. Examples include changes in food preference, or reproduction timing, or chromosomal abnormality.
3. Polyploidy
a. Polyploidy can also cause speciation in plants.
b. Many plants are polyploid because of their ability to reproduce asexually or self-fertilize.
c. Speciation via polyploidy is instantaneous.
d. Polyploidy is rare in animals.