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Evolution of Populations
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When Darwin developed
this theory of evolution he
had no idea how heredity
worked
This left him unable to
explain:
 Source of variation
 How inheritable traits are
passed through generations
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Evolutionary biologists
connect Mendel and
Darwin’s work in the 1930s
Gene pool: consists of all
genes, including all the
different alleles, that are
present in a population
 Typically contain traits with
two or more alleles
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When studying evolution today, biologists
often focus on a particular
Evolution of populations is called
In genetic terms, evolution is any change in
the relative frequency of alleles in a
population

: # of times the allele occurs in
a gene pool
 Can occur because of:
▪ mistakes in
replication/transcription
▪ environmental chemicals or
influences

May or may not affect an
organism’s
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: recombination of genes that
occurs during production of gametes
 Causes most inheritable differences between relatives
 As a result, sexual reproduction is a major source of
variation in organisms
 Despite gene shuffling, the frequency of alleles does not
change in a population
Similar to a deck of cards – no matter
how many times you shuffle, same
cards (alleles) are always there.
controlled by single
gene with two alleles
 Ex.) widow’s peak, hitchhiker’s thumb,
tongue rolling
: controlled by 2 or more
genes, each with 2 or more alleles
 Examples: height, hair color, skin color, eye color
single gene
polygenic
Only two
phenotypes possible
Multiple (many)
phenotypes possible
tongue
roller or non-tongue
roller
height range
4feet to 9 feet all
Natural selection affects
polygenic traits through:
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 Single phenotype is favored,
causing the allele frequency to
continuously shift in one direction.
 Individuals with highest fitness:
those at one end of the curve
Key
Directional Selection
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
 Individuals at the center of the curve have
highest fitness; evolution keeps center in the same
position but narrows the curve
Average phenotype
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: human birth weight
Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Individuals at both ends of the curve
survive better than the middle of the
curve
 Natural selection favors extreme
phenotypes
 Individuals with highest fitness: both
ends of curve
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: birds where seeds are either
large or small
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Disruptive Selection
Largest and smallest seeds become more common.
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
Key
Beak Size
Evolution of Populations
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Natural selection is not the only source of
evolutionary change. In small populations, an
allele can become more or less common by
chance
Genetic drift: random change in allele
frequency that occurs in small populations
So, genetic drift and natural selection both involve
changes in allele frequencies
1. Founder Effect
2. Bottleneck Effect
 Allele frequencies change due
to migration of a small
subgroup of a population
Founder Effect: Beetles
on Hawaiian islands
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Major change in allele
frequencies when population
decreases dramatically due
to catastrophe
Hunted to near
extinction
♦ Population decreased
to 100 individuals in
1900’s
♦ Those 100 repopulated
so today’s population
is roughly 150,000
♦ No genetic variation in
24 genes
♦
Catastrophe
Original
population
Surviving
population
The formation of new biological
species, usually by the division of a
single species into two or more
genetically distinct one.
1.
2.
3.
 Kaibab Squirrel
Abert Squirrel
Two populations are capable of
interbreeding but do not
interbreed because they have
different ‘courtship rituals’ or
other lifestyle habits that differ
 Eastern and Western Meadowlark
populations overlap in the middle of
the US
Male birds sing a mating song that females
like
 East and West have different songs
 Females only respond to their subspecies
song.
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Populations reproduce at
different times
Mates in:
Mates in:
April
July
Geographic, Behavioral, and
Temporal Isolation are all
believed to lead to speciation.
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: large-scale evolutionary
patterns and processes that occur over long
periods of time
Six important topics
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Extinction
Adaptive radiation
Convergent evolution
Coevolution
Punctuated equilibrium
Changes in developmental genes
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More than
of all species that have ever lived
are now extinct
Several times in Earth’s history –
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Examples of mass extinctions and their causes?
What effects have extinctions had? Bursts of
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evolution!
Convergent Evolution
Process by which unrelated organisms
come to resemble one another
Pattern of long, stable periods interrupted by
brief periods of rapid change
 Why would rapid change occur?
 Isolation
 Migration
 Extinction
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Changes in genes for growth and
differentiation during embryological
development could produce transformations
in body shape and size
: insects
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States that allele and genotype frequencies in
a population will remain constant from
generation to generation in the absence of
other evolutionary influences.
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No mutations can happen
Population must be large
No movement in or out
Random mating
Natural selection cannot occur
 Kaibab Squirrel
Abert Squirrel