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CHAPTER 3: EVOLUTION, GENETICS, AND HUMAN VARIATION
Chapter Overview
This chapter discusses evolution and how it relates to human biological variation. Particular
attention is given to mechanisms of genetic evolution, as well as various problems underlying
human racial classification.
Chapter Objectives
1. Understand the differences between creationism and evolution.
2. Know the importance of Mendel’s experiments and their implications for our understanding
of genetics.
3. Understand what population genetics is and how it studies evolution.
4. Be able to discriminate between the key mechanisms of evolution and understand the role
each plays in evolution.
5. Understand the various problems with racial classification.
6. Be able to explain how the distribution of human skin color is affected by natural selection.
7. Be familiar with the American Anthropological Association’s position on “race.”
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. Evolution
1. Evolutionists rejected creationism and catastrophism, believing instead that
species arose from others through a long and gradual process of
transformation, or descent with modification.
2. Charles Darwin, the best known evolutionist, was influenced by Charles
Lyell’s concept of uniformitarianism.
a. Uniformitarianism states that explanations for past events should be
sought in the long-term action of ordinary forces that still work today.
b. Uniformitarianism further asserts that current geological features such
as mountain ranges are the result of long-term natural forces.
3. Darwin applied the ideas of uniformitarianism and long-term transformation
through natural forces to living things, arguing that all life forms are
ultimately related and that the number of species is not immutable but has
increased over time.
F. Natural Selection
1. Darwin and Alfred Russel Wallace offered natural selection as a single
principle that could explain the origin of species, biological diversity, and
similarities among related life forms.
2. Natural selection is the process by which nature selects the forms most fit to
survive and reproduce in a given environment.
3. For natural selection to work on a given population, there must be variety
within that population as well as competition for strategic resources (those
necessary for life) among members of the population.
4. Organisms whose attributes render them fitter to survive and reproduce
(thereby transmitting their genetic material to future generations) in a given
environment will do so in greater numbers than organisms that are less fit.
II. Genetics
A. Genetics helps us understand the causes of the biological variation upon which natural
selection operates.
1. Mendelian genetics studies the ways in which chromosomes transmit genes
across generations.
2. Biochemical genetics examines structure, function, and changes in DNA.
3. Population genetics investigates natural selection and other causes of genetic
variation, stability, and change in breeding populations.
B. Mendel’s Experiments
1. In 1856, Gregor Mendel began a series of experiments that revealed the basic
principles of genetics.
2. By experimenting with successive generations of pea plants, Mendel
discovered that heredity is determined by discrete particles or units.
3. Mendel determined that the traits he observed occurred in two basic forms:
dominant and recessive.
a. Dominant forms mask recessive forms in hybrid (mixed) individuals.
b. Recessive traits can reappear in unaltered form in later generations
because genetic traits are inherited as discrete units.
4. The basic genetic units described by Mendel, now called genes or alleles, are
located on chromosomes.
a. Humans have 46 chromosomes, arranged in 23 pairs, with each parent
contributing one chromosome to each pair.
b. Chromosomes contain multiple positions, or genes, each of which
determines, wholly or partially, a particular biological trait.
c. Alleles are biochemically different forms of a given gene.
d. An individual may be homozygous (possessing two identical alleles) or
heterozygous (possessing different alleles) with respect to a particular
gene.
5. Dominance produces a distinction between genotype, or hereditary makeup,
and phenotype, or expressed physical characteristics.
6. Although some traits have only two forms (dominant and recessive), other
traits—human blood type, for example—are determined by more than two
alleles, some of which may be codominant.
C. Independent Assortment and Recombination
1. Mendel also formulated the law of independent assortment, which states that
traits are inherited independently of one another.
2. The independent assortment and recombination of genetic traits is important
in biological evolution because they create new types on which natural
selection can operate.
3. During mitosis, or ordinary cell division, two identical cells emerge from one.
4. In meiosis, the special process by which sex cells are produced, four cells are
produced from one, with each new cell containing half the genetic material of
the original cell (i.e., 23 chromosomes instead of 46).
5. With fertilization, the 23 chromosomes from one parent combine with the 23
chromosomes from the other parent.
6. Independent assortment of chromosomes is a major source of variety, because
parents’ genotypes can be assorted in more than 8 million (223) different ways.
III. Population Genetics
A. Population genetics studies stability and change in gene frequencies within breeding
populations.
1. Gene pool refers to all the alleles 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, 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.
B.
C.
D.
E.
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.
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 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.
Sickle-Cell Anemia
1. Just as directional selection can reduce variety in a population (by favoring
one allele or trait over another), selective forces also can maintain genetic
variety by favoring a situation in which the frequencies of two or more alleles
of a gene remain constant between generations.
