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Chapter 23~
The Evolution of
Populations
Microevolution – change in allele frequency of a
population over generations (evolution on the
smallest scale)
 Mutations – the only source of new genes and new
alleles
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Point Mutations – change in one base of a gene
AGCCTA  AGCTTA
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Chromosomal Mutations – delete, disrupt,
duplicate, rearrange many loci at once
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Sexual Recombination offers the most common
genetic variation
 1) crossing over during Prophase I of Meiosis
 2) independent assortment of chromosomes
(chromosomes pairing up in different combinations)
 3) fertilization (so many sperm to choose from!)
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Population Genetics – study of how populations change
genetically over time
Population – groups of individuals of the same species living
in the same area at the same time
Gene Pool – all the alleles of all the individuals of a
population
***if all members of a population are homozygous for the same
allele, they are termed: “FIXED”
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Hardy-Weinberg Theorem – used to describe
a population that is NOT evolving
The population will remain constant over
time, unless “forces” change it
Populations remaining the same is not likely,
therefore allele frequencies change…leading
populations to evolve
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In order for Hardy-Weinberg to display no
evolution occurring three conditions must be
met
1.
2.
3.
4.
5.
No mutations
Random mating
No natural selection
Large population
No gene flow
***If there are any of these
things occurring, then HW
equilibrium cannot be met
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p^2 + 2pq + q^2 = 1
p+q=1
 “p” is dominant (so p^2 is homozygous dominant)
 “q” is recessive (so q^2 is homozygous recessive)
 That make “pq” heterozygous
[this equation is used to predict the frequencies
(percentage) of the distribution of alleles]
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1.
2.
3.
4.
5.
6.
Suppose in a plant population that red flowers (R) are
dominant to white flowers (r). In a population of 500
individuals, 25% show recessive phenotype. How many
individuals would you expect to be homozygous dominant and
heterozygous for this trait?
“q^2” frequency is 25% (or 0.25), which means “q” must be
0.5
p + q = 1 (this is reduced from p^2 + 2pq + q^2 = 1), so “p”
must also be 0.5
p^2 + 2pq + q^2 = 1
From the reading 25% were recessive (125 individuals)
We deduced 25% are Homo. Dom. (125 individuals)
That means 50% are heterozygous (250 individuals)
***BUT then again this is a perfect world, in reality it might
not be so nice with the numbers
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Population:
a localized group of individuals
belonging to the same species
Species:
a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring
Gene pool:
the total aggregate of genes in a
population at any one time
Population genetics:
the study of genetic changes in
populations
Modern synthesis/neo-Darwinism
“Individuals are selected, but populations
evolve.”
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Serves as a model for the
genetic structure of a
nonevolving population
(equilibrium)
5 conditions:
1- Very large population size;
2- No migration;
3- No net mutations;
4- Random mating;
5- No natural selection
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p=frequency of one allele (A); q=frequency of the other
allele (a);
p+q=1.0 (p=1-q & q=1-p)
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P2=frequency of AA genotype; 2pq=frequency of Aa plus
aA genotype; q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
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A change in the gene
pool of a population
over a succession of
generations
1- Genetic drift:
changes in the gene
pool of a small
population due to
chance (usually
reduces genetic
variability)
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The Bottleneck
Effect: type of genetic
drift resulting from a
reduction in population
(natural disaster) such
that the surviving
population is no longer
genetically
representative of the
original population
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Founder Effect:
a cause of genetic drift
attributable to
colonization by a
limited number of
individuals from a
parent population
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2- Gene Flow:
genetic exchange due to
the migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
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3- Mutations:
a change in an organism’s
DNA (gametes; many
generations); original
source of genetic
variation (raw material
for natural selection)
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4- Nonrandom
mating: inbreeding
and assortive mating
(both shift
frequencies of
different genotypes)
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5- Natural Selection:
differential success in
reproduction;
only form of
microevolution that
adapts a population
to its environment
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Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
population
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Geographical
variation: differences
in genetic structure
between populations
(cline)
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Prevention of natural selection’s
reduction of variation
Diploidy
2nd set of chromosomes hides
variation in the heterozygote
Balanced polymorphism
1- heterozygote advantage
(hybrid vigor; i.e.,
malaria/sickle-cell anemia);
2- frequency dependent
selection (survival &
reproduction of any 1 morph
declines if it becomes too
common; i.e., parasite/host)
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Fitness: contribution
an individual makes
to the gene pool of
the next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing
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Sexual dimorphism:
secondary sex
characteristic distinction
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Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism