Download CHAPTER 3: EVOLUTION, GENETICS, AND HUMAN VARIATION

CHAPTER 3: EVOLUTION, GENETICS, AND HUMAN VARIATION
CHAPTER OVERVIEW
This chapter discusses the theory of evolution by natural selection, Mendelian genetics, population
genetics, and the key mechanisms of genetic evolution. Particular attention is given to how human
biological variation (e.g., in skin color, blood type, and production of the enzyme lactase) has been
shaped by natural selection acting in a given population and a particular environment.
CHAPTER OBJECTIVES
1.
Know the key dimensions of Darwin’s (and Wallace’s) theory of evolution by natural selection.
Consider the influence of the geologic principle of uniformitarianism on Darwin’s work and
subsequent research in human evolution and other fields. Compare and contrast the other
perspectives on the origin of life that existed in Darwin’s time.
2.
Know the importance of Mendel’s experiments and their implications for evolution. Identify the
key contributions of Mendelian genetics to the study of hereditary traits.
3.
Be familiar with the subject matter of population genetics.
4.
Be able to distinguish the key mechanisms of genetic evolution and understand the role each
mechanism plays in evolution.
5.
Understand how human biological variation (e.g. in skin color, blood type, and production of the
enzyme lactase) has been shaped by natural selection acting in a given population and a particular
environment, keeping in mind changing and competing selective forces.
6.
Consider the role of phenotypical adaptation in lactose tolerance and its relationship to genetic
adaptation organized through natural selection.
CHAPTER OUTLINE
I. The Origin of Species
A. According to creationism, the biological characteristics of all life forms were divinely created
during six days of Creation (described in Genesis, the first book of the Bible) and therefore
are immutable.
B. Carolus Linnaeus developed the first biological classification (taxonomy) of plants and
animals based on similarities and differences in their physical characteristics.
C. In the 18th and 19th centuries, the discovery of fossils from species that no longer existed, as
well as the absence of contemporary species in the fossil record, raised doubts about
creationism.
D. According to catastrophism, a modified version of creationism, ancient species were
destroyed by fires, floods, and other catastrophes and then replaced by new, divinely created
species.
E. Theory and Fact
1. The alternative to creationism and catastrophism was transformism, also called evolution.
2. Darwin was influenced by Sir Charles Lyell’s principle of uniformitarianism.
a. Uniformitarianism states that past geological events can be best explained by
observing the ongoing events of the present and generalizing backward through time.
b. It further asserts that current geological structures are the result of long-term natural
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forces.
3. Darwin applied the theory of the long-term transformation through natural forces to living
things, and, when integrated into evolutionary theory, uniformitarianism cast doubt on
whether the world was only 6,000 years old, as posited in creationism.
4. Like other evolutionists, Darwin argued for the primordial relatedness of all life forms.
5. The fact of evolution was known prior to Charles Darwin's influential research and writing
(e.g., by Darwin's grandfather Erasmus Darwin who proclaimed in 1794 the common
ancestry of all animal species in Zoomania), but Charles Darwin importantly contributed a
theory of evolution through natural selection (explaining how evolution occurred).
6. Darwin posited natural selection as the single theory that could explain the origin of
species, biological diversity, and similarities among related life forms (reaching this
conclusion along with Alfred Russell Wallace).
7. Natural selection is the gradual process by which the forms most fit to survive and
reproduce in a given environment do so in greater numbers than others in the same
population.
8. For natural selection to work on a given population, there must be variety within that
population (as there always is) and competition for strategic resources.
9. The concept of natural selection argues that organisms which have a better fit within their
environmental niche will reproduce more frequently than those organisms that fit less
well.
10. Through a gradual, branching process, involving adaptation to thousands of
environments, natural selection has produced the diverse plants and animals found in
the world today.
II. Genetics
A. The science of genetics explains the origin of the variety upon which natural selection operates.
1. Mendelian genetics studies the way in which chromosomes transmit genes across the
generations.
2. Biochemical genetics examines structure, function, and changes in DNA
(deoxyribonucleic acid).
