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Transcript 16_Evolution_of_Populations_and_Speciation

16 Evolution of Populations
and Speciation
Genetic Equilibrium
• Combining Mendel’s work on genetics with
Darwins ideas of evolution
• Population Genetics: studying evolution
from a genetic point of view
Variation of Traits in a pop.
• Evolution is the change in the genetics of
a group of organisms.
• A population is the smallest grouping that
can evolve.
– What would be a larger grouping?
Bell Curve
• If you measure an
observable trait, there is
often a large average
group, with a few with
more, and a few with less.
• Standard Deviation,
– (Greek letter lower
case sigma σ )
– Shows the spread
around the mean
– How far are you from
Causes of Variation
• Envi. Factors: food
• Genetics: Even two
brothers will look dif.
– Mutations cause some
of this
– Recombination: when
genes get mixed
around during meiosis
– The random pairing of
sperm and eggs
Coen Brothers
Allele Frequencies and the Gene
• Gene Pool: Total genetic information
– If there’s no gene for feathers, then nobody
has feathers.
– If you know what’s there, you can predict what
will be there in the next generation
– Like Punnet squares for groups instead of just
a mom and dad
• Allele frequency: divide the number of a
certain allele by the total number of alleles
of all types in the population.
– Expressed as a percent
– If it’s 20%, then 1 in 5 chromosomes has it
– Shows the amount of diversity in a group
– Bloodtypes, eye colors
Phenotype Frequency
• Number of individuals
with a particular
phenotype divided by the
total number of
individuals in the
• Remember lots of
phenotypes are inner
(proteins, enzymes)
• Is behavior a phenotype?
Is it inheritable?
Variability in mollusc species, (bean
Hardy-Weinberg Equilibrium
• A population that doesn’t change its
– Has no mutation
– Has no immigration or emigration
– Is very large
– Mates are chose randomly
– There is no natural selection
• This population doesn’t evolve or change
• It is hypothetical
Disruption of
• How does
evolution happen
when the HardyWeinberg
equilibrium is
• Doesn’t happen too often
• Mutagens: mutation
causing things
– Latin: Origin of change
– Gamma rays, UV light,
• Mostly mutations are
harmful, gimp you up.
– Often carcinogens: Cancer
– Teratogen: Birth defect
• But a good mutation
influences evolution
• Immigration/emigration
• Behavior affects this
• Gene Flow: Process of
genes moving from one
population to another.
• Baboons: Since only a
few alpha males
reproduce in a group
young males often leave
and find their own group.
If humans have humanity do
baboons have baboonity?
Genetic Drift
• Phenomenon by
which allele freq. in a
population change as
a result of random
events or chance
• how important it is for
everyone to
reproduce depends
on the size of your
disasters change the genetics of a
Non-random mating
• Who’s close by?
• Assortive mating: Mate with someone who
looks like you.
– Tall girls & tall guys
Natural selection
• The most important
factor that disrupts a
genetic stability
• Stabilizing selection:
The average
individual is best
suited for the
environment and has
the most kids
• Everyone becomes
more average
Directional Selection
• One of the extremes
of a trait is more
successful and has
more kids.
Disruptive selection
• Opposite extremes
are more fit than the
• Could lead to two
Sexual Selection
• If you are healthy
enough to have some
weird plumage you’ll
make a good dad,
and your kids will
have even weirder
Tommy Lee,
Formation of Species
• New species show up, old species go
• Speciation: Forming new species
• Well how do you know if you have a new
Morphological concept
• What shape is it?
• If it’s shaped like a
grizzly bear, then it’s
a grizzly bear.
• Except sometimes
one species will have
different shapes.
– They look dif, but
they’ll breed together.
• Pheidole barbata Ants
look dif, but that’s
because they have dif
roles in the colony
Biological species concept
• If a group can breed together, but not with
other groups, then it’s a species
• Can’t see this for extinct animals
The Western meadowlark (left) and the
Eastern meadowlark (right) appear to be
identical, and their ranges overlap, but their
distinct songs prevent interbreeding.
• What about asexual organism?
• What about potential breeding, like they
can’t breed now, but they could if things
were dif?
• Ring species:species
with a geographic
distribution that forms a
ring and overlaps at the
• subspecies of Ensatina
– subtle morphological and
genetic differences
– They all interbreed with
their neighbors, but not
subspecies across the ring
– So where do we mark the
point of speciation?
• Really matters for
endangered species laws
and where you can build
on habitat
• Sit alone in your room
• Two parts of a
formerly interbreeding
popl stop
• Geographicly
– Canyons, rivers,
deserts show up over
the years and
seperage groups.
• Can be shown
experimentally with
Reproductive isolation
• Same area, but suddenly
there are barriers to
– Prezygotic barriers:
Happen before fertilization
• Mating dance isn’t right
• Mating season isn’t right
• These damselfly penises
illustrate just how complex
insect genitalia may be. 
– Postzygotic barrier:
happens after fertilization
• Weak offspring
How long does this take?
• Depends,
• Bananas reached the
Hawaiian islands about a
thousand years ago.
– Now the moths that eat bananas
have several distinct species
– Punctuated equilibrium:
sometimes change comes fast
– The fossil record doesn’t show
gradual change
– The one hopeful monster that
starts a whole new branch.
• What does a bell curve
look like?
• What is meant by the term
gene pool?
• What is genetic
• What is genetic drift?
• How does mutation disrupt
genetic equilibrium? How
does immigration?
• What is sexual selection?
• What are shortcomings of
biological species
• How can isolation lead to
• What is punctuated