Download 11 EVOLUTION AND NATURAL SELECTION

13 EVOLUTION AND NATURAL SELECTION
EXTENDED LECTURE OUTLINE
The Theory for Evolution (p. 270)
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13.0
Evolution: Getting From There to Here (p. 270; Fig. 131, 13.2)
A. Microevolution Leads to Macroevolution
1. Macroevolution is evolution on a grand scale, the formation of new species and major
changes in family lineages.
2. Microevolution, the kind Darwin considered, refers to minor changes in the genetic
composition of a population of interbreeding individuals as influenced by natural forces.
3. Adaptation results from microevolutionary changes that increase the likelihood of
survival and reproduction.
B. The Key Is the Source of Variation
1. Darwin proposed the idea of natural selection as the mechanism of evolution.
2. New species evolve from existing ones because certain individuals have traits that allow
them to produce more offspring that, in turn, carry the traits to the next generation.
3. Variation is not created by experience but already exists when selection acts on it.
C.
The Rate of Evolution
1. Mammals evolve slowly, and certain groups of fish evolve even more slowly.
2. Two opposing viewpoints about how rapidly speciation occurs are now being hotly
debated.
3. A hypothesis of gradual evolutionary change, called gradualism, assumes that
macroevolution occurs at a constant, gradual pace.
4. The contrasting model, punctuated equilibrium, predicts that major environmental
changes trigger bursts of rapid speciation, followed by periods of gradual change.
5. The fossil record supports both of these hypotheses.
6. In some lineages, speciation has been progressive and gradual.
7. In others, many new species appear all at once.
The Evidence for Evolution (p. 272; Figs. 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9)
A. The Fossil Record
1. The most direct evidence of macroevolution is found in the fossil record.
2. Fossils are the preserved remains, traces, or tracks of once-living creatures.
3. In Darwin’s day, the relative age of a rock was determined by its position relative to other
rocks.
4. Today, rocks are dated by measuring the rate of decay of certain radioisotopes contained
in the rock.
5. A History of Evolutionary Change
a. When fossils are lined up according to their age, they often provide evidence of
successive evolutionary change.
b. Many examples serve to illustrate a record of successive change and are one of the
strongest lines of evidence of evolution.
c. Today the fossil record is very complete and few gaps exist.
d. Among the vertebrates, fossils have been found linking all the major groups
B. The Molecular Record
1. The evolutionary past is also evident at the molecular level.
2. Since the record of evolutionary change is linked to changes in DNA, organisms that are
more distantly related will have accumulated a greater number of genetic changes in
DNA.
3. When analyzing nucleotide sequences for the gene encoding the protein cytochrome c,
biologists can construct a molecular clock showing the relatedness of organisms based on
how many nucleotide sequences they are away from each other.
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4. Proteins Evolve at Different Rates
5. Not all proteins evolve at the same rate.
6. Cytochrome c and hemoglobin have changed at relatively constant rates, but other
proteins, like the fibrinopeptides, evolve considerably faster.
7. Phylogenetic (family) trees can be constructed showing the relatedness of organisms
based on both fossil and molecular evidence.
C. The Anatomical Record
1. Many diverse organisms go through the same early stages of embryologic development,
which is evidence for evolutionary relatedness.
2. In vertebrates, homologous structures can be seen from the study of anatomy.
3. Vertebrate forelimbs have diverged to perform different functions, but consist of the same
bone structure, indicating a common ancestry.
4. Sometimes analogous structures are found in animals that have evolved the same solution
to a problem, although they did not share a common ancestor.
5. Vestigial organs—which served a function in an ancestor but have no function in the
modern counterpart (such as the appendix that has no function in humans but functions as
a reservoir for cellulose bacteria in apes)—are also anatomical evidence for evolution.
13.2
Evolution’s Critics (p. 276)
A. Critic’s of evolution raise seven principle objections to evolution
1. Evolution is not solidly demonstrated.
2. There are no fossil intermediates.
3. The intelligent design argument.
4. Evolution violates the Second Law of Thermodynamics.
5. Proteins are too improbable.
6. Natural selection does not imply evolution.
7. The irreducible complexity argument.
How Populations Evolve (p. 277)
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13.4
Genetic Change Within Populations: The Hardy-Weinberg Principle (p. 277; Figs. 13.10)
A. Population genetics is the study of the properties of genes in populations.
B. Genes Within Populations
1. The proportion of alternative forms of a gene, or alleles, in a population can be calculated
and the allele frequencies determined.
