Chapter 21 —Darwinism and the Evidence for Evolution
Lecture Outline
I. Theories of Species Creation and Diversity prior to Evolutionary Theory
A. Theory of Special Creation
1. Pattern Component
—Species are unchanged through time and are independent of one another.
2. Process Component —All species were created independently by God, as recently as 6000 years ago.
B. Theory of Spontaneous Generation
1. Pattern Component —New living organisms may appear suddenly wherever conditions are suitable. Examples: Flies appear on rotting meat; microorganisms appear in old milk and puddles of rainwater.
2. Process Component —Some new life-forms arise spontaneously from streams, soils, rotting meat, and other nonliving materials; not all life arises directly from living organisms.
II. Evidence That Species Are Not Independent of One Another, but Are Related
A. Geographic Proximity of Similar Species —Similar, but distinct, species are often found living close together in the same geographic area. ( Fig. 21.1
)
B. Homologies
1. Structural —similarities in body parts of different organisms. Example: Bone structures in human arm, horse leg, and bat wing. ( Fig.
21.2a
)
2. Developmental
—similarities in the overall morphology of embryos and in the fate of embryonic tissues. Example: Vertebrate embryos, groups of cells in the same position develop into the same structures in different species. ( Fig.
21.2b
)
3. Genetic
—similarities in genes and protein sequences in different organisms.
Example: Similar genes control eye development in fruit flies and mammals.
( Fig. 21.2c
)
4. Distinguishing Homology from Analogy a. Analogous structures appear similar, but are different in structure, organization, or function. Example: Morphology of ichthyosaurs (reptiles) and dolphins (mammals). ( Box 21.2, Fig. 1a ) b. Homologous structures appear similar, and are similar in structure, organization, and function. Example: the HOM genetic loci of insects and the
Hox loci of vertebrates. ( Box 21.2, Fig. 1b )
III. Evidence That Species Have Changed through Time
A. Law of Succession
—A particular geographic area frequently contains fossils that show a striking similarity to living species found in the same area. Example: Fossil sloths and living sloths are found only in South America. ( Fig. 21.3
)

B. Extinction of Species
—Fossil record shows organisms that are unlike any living organisms. Example: "Irish elk" described by Cuvier.
C. Vestigial Traits
—Rudimentary structures that have no function, but are homologous to functioning structures in similar organisms. Examples: Human coccyx to primate tailbone, human appendix to the caecum of other vertebrates. ( Fig. 21.4
)
IV. Theories of Evolution
A. Prior to Darwin: Inheritance of Acquired Characteristics —Lamarck ( Box 21.3
)
1. Pattern Component
—New simple life-forms arise by spontaneous generation and change over time into more complex forms.
2. Process Component —Individuals change in response to their environment, and the changes are passed to the next generation. Example: A giraffe stretches to reach leaves on the highest tree branches; the giraffe's neck grows longer, and the trait of having a longer neck is passed to its offspring.
B. Theory of Evolution by Natural Selection (Darwin and Wallace, 1858)
1. Pattern Component —Species are related to one another, and they change over time. Species existing today have descended from other preexisting species
("descent with modification").
2. Process Component
—Natural selection acts on individuals; individuals with certain favorable characteristics will survive and reproduce more. If the c. Pressure from Wallace spurs Darwin to publish; both papers are read simultaneously at the Linnean Society of London in 1858.
V. How Natural Selection Works
A. Four postulates of natural selection, the outcome of which is evolution:
1. Individual organisms in a population vary in traits (size, shape, color, etc.).
2. Some variations in the population are heritable and can be passed on to offspring.
3. More offspring are produced than can survive; that is only a subset will survive to reproduce.
4. Individuals with traits that confer an advantage will more likely survive to reproduce; and if the favorable traits are heritable, evolution of the population results. advantageous characteristics are heritable, the traits can be passed to their offspring, and over time, the population can change.
3. Darwin vs. Wallace ( Box 21.1
) a. Darwin travels the world on the HMS Beagle , observing organisms in many parts of the world. He writes paper on evolution in 1842, but does not publish for 17 years while he refines it and collects more data. ( Box 21.1
,
Fig. 1a ) b. Wallace studies natural history specimens in Malaysia; writes brief paper outlining the logic of evolution by natural selection, sends it to Darwin for review in 1858. ( Box 21.1
, Fig. 1b )