3. Population genetics investigates natural selection and other causes of genetic
variation, stability, and change in breeding populations.
B. Mendel's Experiments
1. The study of hereditary traits was begun in 1856 by Gregor Mendel, an Austrian monk.
2. By experimenting with successive generations of pea plants, Mendel came to the conclusion
that heredity is determined by discrete particles, the effects of which may disappear in one
generation and reappear in the next.
3. Mendel determined that the traits he observed occurred in two basic forms: dominant and
recessive.
a. Dominant forms manifest themselves in each generation.
b. Recessive forms are masked whenever they are paired with a dominant form of the same
trait in a hybrid individual.
c. It has since been demonstrated that some traits have more than these two forms--human
blood type, for example, has several forms, some of which are codominant.
4. The traits Mendel identified occur on chromosomes.
a. Humans have twenty-three matched pairs of chromosomes, with each parent contributing
one chromosome to each pair.
b. Chromosomes contain several genes, or genetic loci, which determine the nature of a
particular trait.
c. A trait may be determined by more than one gene.
d. Alleles are the biochemically different forms which may occur at any given genetic locus.
e. Chromosome pairs’ loci may be homozygous (identical alleles) or heterozygous (mixed).
5. Dominance produces a distinction between genotype, or hereditary makeup, and phenotype, or
expressed physical characteristics.
C. Independent Assortment
1. Mendel also determined that traits are inherited independently of one another.
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2. The fact that traits are transmitted independently of one another, and hence may occur in new
combinations with other traits, is responsible for much of the variety upon which natural
selection operates.
3. An organism develops from a fertilized egg, or zygote, created by the union of two sex cells, a
sperm and an egg (ovum).
4. A zygote grows through mitosis, which is ordinary cell division wherein one cell splits to form
two identical cells.
5. Sex cells are produced through meiosis, a type of division wherein four cells are produced
from one, each with half the genetic material of the original cell (i.e., twenty-three
chromosomes instead of forty-six).
6. Fertilization allows the products of meiosis from one parent to recombine with those from the
other parent.
7. Chromosomes sort independently, so a human child’s genotype is a random combination of
the DNA of its four grandparents.
III. Population Genetics
A. Population genetics studies stability and change in gene frequencies within breeding
populations.
1. Gene pool refers to all the alleles, genes, chromosomes, and genotypes within a
breeding population.
2. Genetic evolution is defined as change in gene frequency, that is, the frequency of
alleles in a breeding population from generation to generation.
B. The four mechanisms which contribute to changes in gene frequency in a breeding population
are natural selection, mutation, random genetic drift, and gene flow.
IV. Mechanisms of Genetic Evolution
A. Natural Selection
1. Genotype refers to the genetic makeup (genes and chromosomes) of an organism.
2. Phenotype—the organism’s evident biological characteristics—develops over the
years as the organism is influenced by particular environmental forces.
3. Natural selection can operate only on phenotype.
4. Human biology has considerable plasticity; rather than being set at birth, it is
changeable and may be affected significantly by environmental forces.
B. Directional Selection
1. Natural selection affects gene frequencies within a population.
2. Traits that are adaptive (favored by natural selection) in a particular environment are
selected for—that is, organisms containing the alleles responsible for those traits will
reproduce more frequently.
3. As a result of directional selection, or the long-term selection of the same adaptive
trait(s) from generation to generation, maladaptive recessive alleles may be removed
from a gene pool.
4. Directional selection continues as long as environmental forces stay the same.
5. Sexual selection, based on differential success in mating, is the process by which
certain traits of one sex are selected because of advantages they confer in winning
mates.
C. Stabilizing Selection
1. Just as directional selection can reduce variety in a population, selective forces can
also maintain genetic variety by favoring a situation in which the frequency of two or
more alleles of a gene remain constant between generations. This is known as a
balanced polymorphism.