2. The equations of Hardy-Weinberg equilibrium can then be used to predict the frequencies
of genotypes in future populations.
3. According to Hardy and Weinberg, gene frequencies do not change when the size of the
population is large, when mating occurs at random, when natural selection is not
occurring, and while there are no mutations or migration.
4. The symbol p denotes the frequency of the dominant allele, and q stands for the frequency
of the recessive allele.
5. By definition, p + q = 1.
6. By expanding the binomial, (p + q)2 = p2 + 2pq + q2 = 1, where p2 is the proportion of
homozygous dominant individuals in the population, 2pq indicates the proportion of
heterozygotes, and q2 is the proportion of homozygous recessives.
Why Allele Frequencies Change (p. 279; Table 13.1)
A. Five factors alter the proportions of homozygotes and heterozygotes enough to produce
significant deviations from the proportions predicted by the Hardy-Weinberg principle.
B. Mutation
1. Genetic mutations, or alterations in DNA nucleotide sequences, add new combinations to
the genetic makeup of a population.
C. Migration
1. Migration from the movement of individuals into or out of the population can be a source
of new genetic variation.
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13.5
D. Genetic Drift
1. In small populations, by random chance alone, it is possible for the allele frequencies to
change from one generation to the next.
2. Such a phenomenon is termed genetic drift.
3. As local subgroups are isolated from the main population, genetic drift occurs in them as
well. The founder effect occurs when a few individuals are separated from the rest and
give rise, over time, to a new population; this effect often occurs on islands.
4. The founder effect is somewhat similar to the bottleneck effect in which a few members
of a population of species are all that are left to give rise to the next generations of that
species.
5. The bottleneck effect limits the genetic diversity of a population and can lead to the
appearance of recessive mutations.
E. Nonrandom Mating
1. Nonrandom mating and inbreeding (mating with relatives) also lead to changes in gene
frequencies from one generation to the next.
F. Selection
1. Selection, whether artificial selection by humans or natural selection, operates to select
certain fit phenotypes, which add more genes to successive generations.
Forms of Selection (p. 281; Figs. 13.12, 13.13, 13.14, 13.15)
A. How selection changes the population depends on which genotypes are favored.
B. Three types of selection have been observed in natural populations.
C. Stabilizing Selection
1. In stabilizing selection, individuals toward the middle of the range are selected.
D. Disruptive Selection
1. In disruptive selection, both extremes are favored to the demise of the middle phenotype.
2. Disruptive selection is much less common than the other two types of selection.
E. Directional Selection
1. Directional selection favors a phenotype at one extreme or the other of the population.
Adaptation Within Population (p. 283)
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13.8
Sickle-Cell Anemia (p. 283; Figs. 13.16, 13.17, 13.8)
A. Sickle-cell anemia is a hereditary disease in which the homozygous condition is often lethal.
B. The Puzzle: Why So Common?
1. In central Africa, one in 100 people is homozygous for the disorder and develops sicklecell anemia.
C. The Answer: Stabilizing Selection
1. Sickle-cell anemia in humans is an example of stabilizing selection in which the middle
phenotype, in this case the heterozygote with sickle-cell trait but not anemia, is more
adapted to an environment that hosts the malarial parasite.
2. Heterozygotes are resistant to the malarial parasite that otherwise kills the person with
normal hemoglobin; those with sickle-cell anemia perish due to this genetic abnormality.
Selection on Color in Guppies (p. 285; Fig. 13.19, 13.20)
A. Guppies Live in Different Environments
1. Guppies living in high-predation pools exhibit drab coloration and have a small adult size.
2. In absence of predators, male guppies are larger and exhibit gaudy colors.
B. The Experiments
1. A controlled experiment was conducted by John Endler in a laboratory to test the response
to differences in strength of predation.
