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Speciation and macroevolution Chapter
Objectives:
- Review meiosis
-Species
-Repro. Isolating mechanisms
- Speciation
-Is evolution always slow
-Extinction
How Are Populations, Genes,
And Evolution Related?
The gene pool for the coat-color
gene contains 20 copies of allele B
and 30 copies for allele b
Population: 25 individuals
• The gene pool is the sum of
the genes in a population.
Gene pool: 50 alleles
B B B B B B B B
BB
BB
BB
BB
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
bb
bb
bb
bb
bb
bb
bb
bb
B B B B b b b b
– Evolution is the result of
changing allelic frequencies
in a population.
B B B B b b b b
B B B B b b b b
b b b b b b b b
b b b b b b b b
b b
bb
– The Hardy-Weinberg Principle shows that under
certain conditions, allele frequencies and genotype
frequencies in a population will remain constant no
matter how many generations pass; in other words,
this population will not evolve.
– This ideal, non-evolving population is called an
equilibrium population.
• An equilibrium population will occur only if:
– There are no mutations.
– There is no gene flow between different
populations.
– The population is really large.
– All mating is random (i.e., there is no selection of
specific genotypes by other genotypes).
– There is no natural selection; all genotypes
reproduce with equal success.
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What Causes Evolution?
What Causes Evolution?
• Mutations are the original source of genetic variability
in a population.
– An unrepaired mutation in a nucleotide sequence in
DNA occurs rarely
– that change is passed on to gametes
– shows up in the offspring (providing increased
diversity to the gene pool)
• Mutation provides a potential for evolutionary change.
• It is not goal oriented (mutation doesn’t know what
environment will be working on it)
• What if by chance . .
.
BB
BB
BB
BB
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
Bb
bb
bb
bb
bb
bb
– http://upload.wikimedia.org/wikipedia/commons/b/b
6/Random_sampling_genetic_drift.gif
• Population bottlenecks cause genetic drift
• Reduce variation!! (Cheetas!)
.
BB
• Genetic drift
– Chance random events cause allelic frequencies to
change.
– Can occur in small populations.
• A disaster reduces an allelic frequency
• Isolation of a breeding group (can’t pass on
alleles)
• Disease
frequency of B = 50%
frequency of b = 50%
Population size matters.
- If a population is sufficiently large, chance events are unlikely
to alter allelic frequencies much.
- In a small population, a particular allele may be carried by only a
few organisms; chance events could eliminate most or all
examples of such an allele from the population.
What Causes Evolution?
• A founding population has a different allelic frequency
– From a bottleneck event
– From a small break away population
The gene pool of a
population contains
equal numbers of
red, blue, yellow and
green alleles
A bottleneck event
drastically reduces the
size of the population
By chance, the gene
pool of the reduced
population contains
mostly blue and a
few yellow alleles
After the population grows
and returns to its original
size, blue alleles predominate; red and green alleles
have disappeared
Don’t forget!!!!
• Natural selection stems from unequal reproduction (i.e.,
differential reproductive success)
– Natural selection works on individuals, but its populations
that change by evolution
– Natural selection works through those individuals in a
population that leave the most offspring that carry alleles
that were selected by the environment.
– Natural selection does not act directly on the genotype of
an organism but on its phenotype
– Evolution does not make organisms “better.”
So what does the term”survival of the fittest” mean?
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Species (Biological Species Concept)
A group of organisms with similar structural
and functional characteristics that in nature
breed only with each other and have a close
common ancestry
- a common gene pool (isolated from others)
- restricted by reproductive barriers
Species and reproductive isolation
Problems with concept:
1. What about asexual organisms?
2. What about when two species interbreed
and produce an offspring?
• lion + tiger = “liger” (sterile)
• female horse + male donkey = mule
1.
Temporal Isolation
Reproductive isolating mechanisms
- genetic integrity is maintained
Premating barriers interfere with mating! They
are premating!!
1. Temporal isolation
2. Behavioral (sexual) isolation
3. Mechanical isolation
4. Habitat isolation & Geographic isolation
2. Behavioral Isolation
3. Mechanical Isolation
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4. Habitat isolation & Geographic isolation
•Closey related species that occur in different
habitats or vastly separated areas geographically
Species and reproductive isolation
Postmating barriers prevent successful reproduction
when mating does occur:
1. Hybrid inviability: (embryonic hybrid dies)
2. Hybrid sterilty: (hybrid is infertile - no gametes)
3. Hybrid breakdown = F2 Hybrid has defect: F1hybrid
Okay but offspring doesn’t reproduce well
4 Gamete incompatability: gamete doesn’t fertilize egg
at all
Hybrid Sterility
The key to speciation is reproductive isolation.
Speciation:
The evolution of a new species.
Female
horse
Mule
(Sterile)
1. Geographical separation can lead to a new
species
Male
donkey
2.
- Long physical
isolation
- Different selective
pressures
Kaibab squirrel??
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Sypatric Speciation:
• Allopolyploidy: (Plants) Multiple sets of
chromosomes from > 2 spp.
Hybrid
• Changing ecology (fruit maggot flies in
Hudson River valley)
Species A
2n = 6
Species B
2n = 4
P generation
Polyploidy
n= 3
n= 2
Gametes
Hybrid AB
F1 generation
No doubling of
chromosome number
Doubling of
chromosome number
2n = 10
Chromosomes either
cannot pair or go
through erratic meiosis
No gametes or sterile
gametes-no sexual
reproduction possible
Pairing now
possible
during meiosis
n= 5
Viable gametes-sexual
reproduction possible
(self-fertilization)
Slide 3
Is evolutionary change fast or slow?
They eat, mate,
and lay eggs on
different hosts
•
Darwin’s view predicts slow gradual
change?
• Does the fossil record support this?
Hawthorne maggot fly range (parasitic on native hawthorne)
Apple maggot fly and Hawthorn maggot fly range
(parasitic on domestic apples)
Slide 5
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Macroevolution: Large scale phenotypic changes
- radical changes that create new species
Driving forces behind macroevolutionary changes?
1. Evolutionary novelties?
2. Adaptive radiation?
2. Mass extinction?
1. Evolutionary novelties
Evolution works on existing things (its conservative)
Pre-adaptations: variations on a pre-existing structure
- originally filled one role – changed in a way to
become adaptive for a different role
- feathers? Scales (protection/insulation) to
insulation/flight
How might such a novelty occur?
- mutation in a gene?
- e.g. Developmental gene – that regulates
development of an organism mutated
Mutation of a developmental gene
- Axolotl salamander:
mutation prevented maturation
• Remember evolution is conservative
Paedomorphosis
2. Adaptive radiation
• The evolution of many related species from an ancestral
species
a
b
c
d e
z
- adaptive zones (ecological niches)
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3. Extinction!!!!!!!!!!!!!
Is the process of extinction evolution?
Background vs. Mass extinction
6th Great Extinction!
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