B. Fitness
1. The ability of an individual to survive and reproduce = "Darwinian fitness."
2. Fitness is estimated by measuring the number of offspring produced by one individual vs. another in the population.
C. Adaptation
—a heritable trait that increases the fitness of the individual with the trait relative to others that lack the trait.
VI. Evolution in Action: Antibiotic Resistance in the Tuberculosis Bacterium
A. Historical Background
1. Prior to antibiotics, TB caused 25% of all deaths in New York City and 33% of all deaths in Paris in the 1800s. ( Fig. 21.5
)
2. 1950 to 1990 —Antibiotics are successful at treating TB in developed nations.
( Fig. 21.6
)
3. Late 1980s
—Resurgence in TB in developed countries due to strains carrying antibiotic resistance.
B. Case Study
1. Patient admitted to hospital in Baltimore with active TB.
2. Lung cultures taken at week 1 —TB bacteria are present.
3. Patient is given rifampin antibiotic for 40 weeks, released when lung cultures show no bacteria.
4. Week 48 —Patient is readmitted with TB symptoms; lung cultures again show TB bacteria.
5. Patient is given further antibiotic treatment, but dies in 10 days.
C. Evidence That Evolution of Resistance in TB Bacteria Occurred in the Patient
1. TB bacteria from lung cultures at week 1 are sensitive to rifampin, but TB bacteria cultured at week 48 are resistant to rifampin.
2. DNA of TB bacteria from week 1 and week 48 are sequenced and compared; they differ by one nucleotide in the gene coding for RNA polymerase enzyme. a. Rifampin binds to the RNA polymerase enzyme of week-1 TB bacteria and interferes with enzyme activity. This prevents growth of the bacteria. b. Rifampin does not bind to the mutant form of RNA polymerase in the bacteria from week 48, due to a single amino acid change in the enzyme; thus the enzyme functions and the bacteria grow.
3. The development of antibiotic resistance in the TB patient exhibits the 4 postulates of natural selection: a. The two populations of TB bacteria show variability in their characteristics, and the variation is heritable —the difference is in a gene ( natural selection postulates 1 and 2 ). b. Nonrandom selection occurred
—That is, only those bacteria that developed resistance to rifampin survived to reproduce ( natural selection postulates
3 and 4 ).
VII. Evolution in Action: Pollinator Effects on Variation in Alpine Skypilot Plants ( Polemonium viscosum )

A. Observations of Alpine Skypilot Plants Above and Below Timberline ( Fig. 21.7
)
1. Plants growing in the treeless tundra above timberline a. Have larger flowers with longer stalks b. Are sweet-smelling c. Are pollinated by bumblebees
2. Plants growing in the stunted forest at or below timberline a. Have smaller flowers with shorter stalks b. Are "skunky" smelling c. Are primarily pollinated by flies
3. The observations demonstrate natural selection postulate 1 —Variation exists in the population of alpine skypilot plants.
B. Is the Variation Observed in the Alpine Skypilot Plants Heritable?
1. Genes coding for flower size have not been identified.
2. Quantitative traits (size, height, color, etc.) usually involve multiple genes and environmental effects.
3. Heritability Study —determines the proportion of variation in a trait that is due to genetic variation (in cases where genes have not been identified).
4. Heritability Experiment on Alpine Skypilots a. Randomly selected large and small flower plants were taken from the wild, grown in an enclosure, and pollinated by bees. b. 617 seeds were collected from the plants and distributed to random locations in the timberline area habitat. c. Observation —58 plants survived to flower: Small-flowered plants produced seed that grew into small-flowered offspring, and large-flowered plants produced seed that grew into large-flowered offspring. d. Conclusion —Heritability (h 2 ) is close to 1, indicating a strong genetic effect and natural selection postulate 2; flower size in alpine skypilots is a heritable trait. ( Fig. 21.10
)
C. Hypothesis —Natural selection produced the differences in the alpine skypilots due to differences in pollinators: Bees prefer large, sweet-smelling flowers.
1. Prediction 1
—Individuals with larger flowers attract more bees.
2. Prediction 2 —Individuals that attract more pollinators have higher fitness.
3. Experiment —Do bees have a preference for larger flowers? a. Protocol
(1) Transfer randomly picked, sweet-smelling flowers to an enclosure.
(2) Release bees into the enclosure and allow pollination to occur.
(3) Count the number of seeds produced by each plant following pollination. b. Observations
(1) Plants with larger flowers and taller stalks received more bee visits.
( Fig. 21.8
)
(2) Individuals visited by the most bees produced the largest number of seeds. ( Fig. 21.9
) c. The observations provide evidence to support natural selection postulates 3 and 4
—Plants with a certain trait produce more offspring than those that lack the trait.
D. Can Natural Selection of Alpine Skypilots be Observed in the Field?

1. Experimental Protocol a. Allow bees to pollinate skypilot plants in the field, collect the seeds, and plant them at random locations in the field. b. Control —Hand-pollinate wild skypilot plants, collect the seeds, and plant them at random locations in the field.
2. Observation
—Bee-pollinated plants produced offspring with larger flowers than those from hand-pollinated plants. ( Fig. 21.11
)
3. Conclusion —Bees can exert natural selection on the population of alpine skypilot plants.