Evolution of Populations
AP Biology
2007-2008
Doonesbury - Sunday February 8, 2004
Populations evolve
 Natural selection acts on individuals

differential survival
 “survival of the fittest”

differential reproductive success
 who bears more offspring
 Populations evolve
genetic makeup of
population changes
over time
 favorable traits
(greater fitness)
become more common
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Presence of lactate dehydrogenase

Mummichog
Individuals
DON’Tare
evolve…
Individuals
survive
orselected
don’t survive…
Populations
evolve
Individuals
reproduce
or don’t…
AP Biology
2007-2008
Body size & egg laying in water striders
Fitness
 Survival & Reproductive
success

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individuals with one
phenotype leave more
surviving offspring
Variation & natural selection
 Variation is the raw material for natural
selection

there have to be differences within population

some individuals must be more fit than others
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Where does Variation come from?

random changes to DNA
 errors in mitosis & meiosis
Wet year
Beak depth
 Mutation
Dry year
Dry year
 environmental damage
 Sex
mixing of alleles
 recombination of alleles
 new arrangements in every
offspring
 new combinations = new
phenotypes

spreads variation
 offspring inherit traits from parent
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1980
1982
1984
11
Beak depth of
offspring (mm)

1977
Dry year
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
1. Mutation & Variation
 Mutation creates variation

new mutations are constantly appearing
 Mutation changes DNA sequence
changes amino acid sequence
 changes protein’s:

 Structure
 function

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changes in protein may
change phenotype &
therefore change fitness
2. Gene Flow
 Movement of individuals &
alleles in & out of populations
seed & pollen distribution by
wind & insect
 migration of animals

 sub-populations may have
different allele frequencies
 causes genetic mixing
across regions
 reduce differences
between populations
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Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
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3. Non-random mating
 Sexual selection
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4. Genetic drift
 Effect of chance events

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces population to
small number & then population recovers &
expands again
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Founder effect
 When a new population is started
by only a few individuals
some rare alleles may be at high
frequency; others may
be missing
 skew the gene pool of
new population

 human populations that
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started from small group
of colonists
 example:
colonization of New World
Distribution of blood types
 Distribution of the O type blood allele in native
populations of the world reflects original settlement
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Distribution of blood types
 Distribution of the B type blood allele in native
populations of the world reflects original migration
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Out of Africa
Likely migration paths of humans out of Africa
10-20,000ya
10-20,000ya
50,000ya
Many patterns of human traits reflect this migration
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Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
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Cheetahs
 All cheetahs share a small number of alleles
less than 1% diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an important
Peregrine Falcon
concept in conservation
biology of endangered
species
loss of alleles from gene pool
 reduces variation
 reduces adaptability

Breeding programs must
consciously outcross
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Golden Lion
Tamarin
5. Natural selection
 Differential survival & reproduction due
to changing environmental conditions
 climate change
 food source availability
 predators, parasites, diseases
 toxins

combinations of alleles
that provide “fitness”
increase in the population
 adaptive evolutionary change
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5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
Directional, Disruptive, and
Stabilizing Selection
 Selection

Favors certain genotypes by acting on the
phenotypes of certain organisms
 Three modes of selection are



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Directional
Disruptive
Stabilizing
 Directional selection

Favors individuals at one end of the
phenotypic range
 Disruptive selection

Favors individuals at both extremes of the
phenotypic range
 Stabilizing selection

Favors intermediate variants and acts
against extreme phenotypes
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Original population
 The three modes of selection
Original
population
Evolved
population
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
Fig 23.12 A–C
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Phenotypes (fur color)
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
The Preservation of Genetic
Variation
 Various mechanisms help to preserve
genetic variation in a population
 Diploidy

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Maintains genetic variation in the form of
hidden recessive alleles
Heterozygote Advantage
 Some individuals who are heterozygous at
a particular locus

Have greater fitness than homozygotes
 Natural selection

Will tend to maintain two or more alleles at
that locus
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Neutral Variation
 Neutral variation

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Is genetic variation that appears to
confer no selective advantage
Sexual Selection
 Sexual selection


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Is natural selection for mating success
Can result in sexual dimorphism,
marked differences between the sexes
in secondary sexual characteristics
The Evolutionary Enigma of Sexual Reproduction
 Sexual reproduction

Produces fewer reproductive offspring than asexual
reproduction, a so-called reproductive handicap
Sexual reproduction
Asexual reproduction
Generation 1
Female
Female
Generation 2
Male
Generation 3
Generation 4
Figure 23.16
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 If sexual reproduction is a handicap,
why has it persisted?

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It produces genetic variation that may
aid in disease resistance
Any Questions??
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2005-2006
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