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Chapter 12: Diversity Within
Species and Population Genetics
Lecture Outline
Enger, E. D., Ross, F. C., & Bailey, D. B. (2012). Concepts in biology (14th ed.). New York: McGrawHill. 1
12-1
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Populations vs. Species
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A species is all the organisms potentially
capable of naturally breeding among
themselves and having offspring that could
successfully interbreed.
A population is a group of organisms in the
same species in the same geographical
area.
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Population Genetics
and Gene Pools
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Population genetics is the study of the kinds
of genes (alleles) within a population.
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Also accounts for the numbers of alleles in a
population
Predicts and observes how those numbers will
change over time
This data is used to classify organisms and study
evolutionary change.
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Population Genetics
and Gene Pools
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In a population
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Each individual has a set of alleles.
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The population may contain more
different alleles than any one
individual.
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Diploid organisms have 2 alleles at
most.
The human population has 3 alleles
for blood type.
All of the alleles in a population
make up the gene pool.
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Genes, Populations,
and Gene Pools
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Biological Species Concept

According to the biological species concept,
species is a group of organisms
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That share a common gene pool
That are reproductively isolated from other
populations
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They do not exchange genetic information.
Local populations of a single species may
have slightly different allele combinations.
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Gene and Allele Frequencies
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Differences in gene frequencies reflect
genetic differences between populations.
Allele frequency is a measure of how often
an allele is found in a population.
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Expressed as a decimal or percentage
# of times an allele appears in a population/the
total number of alleles in the population
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Allele Frequencies Differ
in the Human Population
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Allele Frequencies, Dominance
and Recessiveness
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Allele frequencies are
unrelated to whether the
allele is dominant or
recessive.
There are many instances
where a recessive allele is
more frequent in a
population.
Blue eyes & light hair are
recessive traits that are
more frequent in European
regions.
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Subspecies, Breeds, Varieties,
Strains and Races
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These all describe different forms of
organisms that are all members of the
same species.
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Dogs have different breeds.
Plants have different varieties.
Bacteria have different strains.
Humans have different races.
All of these are types of subspecies.
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Subspecies of Water Snakes
12-11
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How Genetic Diversity Comes
About
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Genetic diversity describes genetic differences
among members of a population.
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High genetic diversity implies that many different alleles
exist in a population.
Low genetic diversity implies that all of the individuals in the
population have the same alleles.
A gene pool with greater diversity is likely to contain
combinations of alleles that will allow the individuals
to adapt to a changing environment.
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Mutations
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Mutations are changes in the base sequence
of DNA.
Mutations are the source of new alleles.
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All alleles originated with mutations.
Most mutations are harmful.
Occasionally a mutation will change a gene so
that the protein works differently or better.
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Example: Insecticide resistance in mosquitoes
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Sexual Reproduction
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Sexual reproduction generates new genetic
combinations.
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May not necessarily change the frequency of alleles
in a population
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New combinations of alleles in individuals
However, the new combination of alleles in an individual
may create a combination of traits that allows the individual
to survive and reproduce more successfully than other
individuals.
Example: Corn plants that inherit resistance to corn
blight and resistance to insects
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Migration
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The migration is the movement of individuals into
and out of populations.
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Artificial migration is used in zoos to generate
genetic diversity.
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Results in alleles being added or subtracted from a
population
May change allele frequencies in the population
Inbreeding has reduced genetic diversity in small zoo
populations.
Zoos are exchanging animals for breeding to introduce new
alleles into their populations.
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The Importance of Population
Size
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Population size is directly related to genetic
diversity.
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The smaller the population, the less genetic
diversity a population can contain.
 Mutations, migrations, and death can have
dramatic effects on the genetic make-up of a
population.
 Frequently, random events will significantly
change the gene pool.
– This is called genetic drift.
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Genetic Drift
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Why Genetically Distinct
Populations Exist
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Many species have wide geographic
distribution with reasonable distinct
subspecies.
This occurs for several reasons.
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Adaptation to Local
Environmental Conditions
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Genetic diversity allows populations to adapt
to their specific environments.
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Some individuals will have combinations of alleles
that allow them to survive and successfully
reproduce in hostile conditions.
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Death and migration remove or reduce the alleles that
do not contribute to survival.