2. One well-studied example of a balanced polymorphism involves two alleles, HbA and
HbS, that affect the production of the beta strain (Hb) of human hemoglobin.
a. Homozygous HbA produces normal hemoglobin; homozygous HbS produces lethal
sickle-cell anemia; heterozygosity for this gene produces the (in some
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circumstances) deleterious but nonlethal sickle-cell syndrome.
b. It was discovered in certain populations in Africa, India, and the Mediterranean
that HbS existed at surprisingly high frequencies.
c. This is largely explained by the fact that the populations noted were in heavily
malarial areas, and that the heterozygous form produced a phenotype that was
resistant to malaria, and was thus the phenotype most fit for that environment.
3. It is important to note that traits that are maladaptive in one environment, such as how
the heterozygote HbA HbS would be in a malaria-free zone, can be adaptive in a
different environment, and the reverse of this is also true.
D. Mutation
1. Mutations, or changes in the DNA molecules of which genes and chromosomes are
built, provide variety on which natural selection may operate.
2. If a mutation occurs in a sex cell that combines with another as a fertilized egg, the
new organism will carry the mutation in every cell.
E. Random Genetic Drift
1. Random genetic drift is a change in allele frequency that results from chance rather
than from natural selection.
2. Although genetic drift can operate in any population, large or small, fixation due to
drift is more rapid in small populations.
F. Gene Flow
1. Gene flow is the exchange of genetic material between populations of the same
species.
2. Gene flow, like mutation, provides variety on which natural selection can operate.
3. Gene flow is important in the study of speciation, or the formation of new species
(groups of related organisms whose members can interbreed to produce offspring that
can live and reproduce).
a. Speciation occurs when populations of the same species are isolated from
each other (i.e., gene flow is stopped) and, as a result of genetic change
(through natural selection, mutation, or genetic drift), eventually become
incapable of interbreeding.
b. Gene flow tends to prevent speciation unless subgroups of the same species
are separated for a sufficient length of time.
V. Human Biological Adaptation
A. Because of extensive gene flow and interbreeding, Homo sapiens has not evolved subspecies
or distinct races.
1 Biological variation between human populations involves gradual shifts (clines) in
gene frequencies and other biological features, rather than sharp breaks we would
associate with discrete races.
2. Abundant evidence exists for human genetic adaptation and thus for evolution (change
in gene frequency) through selection working in specific environments.
B. Genes and Disease
1. Infectious diseases posed an increasing threat to human mortality with the emergence of
food production around 10,000 years ago.
2. Despite the advances in medical research over the last 100 years, diseases still pose a
significant threat to the health of human populations all over the world. Tropical
diseases, such as malaria, schistosomiasis, and filariasis, affect more than 10 percent of
the world’s population today.
3. Human blood types play an important role in resistance to some diseases.
a. There is evidence that the various alleles producing human blood types interact with
infectious and noninfectious ailments.
b. For example, the presence of type A or AB blood cells seems to make a person more
susceptible to smallpox, while the presence of O or B blood cells appears to increase
resistance to smallpox.
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C. Lactose Tolerance
1. Phenotypical adaptation refers to adaptive changes that occur during an individual’s
lifetime.
2. Genes and phenotypical adaptation together produce differences in the degree of
lactose tolerance (the ability to digest milk) among human populations.
3. Most individuals from herding populations in northern Europe and parts of Africa
maintain their ability to digest milk (because they continue to produce the enzyme
lactase) into adulthood, whereas people from nonherding populations can digest milk
only during childhood.
4. The fact that descendents of herding populations continue to be lactose tolerant, even
though they are not herders, indicates genetic adaptation to a milk-rich diet.
5. The fact that lactose tolerance can vary during an individual's life, depending on how
much milk is consumed, indicates that some phenotypical adaptation also takes place.
VI. Anthropology Today: Three Adaptations to Thin Air
A. People in the Andean Altiplano in South America, the Tibetan Plateau in Asia, and the
Ethiopian Highlands in East Africa, all living at altitudes of at least 8,000 feet (2,500 meters),
have evolved three distinctly different biological adaptations for surviving in oxygen-thin air.
