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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
average
Causes of Variation
• Envi. Factors: food variability
• 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 (gametes)
Coen Brothers
Allele Frequencies and the Gene Pool
• Gene Pool: Total genetic information available.– 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 population.
• Remember lots of phenotypes are inner (proteins, enzymes)
• Is behavior a phenotype? Is it inheritable?
Variability in mollusc species, (bean clams)
Hardy-Weinberg Equilibrium
• A population that doesn’t change its frequencies…– 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 Genetic
Equilibrium
• How does evolution happen when the Hardy-Weinberg equilibrium is broken?
Mutation
• 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
causing– Teratogen: Birth defect
causing
• But a good mutation influences evolution
Migration
• 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 group
disasters change the genetics of a population
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 average
• Could lead to two species
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 plumage.
Tommy Lee, peacocking
Formation of Species
• New species show up, old species go extinct
• Speciation: Forming new species
• Well how do you know if you have a new species?
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.
Problems
• 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 ends.
• subspecies of Ensatina salamanders – 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
Isolating• Sit alone in your room• Two parts of a formerly
interbreeding popl stop interbreeding somehow.
• Geographicly– Canyons, rivers, deserts
show up over the years and seperage groups.
• Can be shown experimentally with flies
Reproductive isolation
• Same area, but suddenly there are barriers to breeding– 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.
Review
• What does a bell curve look like?
• What is meant by the term gene pool?
• What is genetic equilibrium?
• What is genetic drift?• How does mutation disrupt
genetic equilibrium? How does immigration?
• What is sexual selection?• What are shortcomings of
biological species concept?
• How can isolation lead to speciation?
• What is punctuated equilibrium?