Evolution & Speciation
Evolution
Evolution is the change in living things over
long periods of time.
It may occur through the processes of
natural selection and genetic drift.
Natural Selection
Natural selection explains why organisms
are adapted to their environment.
This theory describes a natural process that
can produce new adaptations and new
species.
Natural Selection
4 conditions:
1. Reproduction
2. Variation in traits between individuals
3. Heredity  some of this variation must
be heritable (genetic)
4. Individuals differ in fitness (the number
of offspring they produce)  some
individuals are more likely to reproduce
than others
Natural Selection
If these 4 conditions are met, then natural
selection will result.
Let’s look at an example…
Natural Selection
A cryptic moth:
1. These moths reproduce
2. There is variation in their wing color
pattern
3. This color pattern variation is heritable
(offspring resemble their parents)
http://www.bio.umass.edu/biology/kunkel/catocala.html
Natural Selection
4. Because they use their color pattern as
camouflage from bird predators, if
individuals do not match their background
they will get eaten  only moths that look
cryptic will survive and reproduce
http://www.bio.umass.edu/biology/kunkel/catocala.html
Natural Selection
Other examples:
•
Pesticide resistant insects:
- only those that can tolerate the pesticide will
survive and reproduce
- if this pesticide tolerance is heritable, then their
offspring will also be tolerant to the pesticide
pesticide
Surviving insects
Pesticide resistant insects
e.g. DDT used to be a very effective pesticide and initially wiped
out ~90% of mosquitoes in India.
After about 10years, mosquitoes evolved resistance to DDT
and now only ~20% of mosquitoes are wiped out by DDT.
Natural Selection
Other examples:
2. Antibiotic resistant bacteria:
- only those that can tolerate the antibiotic will
survive and reproduce
- if this antibiotic tolerance is heritable, then their
offspring will also be tolerant to the antibiotic
- have become increasingly prevalent as the use
of antibiotics has become widespread
Add antibiotic
Surviving bacteria
Antibiotic resistant strains
e.g. About 70% of bacteria that cause infections in hospitals are
resistant to at least one of the drugs most commonly used
to treat infections.
Natural Selection
Types of selection:
•
Directional selection
–
•
Favors one extreme and shifts the
population mean
Stabilizing selection
–
•
Favors the mean and decreases variation
around the mean
Disruptive selection
–
Favors the extremes and selects against
the mean
Responses to Selection
Coloration
Directional selection
Coloration
light
dark
light
dark
Responses to Selection
Directional selection
Stabilizing selection
Coloration
light
dark
Responses to Selection
Directional selection
Stabilizing selection
Coloration
light
dark
Disruptive selection
Genetic Drift
Genetic drift is the fluctuation of genotypes
in the population due to chance.
In small populations, genetic drift can result
in non-adaptive evolution.
Genetic Drift
Genotype A
Genotype B
Population 2
6:1
Population 4
1:6
Population 1
1:1
Population 3
1:1
For the population as a
whole, the frequency of the
2 genotypes is 1:1
Genetic Drift
Genotype A
Genotype B
Population 2
6:1
Population 4
1:6
Population 1
1:1
Population 3
1:1
For the population as a
whole, the frequency of the
2 genotypes is 6:1
 the frequency of
genotype A has increased
due to chance
Genetic Drift
Genotype A
Genotype B
Population 2
6:1
Population 4
1:6
Population 1
1:1
Population 3
1:1
For the population as a
whole, the frequency of the
2 genotypes is 1:1
 the frequency of the 2
genotypes has remained
the same due to chance
Genetic Drift
• Endangered species are often reduced to dangerously
small numbers and restricted to small, isolated habitats.
• In small populations, genetic drift erodes genetic
variation which limits a species ability to adapt to
environmental change (e.g. disease, global warming, etc)
and increases its probability of going extinct due to
chance events.
Speciation
What is a species?
• A group of interbreeding individuals of
common ancestry that are reproductively
isolated from other groups
Speciation
How might speciation (formation of new
species) occur?
• Allopatric speciation
– A barrier divides a continuous population into 2
parts
• Parapatric speciation
– A hybrid zone divides a continuous population into
2 connected parts
• Sympatric speciation
– A population splits in 2 without any geographic
separation
Speciation
Ancestral population
A physical barrier divides the
ancestral population. Over time,
each isolated population diverges
due to natural selection or drift
Allopatric
speciation
Speciation
Ancestral population
A physical barrier divides the
ancestral population. Over time,
each isolated population diverges
due to natural selection or drift
There is some kind of habitat
gradient that results in locally
adapted individuals.
hybrid zone
Allopatric
speciation
Parapatric
speciation
Speciation
Ancestral population
A physical barrier divides the
ancestral population. Over time,
each isolated population diverges
due to natural selection or drift
The extremes in the population
are selected for and there is
assortative mating.
There is some kind of habitat
gradient that results in locally
adapted individuals.
hybrid zone
Allopatric
speciation
Parapatric
speciation
Sympatric
speciation
Allopatric Speciation
Blue-headed wrasse
Isthmus
of
Panama
Cortez rainbow wrasse
http://cas.bellarmine.edu/tietjen/images/speciation.htm
Parapatric Speciation
http://cas.bellarmine.edu/tietjen/images/speciation.htm
Sympatric Speciation
Is thought to have
occurred in Cichlids
in a lake in Central
America (also in
Africa).
http://www.cichlidnewsmagazine.com/issues/2001apr/nicaragua.html
The diversity of the animals we see today is the
result of processes such as natural selection and
genetic drift that lead to speciation over time.
Hawaiian honey-creepers