20.0 Introduction

1. Evolution Well Known, But Poorly Understood by Public
1. Many Feel Evolution Challenges Their Religious Beliefs fig 20.1
2. Trends to Teach Religious Dogma as Scientific Creationism

1. Sickle-Cell Anemia
1. Historical Description
1. First detected by Iron in 1904 fig 20.2
2. Causes red blood cells to assume irregular, elongated shapes
3. Common among African Americans
1. Disorder affects 3 in 1000 individuals
2. Determine frequency with Hardy-Weinberg: Ö 0.003 = 0.054
3. Frequency in white Americans is only 0.001

2. Molecular Basis of the Disease
1. Disease is often fatal
1. Recent therapies enable individuals to survive through childhood
2. 31% of affected individuals in U.S. die by age of 15

2. Disease affects shape of hemoglobin molecule
1. Hydrophobic valine substituted for polar glutamic acid
2. Creates "sticky" patch on surface of hemoglobin
3. Oxygen shields patch, no unusual interactions
4. Without oxygen "sticky" patches bind to other patches
5. Molecules form long, fibrous clumps that deform blood cell fig 20.3

3. Malaria and Balancing Selection
1. Sickle-cell trait
1. Heterozygous, Ss individuals
2. Produce few sickle-shaped cells

2. Frequency of recessive allele in Central African population = 0.12
1. 1 per 5 are heterozygous
2. 1 per 100 heterozygous recessive and die before reproducing
3. Recessive homozygotes die before reaching reproductive age

3. Recessive allele maintained at unusually high levels
1. Heterozygotes less susceptible to malaria
2. Heterozygous women more fertile than homozygotes

4. Environment acts to maintain allele frequency
1. Selective force in Africa is presence of malaria
2. Maintenance of allele has adaptive value in Africa fig 20.4
3. No such selective force in U.S. black population
4. Selection acts to eliminate allele in U.S.

2. Peppered Moths and Industrial Melanism
1. Description of the Peppered Moth
1. European moth that rests on trees during daytime
1. Prior to 1850 most had light-colored wings
2. After 1850 most had dark-colored wings
1. Possess dominant allele
2. Allele rare in populations until then

2. Observed dark tree trunks in industrial areas
1. Dark moths less conspicuous on their surfaces
2. Air pollution killed light-colored lichens

2. Selection for Melanism
1. Tutt hypothesis explained decrease in light-colored moths
1. Peppered forms more visible on sooty trees without lichens
2. Birds ate peppered moths resting on trees during the day
3. Black forms advantageously camouflaged fig 20.5

2. Kettlewell tested hypothesis
1. Reared populations of moths with equal representation of light and dark forms
2. Released into to sets of woods, polluted and unpolluted
3. More dark, melanic moths survived in polluted areas
4. More light moths survived in unpolluted areas
5. Further support by direct observation of predation

3. Industrial Melanism
1. Process where darker individuals replace lighter individuals
1. Occurred due to lack of predation of dark forms
2. Habitats darkened by industrial pollution

2. Dozens of species exhibit similar changes through Eurasia and North America

4. Selection Against Melanism
1. Trends reversing due to pollution controls
2. Example: England
1. Enactment of Clean Air Act in 1956
2. Sample population outside Liverpool since 1959
3. Frequency of dark form dropped from 94% in 1960 to 19% in 1994 fig 20.6
4. Similar reversals throughout Europe

3. Example: America
1. Industrial melanism of American subspecies not as widespread as in England
2. Well-documented in rural field station near Detroit
3. 576 peppered moths collected from 1959 to 1961: 89% melanic
4. American Clean Air Act passed in 1963
5. Resampling in 1994 found 15% melanic form fig 20.6

4. Both examples provide strong evidence of natural selection

5. Reconsidering the Target of Natural Selection
1. Reevaluation of Tutt's hypothesis
2. Selection against melanism does not correlate with changes in lichens
1. In England light moths appeared before lichens reappeared
2. In Detroit no significant changes in lichens
1. Can't find moths on trees at all
2. May rest in leaves on tree tops

