Download AP Biology 1. Small Population

Chapter 23
Hardy-Weinberg & Evolutionary Forces
What changes populations?
AP Biology
2007-2008
Review…
 So far we have learned that evolution can occur
through natural selection.

Evolution - changes in allele frequencies (the % of a
specific allele of a gene) that occur in a gene pool
over time / change in genetic makeup of populations
over time
 We have explored evidence that supports
evolution…but how do we know if evolution is
occurring or not?
If allele frequencies remain constant from
generation to generation, population is not
undergoing any evolutionary change (genetic
equilibrium).
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 If allele frequencies are changing, then…maybe!

Hardy-Weinberg Principle/Equilibrium/Law
 Mathematical model that shows that under

certain conditions, allele (& genotype)
frequencies in a population will remain
constant no matter how many generations
have passed (genetic equilibrium) =
evolution not occurring
Represents an ideal situation that never
really occurs in the real world
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Hardy-Weinberg Principle/Equilibrium/Law
 5 conditions must be met for genetic equilibrium
(evolution not occurring):
1)
2)
3)
4)
5)
Large population size
Random mating
No mutations
No gene flow
No natural selection
 These conditions remove all agents of

evolutionary change (all mechanisms of
evolution).
Under these conditions, allele frequencies will
remain the same indefinitely.
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Hardy-Weinberg Principle/Equilibrium/Law
p2 + 2pq + q2 = 1
p+q=1
p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
p2 = % of homozygous dominant (AA)
2pq = % of heterozygous (Aa)
q2 = % of homozygous recessive (aa)
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If H-W has conditions for a non-evolving population,
then a population can evolve by…
1. Small Population 2. Non-Random Mating
5 Agents of
Evolutionary
Change
Mechanisms of Evolution
Genetic Drift
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Sexual Selection
3. Mutation
4. Gene Flow
5. Natural Selection
Changes to DNA
Migration
Differential Survival
1. Small Population - Genetic Drift
 Small populations are more subject to the effects
of genetic drift
 Genetic Drift - Random chance events can change
frequency of traits in a population





not adaptation to environmental conditions (not
selection)
Can result in a reduction or elimination of an allele
(whether or not it was beneficial or harmful)
Decreases genetic variation within the population
Increases genetic variation between different
populations
2 types:
QuickTime™ and a
 founder effect
 bottleneck
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decompressor
are needed to see this picture.
Founder Effect
 When a new population (colony) is started by
only a small group of individuals.
 Who joins the group is random.


just by chance some rare alleles may
be at high frequency;
others may be missing
gene pool of new
population is skewed
 human populations that
started from small group
of colonists
 examples:
- birds get lost or blown off
course by storm
- the Amish
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QuickTime™ and a
decompressor
are needed to see this picture.
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
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Many patterns of human traits reflect this migration
Bottleneck Effect
 A disaster reduces population to small number &
population recovers and expands again but from a
limited gene pool.
 Who survives the disaster is random.


famine, natural disaster, human impact (loss of
habitat, hunting)…
loss of variation by chance event
 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
 new population very
different from original
population
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Bottleneck Effect - 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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Bottleneck Effect - Conservation Issues
Peregrine Falcon
 Bottlenecking is an important
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
2. Non-Random Mating - Sexual Selection
 Random mating - pairing by chance
 Non-random mating / sexual selection - individuals
select mates based on phenotype. Can lead to
changes in allele frequencies in gene pool



attractiveness to potential mate
fertility of gametes
successful rearing of offspring
Sexual selection may
act in opposition to
natural selection!
competition or
female choice
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3. Mutations
 Mutations are a source of new alleles
on which natural selection can work
 Provide potential for evolution
QuickTime™ and a
decompressor
are needed to see this picture.
Sickle Cell Anemia
Can be lethal if
homozygous recessive, but
beneficial if heterozygous!
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HETEROZYGOTE ADVANTAGE
4. Gene Flow
 Movement of alleles
between populations changes allele frequencies,
introduces new variation
(Ex:migration,immigration/emigration)
 Increases genetic variation
within the population
 Decreases genetic variation
between different populations
 Tends to counteract the effects of natural selection &
genetic drift (these tend to cause populations to
become more distinct).
 Helps maintain all the organisms over a large area as
one species.
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5. Natural Selection
 Traits that improve survival or reproduction will
accumulate in the population - adaptive change
 Over time the proportion of favorable alleles
increases in the population


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Ex: predation selection acts on both predator and
prey (speed, camouflage, defense)
Ex: physiological selection acts on body functions
(disease resistance, efficient use of nutrients)
Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice
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QuickTime™ and a
decompressor
are needed to see this picture.
AP Biology