Chapter 11– Evolution of
Populations
Darwin knew traits had to be heritable, but didn’t know
how heredity worked  scientists started to connect
Darwin and Mendel’s work for a better understanding
Genetic variation is seen in populations
since they share common genes
Gene pool = all the genes/alleles
in a population
Allele frequency is how common an allele is in the
population
Dominant or recessive have nothing
to do with frequency, its just how
many times the allele appears
To find frequency:
# of time allele occurs
Total # of alleles
Genetic variation has 2 main sources:
1. Mutations – random DNA changes
2. Recombination – results from meiosis and sex
How would the fitness of
this alligator be affected
by his mutation?
Natural selection acts on the phenotypes produced by
the alleles
Single gene traits will only have 2 phenotypes;
dominant or recessive
Polygenic traits have many phenotype options
since they are controlled by more than one gene
Polygenic traits usually show a normal, or bell-like shape
distribution
This usually indicates an equal chance
of survival for all phenotypes
Mean/average
Low extreme
High extreme
An environmental change can cause natural
selection to favor certain phenotypes  this
changes their distribution
1. Directional selection favors phenotypes
of either extreme
Causes the whole curve to shift in 1 direction
Possible causes: Limited resources, new
element introduce to environment
2. Stabilizing selection occurs when individuals
with the average phenotype are favored
Causes the curve to slim around the mean
Example
Example
3. Disruptive selection favors both extremes and selects
against the average
Causes the curve to start to split
If natural selection is strong enough, the
curve fully splits forming a new species
Sometimes natural selection isn’t the only cause of
change in a population
Gene flow = alleles move from one pop to another
This means new alleles in a pop and
more genetic diversity
Genetic drift = the random change in allele
frequency due to chance
Generally affects small populations
2 common causes are the bottleneck
effect and the founder effect
Bottleneck effect = a much
smaller population size
after a major disruption
Founders effect = small
number of individuals
colonize a new area
Sexual selection = when certain traits increase
mating success
The traits aren’t always adapted for
survival, but seem to stick around
Male frigate bird
Just like scientists compare experimental results to a
control, they often compare population data to models
The Hardy-Weinberg equilibrium model
is a go to comparison and prediction
It says a pop will remain in equilibrium if
certain requirements are met
Weinberg
Hardy
5 conditions for Hardy-Weinberg
(remember, if all 5 are met  no evolution)
1. Large population
2. No moving in or out of the pop
3. No mutations
4. Random mating
5. No natural selection
If one pop is isolated from another, they no longer share
genes, they could become 2 different species
Speciation = the formation of a new
species from an existing species
There are several ways populations can become isolated
1. Behavioral isolation - caused by different
courtship/mating rituals
2. Temporal isolation – caused by mating at
different times
3. Geographic isolation - caused by physical
barriers that separate pops
Certain geographic barriers may not
separate all species
Why might a river geographically
isolate a lizard population, but not
a bird population?
4. Reproductive isolation - when different pops
can no longer mate successfully
Considered final stage of speciation 
other isolation types feed into this
Macroevolution looks at evolution on larger scale
 6 major topics/trends have been noticed
1. Convergent evolution – evolution toward similar
characteristics in unrelated species
Ex. Wings in birds, insects, bats (analogous)
2. Divergent evolution – related species evolve in
different directions
Generally due to different environments
Kit Fox
Red Fox
3. Coevolution – 2 species evolve in response to
changes in each other over time
4. Extinction – a species no longer exists
Background extinctions occur constantly, but
at a low rate
Mass extinctions happen on large scale and
are usually sudden
5. Punctuated equilibrium – a burst
of evolution, followed by a long
period of stability
6. Adaptive Radiation – the diversification of 1 ancestral
species into many descendent species