2. One example of this process involves two alleles, HbA and HbS, that affect the
production of the beta strain (Hb) of human hemoglobin.
a. People homozygous for HbA have normal hemoglobin, whereas those
who are homozygous for HbS suffer from sickle-cell anemia, a disease
that is usually fatal.
b. Surprisingly, certain populations in Africa, India, and the
Mediterranean had high frequencies of the HbS allele.
c. The HbS allele persisted because the heterozygote, with one sickle-cell
allele and one normal one, was the fittest phenotype for the malarial
environments in which these populations lived (heterozygotes have
enough abnormal hemoglobin, in which malaria parasites cannot
thrive, to protect against malaria, but they also have enough normal
hemoglobin to fend off sickle-cell anemia).
3. This example demonstrates that adaptation and fitness are in relation to
specific environments—that traits are not adaptive or maladaptive for all times
and places.
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.
Random Genetic Drift
1. Random genetic drift is a change in allele frequency that results from chance
rather than from natural selection.
2. Random genetic drift is most common in small populations, because alleles
are more likely to be lost by chance in smaller groups.
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. Race: A Discredited Concept in Biology
A. Racial classification, now largely abandoned, is the attempt to assign humans to
discrete categories (purportedly) based on common ancestry.
B. In biological terms, a race is a geographically isolated subdivision of a species that is
capable of interbreeding with other subspecies of the same species, but does not
because of its geographic isolation.
1. Humanity lacks such races because human populations have not been isolated
enough from one another to develop into discrete groups.
2. 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.
C. Racial classifications based on phenotype
1. Although race is supposed to reflect shared genetic variation inherited from a
common ancestor, racial classifications (such as those developed by early
scholars) are usually based on phenotypical traits such as skin color.
2. Because there is not a clear-cut hierarchy of phenotypic traits, it is difficult to
determine which trait(s) should be considered primary in a racial
classification.
3. The so-called three great races (white, black, and yellow) are more a reflection
of European colonialist politics than an accurate representation of human
biological diversity.
4. Skin color-based race models do not accurately represent the wide range of
skin color diversity among human populations.
5. Racial classifications based on combinations of physical traits are problematic
because particular traits (e.g., skin color, stature, skull form, facial features) do
not necessarily co-occur.
6. The assumption that the phenotypical characteristics on which races are based
reflect genetic material that is shared and that has stayed the same for long
periods of time is likewise problematic.
a. Phenotypical similarities and differences do not necessarily have a
genetic basis.
b. The range of phenotypes characteristic of a population may change
without any genetic change because environmental variables can
greatly affect the growth and development of individuals.
D. Explaining Skin Color
1. Variation in skin color depends primarily on the amount of melanin in the
skin, which is determined genetically.
2. Prior to the sixteenth century, very dark-skinned populations lived in the
tropics, while increasingly lighter-skinned populations lived in regions closer
to the poles.
3. Natural selection provides an explanation for the geographic distribution of
different skin colors.
a. Light skin is selected against in the tropics because it is more
susceptible to sunburn (thereby increasing light-skinned individuals’
susceptibility to disease, and decreasing their ability to sweat) as well
as skin cancer.
b. Lighter skin helps to ensure adequate production of vitamin D in
cloudy and cold environments (thus providing protection against
rickets), while darker skin helps to prevent overproduction of vitamin
D in the sunny tropics (thus providing protection against
hypervitaminosis D).
E. Lactose Tolerance
1. Phenotypical adaptation refers to adaptive changes which 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. Box: American Anthropological Association (AAA) Statement on "Race"
A. Human populations are not unambiguous, clearly demarcated, biologically distinct
groups.
B. There is greater genetic variation within “racial” groups than between them.
C. Physical variations tend to occur gradually rather than abruptly over geographic areas.
D. Physical variations in the human species have no meaning except the social ones that
humans attribute to them.
E. Historically, racial categories have been used to divide, rank, and control nonEuropean populations (and even European groups themselves).
F. Racial beliefs constitute myths about the diversity in the human species and about the
abilities and behavior of people homogenized into “racial” categories.
G. Human cultural behavior is learned and always subject to modification.
H. Present-day inequalities between so-called “racial” groups are not consequences of
their biological inheritance but products of historical and contemporary social,
economic, educational, and political circumstances.
Lecture Topics
1. Explain the various mechanisms of genetic evolution and how each may affect genetic
diversity. Also discuss the relationships between genotype, phenotype, and selection.
2. Explain how natural selection is “blind,” not teleological. Discuss the relationships between
phenotypical diversity, competition for key resources, differential survival/reproduction, and
changes in gene frequencies.
3. Challenges to the teaching of evolutionary theory have become increasingly common and
threatening to free speech in the classroom. Many of these challenges dismiss evolution as
“just a theory.” Discuss the scientific method, using the theory of evolution as your case
study. Encourage your students to think about the necessary attributes of critical inquiry, and
how to decide whether evidence supports or refutes a hypothesis.
Suggested Films
DNA and the Evidence for Evolution
20 minutes
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 Richard Dawkins’s book, 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.
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; and 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 mini-course 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.