Transcript Population genetics

Window on Humanity
Conrad Phillip Kottak
Third Edition
Chapter 3
Evolution, Genetics, and Human
Variation
© 2008 McGraw-Hill Higher Education. All right reserved.
Overview
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•
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Evolution and natural selection
Genetics
Mechanisms of genetic evolution
Human biological adaptation
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The Origin of Species
– Creationism
• Divine creation of life forms as described in Genesis
• Biological characteristics of life forms immutable
– Carolus Linnaeus – first biological classification
(taxonomy) of plants and animals
– Fossil record – raised doubts about creationism
– Catastrophism
• Modified version of creationism
• Ancient life forms destroyed by catastrophes (e.g., fires,
floods) then replaced by new, divinely created species
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Theory and fact
• Alternative to creationism and catastrophism was
transformism, also called evolution
– Charles Darwin
• Influenced by Lyell’s concept of uniformitarianism
– Explanations for past events should be sought in the long-term
action of ordinary forces observable today
– Current geological features are the result of long-term natural
forces
• Darwin applied the notions of uniformitarianism and long-term
transformation to living things
– All life forms are ultimately related
– Number of species has increased over time
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Natural selection
• Process by which nature selects the forms
most fit to survive and reproduce in a given
environment
• Necessary conditions for selection to occur:
• Variation within a population
• Competition among individuals for strategic resources
– Individuals whose attributes render them fitter to survive and
reproduce in a given environment will do so in greater numbers
than individuals that are less fit
– Differential reproductive success (rather than survival of the fittest)
• Evolutionary theory is used to explain, to show
how and why
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Genetics
• Helps us understand the causes of the
biological variation upon which natural
selection operates
– Mendelian genetics – studies how chromosomes
transmit genes across generations
– Biochemical genetics – examines structure, function,
and changes in DNA
– Population genetics – investigates natural selection and
other causes of genetic variation, stability, and change
in breeding populations
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Genetics
• Mendel’s experiments
– Gregor Mendel – discovered that heredity is determined
by discrete particles or units
– Two basic forms of traits: dominant and recessive
• Dominant forms mask recessive forms in hybrid (mixed)
individuals
• Reappearance of recessive traits in later generations
– “Dominance produces a distinction between genotype,
or hereditary make up and phenotype, expressed
physical characteristics.”
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Genetics:
– Basic genetic units (genes or alleles) are located on
chromosomes
• Humans have 46 chromosomes (23 pairs)
• Genes – positions on chromosomes that wholly or partially
determine biological traits
• Alleles – biochemically different forms of a given gene
– Individuals may be homozygous or heterozygous with
respect to a particular gene
• Homozygous – possessing two identical alleles (TT) (tt)
• Heterozygous – possessing different alleles (Tt)
© 2008 McGraw-Hill Higher Education. All right reserved.
Genetics
• Dominance produces a distinction between
genotype and phenotype
– Genotype – hereditary makeup
– Phenotype – expressed physical characteristics
• Some traits have only dominant and
recessive forms
• Other traits (e.g., human blood type) are
determined by more than two alleles
– Alleles may be codominant
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Independent assortment
• Law of independent assortment
– Formulated by Mendel
– Traits are inherited independently of one another
• The fact that traits are inherited independently and are
recombined is one of the ways that variety is produced in a
population.
• Create new types on which natural selection can operate
– Mitosis – ordinary cell division
– Meiosis – production of sex cells, each with only 23 chromosomes
– Fertilization – combination of 23 chromosomes from each parent
• Parents’ genotypes can be assorted in more than 8 million (223)
different ways
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Population genetics
• Studies stability and change in gene frequencies within
breeding populations
• Gene pool – all the alleles, genes, chromosomes, and
genotypes within a breeding population
• Genetic evolution – change in gene frequency (the
frequency of alleles) in a breeding population from
generation to generation
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Four mechanisms of genetic evolution:
1.
2.
3.
4.
Natural selection
Mutation
Genetic drift
Gene flow
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1. Natural Selection
• Genotype vs. phenotype
– Phenotype is influenced by both genotype and the environment
– Human biological plasticity
• Natural selection operates only on phenotype (not
on genotype)
• Natural selection affects gene frequencies within a
population
• Traits that are adaptive in a given environment
will be selected for
• Individuals possessing the alleles responsible for those traits
will reproduce more frequently
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1. Natural Selection, cont.
– Directional selection
• Long-term selection of the same adaptive trait(s)
from generation to generation
• Maladaptive alleles may be removed from a gene
pool
• Continues as long as environmental forces stay the
same
– Sexual selection – the process by which certain
traits of one sex are selected because of
advantages they confer in winning mates
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1. Natural Selection, Cont.
Stabilizing selection
• Selective forces may favor a balanced polymorphism in
which the frequencies of two or more alleles remain
constant between generations
– Example: two alleles (HbA and HbS) that affect the production of
human hemoglobin
• Homozygous for HbA = normal hemoglobin
• Homozygous for HbS = sickle-cell anemia
• High frequencies of the HbS allele exist among certain populations in
Africa, India, and the Mediterranean
• HbS allele persists because the heterozygote is the fittest phenotype in
malarial environments
– Adaptation and fitness are in relation to specific environments
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2. Mutation
• Mutation
– Changes in the DNA molecules of which genes
and chromosomes are built
– Occurs spontaneously and regularly
– Produces variety on which natural selection
may operate
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3. Random genetic drift
• Change in allele frequency that results from chance
rather than from natural selection
• May operate in any population, large or small, but
more rapid in small populations
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4. Gene flow
• Exchange of genetic material between populations of the
same species
• Produces variety on which natural selection can operate
• Tends to prevent speciation (the formation of new species)
– Species – groups of related organisms whose members can
interbreed to produce offspring that can live and reproduce
– Speciation occurs when populations of the same species are
isolated from each other (lack of gene flow) and, as a result of
genetic change, become incapable of interbreeding
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Human biological adaptation
– Because of extensive gene flow and interbreeding,
Homo sapiens has not evolved subspecies or distinct
races
– Biological variation between human populations
involves gradual shifts (clines) in gene frequencies and
other biological features
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Human biological adaptation
• Genes and disease
– Infectious diseases posed an increasing threat to
human populations beginning with food
production (10,000 years ago)
– Tropical diseases (e.g., malaria,
schistosomiasis, and filariasis) affect more
than 10 percent of the world’s population today
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CHAPTER 3
Evolution, Genetics, and Human Variation
• Genes and Disease
– Human blood types play an important role in
resistance to some diseases
• Various alleles producing human blood types
interact with infectious and noninfectious
ailments
• For example, type A or AB blood cells seem to
make a person more susceptible to smallpox,
while the presence of O or B blood cells
appears to increase smallpox resistance
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Human biological adaptation
• Lactose tolerance
– Phenotypical adaptation – adaptive changes which
occur during an individual’s lifetime
– Lactose tolerance (ability to digest milk) determined
by both genes and phenotypical adaptation
• People from herding populations remain lactose tolerant into
adulthood, while those from nonherding populations do not
• Lactose tolerance among descendents of herding populations –
genetic adaptation to a milk-rich diet
• Lactose tolerance can vary during an individual's life,
depending on amount of milk consumed – phenotypical
adaptation
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