2. The results established that predation can lead to rapid evolutionary change.
3. A field experiment was conducted by Ender that revealed that natural selection can lead to
rapid evolutionary change.
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How Species Form (p. 287)
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The Biological Species Concept (p. 287; Table 13.2)
A. According to Darwin's ideas, species form slowly over time as microevolutionary changes
accumulate and give rise to macroevolution, or the formation of new species.
B. A species is defined as a group of organisms that is unlike other such groups and that does not
integrate extensively with other groups in nature.
C. Species formation is the final step of a long process in nature.
1. Two separate local populations become adapted to the unique aspects of their own
environments and, after a period of time, they are considered different enough to be
called ecological races.
2. Natural selection reinforces the differences; these changes are isolating mechanisms.
Isolating Mechanisms (p. 288; Fig. 13.21, 13.22)
A. Prezygotic isolating mechanisms, whether physical or behavioral, prevent interbreeding, and
can lead to reproductive isolation after which separate species might exist.
B. Prezygotic isolating mechanisms lead to reproductive isolation by preventing the formation of
hybrid zygotes.
C. Postzygotic isolating mechanisms include improper development of hybrids and failure of the
hybrids to become established in either parental habitat.
Working With the Biological Species Concept (p. 290; Fig. 13.23)
A. Speciation is a two-part process.
1. Identical populations must diverge.
2. Reproductive isolation must evolve to maintain the differences.
B. Allopatric Divergence
1. Ernst Mayr demonstrated that geographically separated populations appear much more
likely to have evolved substantial differences leading to speciation.
C. Sympatric Speciation
1. Instantaneous sympatric speciation occurs when an individual is born that is
reproductively isolated from all other members of its species through polyploidy.
D. Problems With the Biological Species Concept
1. One criticism concerns the extent to which all species are truly reproductively isolated.
E. Natural Selection and the Ecological Species Concept
1. This concept is that natural selection is not likely to have any fewer exceptions than the
biological species concept, although it may prove more successful for certain types of
organisms or habitats.
F. Other Problems with the Biological Species Concept
1. It can be difficult to apply the concept to populations that do not occur together in nature.
2. Because of the diversity of living organisms, no single definition of what constitutes a
species may be universally applicable.
The Pace of Evolution (p. 290)
A. Evolution in Spurts
1. Evolutionary change can be slow and gradual, or rapid and discontinuous, separated by
long periods of stasis, which may result from stabilizing selection.
LEARNING OBJECTIVES
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Explain how microevolutionary changes can accumulate, leading to macroevolution.
Discuss how natural selection on variants within populations leads to the evolution of different species.
Contrast the punctuated equilibrium model with the gradualism model concerning the pace of
evolution.
Describe evidence for evolution as supported by the fossil record.
Describe how molecular evidence for evolution supports the fossil record.
List the types of evidence for evolution shown by comparative anatomy.
Know how to use the Hardy-Weinberg equilibrium equation.
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Understand how gene frequencies change from one generation to the next due to mutation, migration,
genetic drift, nonrandom mating, and selection.
Give examples of the three types of selection.
Discuss the case of sickle-cell anemia as an example of evolutionary adaptation.
Explain how natural selection favors different morphs of peppered moths in different situations.
Define a species.
Differentiate between how prezygotic and postzygotic isolating mechanisms lead to reproductive
isolation.
KEY TERMS
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macroevolution (p. 270) Evolutionary change on a grand scale; new kinds of organisms; episodes of
extinction.
microevolution (p. 271) Many people who refuse to acknowledge the evidence for macroevolution are
able to accept the notion of microevolutionary changes (i.e., antibiotic resistant strains of bacteria).
