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Transcript
CHAPTER 3
EVOLUTION, GENETICS, AND HUMAN VARIATION
CHAPTER OVERVIEW
This chapter discusses evolution and how it relates to human evolution and human variation. Particular
attention is given to mechanisms of genetic evolution.
CHAPTER OBJECTIVES
1.
Understand the differences between creationism and evolution.
2.
Know the importance of Mendel’s experiments and their implications for evolution.
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
mechanism plays in evolution.
CHAPTER OUTLINE
I. Creationism and Evolution
A. Creationism and Catastrophism
1. Creationism accounts for biological diversity by referring to the divine act of Creation as
described in Genesis.
2. The discovery of fossil remains of creatures clearly unknown to modern humans was not
accountable within the terms of simple creationism.
3. Catastrophism is a modified version of creationism, which accounts for the fossil record by
positing divinely authored worldwide disasters that wiped out the creatures represented in the
fossil record, who were then supplanted by newer, created species.
4. Both versions of creationism describe the different species of plants and animals as essentially
different, having distinct, separate moments of creation.
B. Evolution
1. An alternative term for early evolutionism was “transformism.”
2. Darwin was influenced by the geological concept 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 forces.
3. Transformism had posited the primordial relatedness of all life forms.
4. Darwin posited natural selection as the mechanism through which speciation takes shape
(reaching this conclusion along with Alfred Russell Wallace).
5. Natural selection is the gradual process by which nature selects the forms most fit to survive
and reproduce in a given environment.
6. For natural selection to work on a given population, there must be variety within that
population and competition for strategic resources.
7. 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.
1
II. Genetics
A. The science of genetics explains the origin of the variety 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. 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).
C. Independent Assortment and Recombination
1. Mendel also determined that traits are inherited independently of one another.
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. Mitosis is ordinary cell division, wherein one cell splits to form two identical cells.
4. Meiosis is the type of division particular to sex cells, 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).
5. Fertilization allows the products of meiosis from one parent to recombine with those from the
other parent.
6. Because genes sort independently during recombination, the number of possible combinations
is exponentially high (223): a major source of variety.
III. Population Genetics
A. Population genetics looks at changes in gene frequencies at the level of the community or
breeding population
1. Gene pool refers to all of the alleles and genotypes within a breeding population.
2. Genetic evolution is defined as change in the frequency of alleles in the breeding population
from generation to generation.
B. There are four basic mechanisms which produce changes in gene frequency in a population:
natural selection, mutation, genetic drift, and gene flow.
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IV. Mechanisms of Genetic Evolution
A. Natural Selection
1. Genotype refers to the genetic makeup of an organism.
2. Phenotype is the expression of the genotype as it has been influenced through development by
interacting with its environment.
3. Environmental influence in this interaction is extremely important, and lends great plasticity
to human biology.
4. Natural selection acts upon phenotypes.
B. Directional Selection
1. Natural selection affects gene frequencies within a population.
2. Adaptive genes are selected for (organisms containing them reproduce more frequently).
3. Maladaptive genes are selected against (organisms containing them reproduce less
frequently).
4. When specific adaptive genes are selected for over a long time period, causing a major shift in
gene frequency, this is called directional selection.
5. Directional selection continues until equilibrium is reached (due to the effects of
contradictory selective forces, the base mutation rate, or both).
6. Directional selection, in favoring one gene, can reduce variation in a gene pool.
C. Sickle-Cell Anemia
1. Just as directional selection can reduce variety, it can also maintain genetic variety by
favoring a situation in which the frequency of certain alleles remains constant between
generations.
2. Hemoglobin in Africa
a. HbA and HbS are two alleles for a gene which largely determines hemoglobin production in
humans.
b. Homozygous HbA produces normal hemoglobin; homozygous HbS produces lethal sicklecell anemia; heterozygosity for this gene produces (in some circumstances) the deleterious
but nonlethal sickle-cell syndrome.
c. It was discovered in certain populations in Africa, India, and the Mediterranean that Hb S
existed at surprisingly high frequencies.
d. 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 the sickle
cell 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. Mutation introduces genetic variation into a breeding population.
2. Chemical alterations in genes may provide a population with entirely new phenotypes, with
possible concomitant selective advantages.
3. The spread of HbS in heavily malarial environments is one example.
E. Random Genetic Drift
1. Random genetic drift is the loss of alleles from a population's gene pool through chance.
2. There is no set form for this chance; it may simply occur through a statistical fluke in sexual
reproduction patterns, or through the effects of a catastrophe on the population as a whole.
