Download Population Genetics

The Gene Pool
• Members of a species
can interbreed &
produce fertile offspring
• Species have a shared
gene pool
• Gene pool – all of the
alleles of all individuals
in a population
The Gene Pool
• Different species
do NOT exchange
genes by
interbreeding
• Different species
that interbreed
often produce
sterile or less
viable offspring e.g.
Mule
Populations
• A group of the
same species
living in an area
• No two individuals
are exactly alike
(variations)
• More Fit individuals
survive & pass on
their traits
Speciation
• Formation of new
species
• One species may
split into 2 or more
species
• A species may
evolve into a new
species
• Requires very long
periods of time
Modern Synthesis Theory
• Today’s theory on evolution
• Recognizes that GENES are responsible for
the inheritance of characteristics
• Recognizes that POPULATIONS, not
individuals, evolve due to natural selection &
genetic drift
• Recognizes that SPECIATION usually is due
to the gradual accumulation of small genetic
changes
Microevolution
• Changes occur in gene pools due to
mutation, natural selection, genetic drift,
etc.
• Gene pool changes cause more
VARIATION in individuals in the population
• This process is called MICROEVOLUTION
• Example: Bacteria becoming unaffected
by antibiotics (resistant)
Population - a localized group of individuals of the same
species.
Species - a group of populations whose individuals have
the ability to breed and produce fertile offspring.
Individuals near a population center are, on average,
more closely related to one another than to members of
other populations.
A population’s gene pool is the total of all
genes in the population at any one time.
If all members of a population are
homozygous for a particular allele, then
the allele is fixed in the gene pool.
we know that evolution does occur within populations.
Evolution within a species/population = microevolution.
Microevolution refers to changes in allele frequencies in a
gene pool from generation to generation. Represents a
gradual change in a population.
Causes of microevolution:
1) Genetic drift
2) Natural selection (1 & 2 are most important)
3) Gene flow
4) Mutation
1) Genetic drift
Genetic drift = the alteration of the gene pool of a small
population due to chance.
Two factors may cause genetic drift:
a) Bottleneck effect may lead to reduced genetic
variability following some large disturbance that
removes a large portion of the population. The
surviving population often does not represent the
allele frequency in the original population.
b) Founder effect may lead to reduced variability when a
few individuals from a large population colonize an
isolated habitat.
*Yes, I realize that this is not really a cheetah.
2) Natural selection
As previously stated, differential success in reproduction
based on heritable traits results in selected alleles being
passed to relatively more offspring (Darwinian
inheritance).
The only agent that results in adaptation to environment.
3) Gene flow
-is genetic exchange due to the migration of fertile
individuals between populations.
4) Mutation
Mutation is a change in an organism’s DNA and is
represented by changing alleles.
Mutations can be transmitted in gametes to offspring,
and immediately affect the composition of the gene pool.
The original source of variation.
Genetic Variation, the foundation of Natural Selection
Genetic (inheritable) variation within and between
populations: exists both as what we can see (e.g., eye
color) and what we cannot see (e.g., blood type).
Environment also can alter an individual’s phenotype [e.g.,
the hydrangea we saw before, and…
…Map butterflies (color changes are due to seasonal
difference in hormones)].
Variation between populations
Geographic variations are differences between gene pools
due to differences in environmental factors.
Natural selection may contribute to geographic variation.
It often occurs when populations are located in different
areas, but may also occur in populations with isolated
individuals.
Geographic variation
between isolated
populations of house
mice.
Normally house mice are
2n = 40. However,
chromosomes fused in
the mice in the example,
so that the diploid
number has gone down.
Cline, a type of geographic variation, is a graded variation
in individuals that correspond to gradual changes in the
environment.
Example: Body size of North American birds tends to
increase with increasing latitude. Can you think of a
reason for the birds to evolve differently?
Example: Height variation in yarrow along an altitudinal
gradient. Can you think of a reason for the plants to
evolve differently?
Mutation and sexual recombination generate genetic
variation
a. New alleles originate only by mutations (heritable only in
gametes; many kinds of mutations; mutations in functional
gene products most important).
- In stable environments, mutations often result in little or
no benefit to an organism, or are often harmful.
- Mutations are more beneficial (rare) in changing
environments. (Example: HIV resistance to antiviral drugs.)
Heterozygote advantage is one example of a
balanced polymorphism, where the
heterozygote has greater survival and
reproductive success than either
homozygote (Example: Sickle cell anemia
where heterozygotes are resistant to
malaria).
Sickle Cell and Malaria
Relative fitness
a. Directional selection favors
individuals at one end of the
phenotypic range. Most common
during times of environmental change
or when moving to new habitats.
Directional selection
Diversifying/Disruptive selection favors extreme over
intermediate phenotypes.
- Occurs when environmental change favors an extreme
phenotype.
Stabilizing selection favors intermediate over extreme
phenotypes.
- Reduces variation and maintains the current average.
- Example = human birth weights.
Diversifying selection
Genes Within Populations
Forms of Selection
• Disruptive selection
– Selection eliminates intermediate types.
• Directional selection
– Selection eliminates one extreme from a
phenotypic array.
• Stabilizing selection
– Selection acts to eliminate both extremes
from an array of phenotypes.
Kinds of Selection
Why is genetic variation
important?
variation
global
warming
survival
EXTINCTION!!
no variation
Why is genetic variation
important?
variation
no variation
Why is genetic variation
important?
divergence
variation
no variation
NO DIVERGENCE!!
Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resista
0.00 resistant
Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resista
0.00 resistant
Natural
selection
mutation!
Resistance to antibacterial soap
Generation 1:
1.00 not resist
0.00 resistant
Generation 2:
0.96 not resist
0.04 resistant
Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resist
0.00 resistant
Generation 2:
0.96 not resist
0.04 resistant
Generation 3:
0.76 not resist
0.24 resistant
Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resist
0.00 resistant
Generation 2:
0.96 not resist
0.04 resistant
Generation 3:
0.76 not resist
0.24 resistant
Generation 4:
0.12 not resist
0.88 resistant
Natural selection can cause
populations to diverge
divergence
Selection on sickle-cell allele
aa – abnormal ß hemoglobin very low
fitness
sickle-cell anemia
AA – normal ß hemoglobin intermed.
fitness
vulnerable to malaria
Aa – both ß hemoglobins
resistant to malaria
high
fitness
Selection favors heterozygotes (Aa).
Both alleles maintained in population (a at low level).
How does genetic structure
change?
• mutation
• migration
genetic change by chance alon
• natural selection
• genetic drift
• non-random mating
• sampling error
• misrepresentation
• small populations
Genetic drift
Before:
8 RR
0.50 R
8 rr
0.50 r
After:
2 RR
6 rr
0.25 R
0.75 r
How does genetic structure
change?
• mutation
• migration
• natural selection
• genetic drift
• non-random mating
cause changes in
allele frequencies