Convince your students that an accumulation of microevolutionary changes can lead to macroevolution
over a long period of time.
adaptation (p. 271) Adaptation results from microevolutionary changes that increase the likelihood of
survival and reproduction.
punctuated equilibrium (p. 271)
gradualism (p. 271) Also known as “phyletic gradualism”, this model assumes that evolution proceeds
gradually, and change is progressive in the fossil record.
fossil (p. 272)
radioisotopic dating (p. 273) Radioisotopic dating is explained in greater detail in Chapter 3.
molecular clock (p. 273) Particularly in reference to cytochrome c, changes in nucleotide sequences
appear to occur at a constant rate, providing a “molecular clock”.
phylogenetic tree (p. 274) A pattern of descent that represents the evolutionary history of a gene.
homologous structure (p. 274) These are derived from the same ancestor.
analogous structure (p. 274) These are the result of similar environmental pressure but with separate
evolutionary histories.
population genetics (p. 277) The study of the properties of genes in populations.
allele frequency (p. 277) Allele frequency refers to the proportion of alleles of a given gene.
Hardy-Weinberg equilibrium (p. 277)
stabilizing selection (p. 281) Human infant birth weight is a good example of this type of selection.
disruptive selection (p. 282) This form is less common than the other two.
directional selection (p. 282)
sickle-cell anemia (p. 283)
heterozygote advantage (p. 284)
speciation (p. 287)
isolating mechanism (p. 288) There are a number of isolating mechanisms, but those that are
reproductively isolating in the end are most important.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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Fossils. Demonstrate what a fossil is by making an imprint of a leaf in some sand or soil. Describe how
scientists study imprints of soft structures. Then show students a number of examples of fossils,
describing what they are and, if possible, their ages. Discuss how the age of fossils is determined and
describe other types of organisms that lived at the same time as the fossils they are viewing.
Changing Gene Frequencies. Divide students into groups and pass out 10 red and 10 black jellybeans
to each group (any pair of color combinations will do). With a population of 20, they can easily see that
the dominant gene has a frequency of 0.5, the same as the frequency of the recessive phenotype. Devise
a scenario to change gene frequencies in the succeeding generations. For example, a certain type of
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predator (jelly bean jackal) prefers red jellybeans to black, and as long as they are fairly abundant in
the population, the jackal will take the time to seek out, chase down, and consume the reds. Remove 5
red jellybeans and replace them with 5 black ones (keep the total population at 20, assuming that's the
carrying capacity for the environment). Show how gene frequencies are changed. In the next
generation, remove 3 more red jellybeans and replace them with black ones, again illustrating the
change. Now, with few red jellybeans to be found, the jellybean jackal must alter his eating habits a bit
and select equally from blacks and reds. Have students remove 10 jellybeans at random and then
calculate the gene frequencies of the remaining group of 10. You should see differences among groups
as to gene frequencies, illustrating both what can happen by random chance as well as what can happen
through time as gene frequencies are affected by natural selection.
CHANGES TO THE NEW EDITION
Refer to the Johnson instructor web site at http://www.mhhe.com/biosci/genbio/tlw4 for a complete list of
changes to this edition.
CRITICAL THINKING QUESTIONS
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Based on what you know about the pace of evolution, argue whether punctuated equilibrium or
gradualism is the appropriate model. Comment on the suggestion that both types of evolution have
occurred.
Devise a scenario where two ecological races become reproductively isolated.
Explain how the bottleneck effect, such as is now experienced in living cheetahs, could make this
species more susceptible to disease.
FILMS/MEDIA SUGGESTIONS
(Telephone and fax numbers and/or web sites for the sources of the following materials are listed in the
Appendix.)
Theories of Evolution. The theories of Lamarck and Darwin are explored in this video along with the
major categories of evidence for evolution. The competing theories on the pace of evolution are discussed.
27 min.
CLEARVUE/eav, WW6VH 1447
Evolution. This two-part video teaches the basics of modern evolutionary theory. Part I explains how the
theories of evolution were developed. Part II shows evolution by natural selection. 1994. 38 minutes.
Hawkhill Video, #108
Charles Darwin Explains the Diversity of Life. This video explains the work of Charles Darwin and how
the theory of evolution laid the foundation for all modern biological thought. 1994. 21 minutes.
Hawkhill Video, #192
The Origin of Species- Creationism and Evolutionism. Travel along Darwin’s journey and see what he
encountered that led him to his ideas on natural selection.
CLEARVUE/eav, WW6VH 2902
Natural Selection. This video demonstrates how continental drift and isolation can lead to speciation. 20
minutes.
Carolina Biological Supply, CE-49-2206V
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