3. Action of selection may depend on differences between forms other than wing coloration
4. Report difference in ability to survive as caterpillars in a variety of conditions
5. Natural selection may be targeting caterpillars, not adults

3. The Beaks of Darwin's Finches
1. Classic Example of Evolution by Natural Selection
1. Darwin collected specimens while visiting Galapagos Islands in 1835
2. Identified birds by examining beaks fig 20.7

2. The Importance of the Beak
1. Collection reexamined by Gould on return to England
1. All very similar except for bills
2. Categorized into 13 species

2. Categorization associated with feeding habits
1. Two ground finches with larger bills feed on seeds
2. Two ground finches with narrower bills eat insects
3. One is fruit eater
4. One is cactus eater
5. One drinks blood from sea birds
6. Woodpecker finches pick up twig, use it to probe for grubs

3. Correspondence of beaks and food source suggested shaping by evolution

3. Was Darwin Wrong?
1. If Darwin's suggestion is correct
1. Should find different species of finches with different evolutionary roles
2. Four seed-eaters should feed on different seeds
3. Stouter-billed finches should eat harder-to-crush seeds

2. Lack reexamined Galapagos in 1938, observations seemed to contradict Darwin
1. Many different species feed together on same seeds
2. Stout and slender-beaked birds fed on same seeds

3. Flaw in Lack's observations
1. He visited during a wet year with plentiful food
2. Both types of beak collect tender seeds equally well
3. Later observations revealed different picture
4. When few seeds are available differences are ore pronounced

4. A Closer Look
1. Examination of a particular medium species by Grants in 1973
1. Preferentially feed on small, tender seeds when abundant in wet years
2. Resort to larger, drier seeds, harder to crush when small seeds hard to find
3. Lean times come during periods of dry weather

2. Quantified beak shape among medium ground finches
1. Measured beak depth
2. Found that beak depth changed from year to year in predictable fashion
3. Beak depth increased in droughts, deceased with wet seasons fig 20.9

3. Test whether beak size is related to gene frequencies
1. Assume beak changes due solely to diet, poorly fed birds have stout beaks
2. Measured bill size in parents and offspring over many years
1. Depth of bill size passed from generation to next
2. Reflected gene differences

4. Switching eggs from one nest to another valuable, but not practical experiment

5. Darwin Was Right After All
1. Observations supported Darwin's conclusions
2. Changes can be predicted by pattern of dry years
1. Birds with stout beaks have advantage in dry years
2. In wet years slender beaks collect small seeds more efficiently

1. The Fossil Record
1. Provides Best Evidence of Macroevolution tbl 20.1
1. More complete understanding than in Darwin's time
2. Formation of fossils
1. Organisms buried in sediment
2. Calcium in bone and hard tissue is mineralized
3. Sediment converted to rock

2. Dating Fossils
1. Date of rocks reflects age of fossils
2. Dating in Darwin's day solely by relative position in sedimentary rocks
3. Recent dating uses more accurate absolute techniques
1. Measure rate of decay of radioisotopes
2. Rate constant over time, not affected by temperature or pressure
3. Rock acts as internal clock

3. A History of Evolutionary Change
1. Array fossils from old to young to provide evidence of progressive evolutionary change
2. Examples
1. Hoofed mammals fig 20.10
2. Horse evolution shows decreasing number of toes from four to one
3. Oyster shell shape fig 20.11

4. Gaps in the Fossil Record
1. Fossil record is more complete today
2. Fossils exist linking all major groups of vertebrates
3. Example: Extinct whale "missing links" recently discovered

2. The Molecular Record
1. Traces of Evolutionary Past Exists at Molecular Level
1. Progressive evolutionary change implies a change within DNA
1. Result from accumulation of genetic changes
2. Distant relatives have greater number of differences

2. Example: Evolution of human hemoglobin polypeptide fig 20.13

2. Molecular Clocks
1. Comparison of DNA sequences between organisms show same pattern
2. Greater time since divergence associated with more nucleotide changes
1. Example: Cytochrome c fig 20.14
2. Changes appear at constant rate, referred to as a "molecular clock"