Example: Lizards in the desert have lighter
coloration than those that live in other
environments.
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Founder Effect
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The founder effect is a type of genetic drift
that occurs when a new population is
established by a few colonizing individuals.
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The small colonizing group may have different
allele frequencies than the original population.
When the colonizing individuals mate and
multiply, their allele frequencies will tend to
persist, making the new population different from
the parent population.
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Genetic Bottleneck
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Genetic bottleneck is another form of
genetic drift.
Occurs when there is a dramatic reduction
in population size
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Usually due to some chance event like a
natural disaster
Could be due to over-hunting by humans
The remaining members of the population
will mate and pass on their alleles, limiting
their genetic diversity.
Many endangered species are undergoing
genetic bottlenecks.
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Barriers to Movement
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When migration is limited, populations
become geographically and reproductively
isolated.
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Perpetuates the effects of genetic drift caused by
founder effect and bottleneck
Limits genetic diversity and generates subspecies
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Genetic Diversity in
Domesticated Plants and Animals
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Domesticated plants and animals are
populations maintained by humans for
human purposes.
Several techniques are used to produce
plants and animals with specific
characteristics.
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High productivity, disease resistance, etc.
Low genetic diversity is an undesirable side
effect.
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Cloning
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Cloning is the process of reproducing
organisms asexually so that they are all
genetically identical.
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Generates clones
In plants, cuttings are used for this process.
Animals have recently been cloned by somatic
cell nuclear transfer.
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Cloning Plants from Cuttings
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Selective Breeding
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Humans can bring together certain desired
alleles by selective breeding.
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Selective breeding has been used to
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Involves the careful selection of individuals with
specific desirable traits and their controlled mating
Generate chicken breeds that grow quickly and
generate a lot of meat or lay a lot of eggs
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Selective Breeding
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Intraspecific hybrids result from the crossing
of two different varieties of the same species.
Interspecific hybrids result from the crossing
of two species.
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Tangelos come from crosses of tangerines and
oranges.
Mules come from crosses between horses and
donkeys.
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Genetic Engineering
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Recombinant DNA technology has given us
the ability to introduce alleles and modify the
characteristics of domesticated species.
This has allowed agricultural scientists to
engineer specific, valuable characteristics
into certain species.
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Generates genetically modified organisms
(GMOs)
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The Impact of Monoculture
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Generates domesticated populations that are
all similar with specific desirable traits.
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Necessarily limits genetic diversity
Planting large crops of identical plants is
called monoculture.
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These are easy to maintain until a new disease
comes along.
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It may kill the entire crop because they are all identical.
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Monoculture
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The Impact of Monoculture
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As a solution, scientists have generated
gene banks.
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These contain populations of primitive ancestors
of domesticated species.
If a disease eliminated a given domesticated
variety, then the ancestor could be used to
regenerate the domesticated variety.
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The Banking of Genes
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Is it a species or not?
The Evidence
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Testing the biological definition of a species may
not be practical.
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We cannot test each individual by breeding it with
another.
Fossils cannot be tested.
Organisms that reproduce asexually cannot be
tested.
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Is it a species or not?
The Evidence
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Therefore, we employ other species concepts to
identify species.
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The morphological species concept uses physical
traits to identify species.
Behavioral differences can be used to
differentiate species.
Differences in metabolism can be used to help
distinguish among species.
The most precise way is to analyze differences in
DNA sequences.
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Human Population Genetics
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All humans belong to the same species, but races
and regional differences exist because our ancestors
were more geographically isolated.
We are more mobile now, but non-random mate
selection still occurs.
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We tend to mate with people who are like us.
This means that certain diseases are prevalent in certain
people groups.
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Tay-Sachs disease and Ashkenazi Jews
Sickle-cell anemia and African ancestry
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The Frequency of the Tay-Sachs
Allele
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Normal and Sickle-shaped Cells
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Ethics and Human Population
Genetics
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Understanding inheritance patterns has led
to both bad and good human responses.
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Eugenics (bad)
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Disease treatment (good)
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A movement motivated to purge the “bad” genes from
society
Based on a fundamental misunderstanding of the power
of inheritance
Understanding genetic diseases has allowed us to
develop treatments.
PKU does not have to lead to mental retardation if foods
with phenylalanine are avoided.
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