3
F. Gene Flow
1. Gene flow occurs through interbreeding: the transmission of genetic material from one
population to another.
2. Gene flow inhibits speciation, the formation of new species.
a. A species is an internally interbreeding population whose offspring can survive and are
capable of reproduction.
b. Speciation occurs when populations of the same species become isolated from each other
(thus stopping gene flow) allowing natural selection and genetic drift gradually to produce
gene pools that are different, to the extent that successful interbreeding is no longer
impossible.
V. Race: A Discredited Concept in Biology
A. In biological terms, a race is a geographically isolated subdivision of a species that can reproduce
with individuals from other subspecies of the same species, but does not because of its
geographic isolation.
1. Human populations vary biologically, but there are no sharp breaks between populations.
2. Human biological variation is distributed gradually between populations along clines.
B. Ethnicity and race are not synonymous, although American culture does not discriminate between
the two terms.
C. Races Are Not Biologically Distinct
1. Race is supposed to describe genetic variation, but racial categories (particularly early on) are
based on phenotypes.
a. Phenotypes are the product of genetic, developmental, and environmental factors.
b. There is no clear logical hierarchy to phenotypic traits, thus it is difficult to demonstrate
which should be a definitive racial feature.
2. 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.
3. Even skin color-based race models that include more than three categories do not accurately
represent the wide range of skin color diversity among human populations.
4. Fundamental Problems with Phenotype-Based Race.
a. Populations grouped into one race based upon phenotypic similarity may be genetically
distinct; such similarities may be the result of parallel evolution or other factors.
b. Genetic traits occur together due to the selective forces of the environments in which they
evolved, and therefore do not constitute an internally coherent “type.”
D. Explaining Skin Color
1. Natural selection “is the process by which nature selects the forms most fit to survive and
reproduce in a given environment.”
2. Variation in skin color is determined by the amount of melanin in the skin cells, which in turn
isgenetically determined.
3. Prior to the sixteenth century, darker skinned populations were closest to the equator, while
lighter skinned populations were closer to the poles.
4. Selective Advantages and Disadvantages.
a. Light skin in the tropics is selected against because it burns more easily, thus subjecting
light-skinned individuals to a greater likelihood of infection and disease.
b. Sunburn impairs the body's ability to withstand heat by reducing the skin’s ability to sweat.
c. Light skin is more susceptible to skin cancer.
d. The effect of sunlight on vitamin D formation indicates how dark skin might have been
selected for in tropical environments (protection against hypervitaminosis D), and against
in lower-sunlight environments (protection against rickets); and it further indicates how
light skin might have been selected for in low-sunlight environments, and against in the
tropics.
4
E. Lactose Tolerance
1. The term phenotypic adaptation refers to changes which occur to an individual organism
during it's lifetime which enhance its reproductive fitness.
2. Individuals from herding populations in northern Europe and parts of Africa maintain their
ability to digest milk (continue to produce the enzyme, lactase) into adulthood, whereas
people from other populations can digest milk (specifically, milk sugar, called lactose) only
during childhood.
3. The fact that descendents of these herding populations who no longer herd continue to be
lactose tolerant as adults indicates genetic adaptation to a milk-rich diet.
4. The fact that lactose intolerance can vary during an individual's adult life, depending on how
much milk is consumed, indicates that some phenotypic 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 are distributed gradually rather than abruptly through space.
D. Physical variations in human populations have no meaning other than the social ones societies
attribute to them.
E. Historically, racial categories have been used to divide, rank, and control populations ethnically
separate from Western Europe.
1. Some populations have been assigned to a perpetual low status (e.g., African-Americans).
2. Other populations have been assigned to a perpetual high status with access to privilege,
power, and wealth.
LECTURE TOPICS
1.
Use the sources of 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.
2.
Explain how natural selection is blind, not working toward a predetermined end. Explain the
relationships among differential reproduction, key resources, key hazards, and natural selection.
3.
Challenges to the teaching of evolutionary theory have become increasingly numerous and
threatening to free speech in the classroom. The crux of many of these challenges has to do with
the claim that evolution is “just a theory.” Build on the discussion of the scientific method in
Chapter 2 by using the theory of evolution as your case study. Give your students some
indications about the necessary attributes of critical inquiry by telling them 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.
5
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’ 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; 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 represent 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.
6