16 Evolution of Populations and Speciation

Mr. Brascia’s Biology Class

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

http://upload.wikimedia.org/wikipedia/commons/5/53/Speciation_modes.svg

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?