B. Andeans adapted to the thin air by developing an ability to carry more oxygen in each red
blood cell.
C. Tibetans increase their oxygen intake by taking more breaths per minute than people who live
at sea level. A second biological adaptation may expand their blood vessels and allow them to
deliver oxygen more effectively throughout the body than those who live at sea level.
D. Scientists have yet to discover how Ethiopian highlanders’ bodies have evolved to survive at
high altitude.
E. Knowing how long the populations have been living in high altitude environments is crucial
to answering the evolutionary question of whether these adaptations are the result of
differences in the founding populations, random genetic mutations, or the passage of time.
LECTURE TOPICS
1.
Discuss the significance of the cline for understanding human variation and adaptation.
2.
Identify the sources of biological diversity to explain the genetic mechanism and the relationship
between genotype and phenotype. List the types of diversity and the sources of diversity.
Explain the relationships among diversity, adaptation, and selection.
3.
Explain how natural selection is "blind," or not working toward a predetermined end. Explain the
relationships among differential reproduction, key resources, key hazards, and natural selection.
4.
Discuss the relationship of evolutionary theory and the other perspectives on the origin of life
which existed in Darwin’s time and those which exist today (e.g., intelligent design, creationism,
scientific creationism). Address the intertwined European social history of science and religion,
and the relationship of science and religion in the early history of the discipline of anthropology.
Consider the overlapping, perhaps competing, forms of explanation provided by science and
religion today. At the same time, how do science and religion respond to different problems and
questions?
SUGGESTED FILMS
DNA and the Evidence for Evolution
20 minutes
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This film presents the structure and replicating processes of DNA and the effect of genetic mutation. The
film uses DNA and fossil evidence to demonstrate that the process of adaptation and the selection of
adaptors rest on a wide range of genetic variability. From Films for the Humanities and Sciences.
Lifelines: Darwin and the Theory of Inheritance
1998 30 minutes
In this film, experts in Darwinian evolution discuss three important researchers and their contributions to
evolutionary genetics: Charles Darwin, Gregor Mendel, and Friedrich Meischer. From Films for the
Humanities and Sciences.
The Blind Watchmaker: The Evolutionary Ideas of Richard Dawkins
1987 49 minutes
This film presents the views of both creationists and Darwinists regarding evolution. Based on and
inspired by the book by Richard Dawkins, this film argues that the diversity and complexity we see in
biological life can best be explained by cumulative natural selection over long periods of time. A BBC
Production.
The Anthropologist
1992 50 minutes
This film examines how Rudolph Poch's anthropological studies were used during Hitler's eugenics
movement in WWII. Combining drama and documentary, the film raises important questions about the
use and misuse of anthropological research. From the University of California Extension Center for
Media and Independent Learning.
.
Series: The Evolution of Darwin
6-part series
26 minutes each
This series is an introduction to the evidence for and against the Darwinian view of the origin of species.
The series includes: The Origins of Darwin’s Theory; The Theory of Inheritance; The Evolution of
Human Purpose; Darwin’s Theory Today; The Creationist Argument; The Evidence for Evolution. From
Films for the Humanities and Sciences.
Series: Evolution: The Evidence for Modern Ideas on Evolution
11-part series 20 minutes each
This series represents a minicourse in evolution. Titles in the series: The Evolution of Man; The Record
of the Rocks; Fossils: Plants and Tetrapods; Fossils: Reptiles and Mammals; Relationships: Structural
Homologies and Coevolution; Behavior and the Protein Record; Selection in Action: Natural Selection;
Selection and Adaptation; The Human Influence; Origins of Change: Heredity and Mutation; DNA and
the Evidence for Evolution. From Films for the Humanities and Sciences.
Cure from the Crypt: Fighting Tuberculosis, Again
2001 27 minutes
When a crypt containing 200 extraordinarily preserved bodies was discovered in 1994 in the Hungarian
town of Vac, it caught the interest of a scientist fighting tuberculosis on the other side of the globe. From
Films for the Humanities and Sciences.
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