3. Proteins Evolve at Different Rates
1. Highly conserved proteins (hemoglobin, cytochrome c) are best clocks
2. All proteins thus far show accumulation of change over time
3. Different proteins evolve at different rates fig 20.15
1. Fastest rate of change in fibrinopeptides
2. Most highly conserved rate is histone H4

4. Even faster rate of change in pseudogenes
1. Not transcribed
2. Evolution proceeds more quickly when not constrained by selection

4. Phylogenetic Trees
1. Pattern of genetic descent fig 20.16
1. Determined by comparing nucleotide sequences
2. Represents evolutionary history of gene

2. Often similar to relationships predicted by anatomy

3. The Anatomical Record
1. Homology
1. Same bones in vertebrates put to different uses
2. Homologous structures are derived from common form
1. May appear differently
2. Functions are variable
3. Example: Forelimbs of mammals fig 20.17

3. Analogous structures resemble each other, have similar function
1. Caused by parallel evolution in different linages
2. Example: Flippers of penguins and dolphins
3. Example: Similarities between marsupial and placental mammal populations

2. Development
1. Evolutionary history reflected in development of embryo
2. Embryo exhibits characteristics of its ancestors' embryos fig 20.18
3. Example: Human development
1. Possess fish-like gill slits early in development
2. Exhibit tail, its vestige becomes coccyx
3. Possess fine fur during fifth month of development

4. Compare larval forms of slug and giant squid to examine similarities

3. Vestigial Structures
1. Structures with no apparent function resembling those of presumed ancestors
2. Examples
1. Human ear-wiggling muscles tbl 20.2
2. Hip bones and rudimentary limbs in boa constrictors
3. Fingernails on fins of manatees
4. Baleen whale pelvic bone fig 20.19
5. Human vermiform appendix

3. Indicate presumed common ancestry of related organisms

4. Convergent Evolution
1. Different Areas Possess Only Distantly Related Communities with Similar Appearance
1. Unlikely that similarities result from coincidence
2. Natural selection favors parallel (convergent) evolution
3. Selection favors changes that makes phenotypes more alike
1. Similarity due to analogy, not homology

2. The Marsupial-Placental Convergence
1. Groups have live independently on separate continents
1. Separated over 50 million years ago
2. Marsupials were the only mammals present in Australia

2. Characteristics of marsupials
1. Young are born in immature condition
2. Held in pouch to complete development
3. Marsupials evolved prior to placental mammals

3. Placental mammals dominant on other continents, recently introduced to Australia
4. Appearance of both groups similar fig 20.20
1. Comparisons between individuals supports convergent evolution
2. Experienced similar selective pressures

3. Homology versus Analogy
1. Adaptation favoring different functions can obscure homologies
2. Convergent evolution creates analogs that appear as similar as homologs
3. Embryonic development often reveals features not apparent in adult forms
4. More complex structures are less likely to evolve independently

4. Darwin and Patterns of Recent Divergence
1. Organisms on islands most closely resemble forms on nearest continent
1. Example: Galapagos turtle fig 20.21
2. Island forms evolved from ancestor from adjacent mainland
3. Example: Galapagos finches fig 20.7

2. Relationship provides strong evidence for macroevolution
3. Example: Solitary finch from Cocos Island
1. Remote volcanic island north of Galapagos
2. Bird most resembles finches from Costa Rica, 500 kilometers to east

4. Forms not identical, but diverged over time
5. Difference in shell shape of tortoises
1. Dome-shaped shells in moist habitats
2. Saddle-backed shells in dry places

6. Island and continental forms have diverged, not converged

1. Scientific Creationism
1. Acceptance of Evolution
1. Universally supported by biologists to explain diversity
2. Supported by observations and experiments
3. Controversy even among scientists as to details of how evolution occurs

2. The Creationism Movement
1. Literal interpretation of the Bible
2. Religious, non-scientific perspective
1. Earth much younger than scientists believe
2. All organisms created as they exist today

3. Arguments to present as theory comparable to evolution
4. Acceptable premise if "scientific creationism" were indeed scientific
5. Confusion not in beliefs, but in labeling them as scientific

3. Creationism Is Not Science
1. Lacks empirical scientific observations
2. Does not infer principles from observation
3. Assumptions do not lead to testable, falsifiable hypotheses