I got nothin’ witty.

Evolutionary Theory

How evolution takes placeHow evolution takes placeBIG IDEA: The theory of evolution is constantly

changed as new evidence is discovered.One thing is known – evolution occurs at the population level.

There are five major mechanisms of evolution:There are five major mechanisms of evolution:1.Gene flow – caused by organisms migrating in or out

of a population2.Mutations – random changes in DNA3.Natural selection4.Nonrandom mating – you pick your mate for a

reason5.Genetic drift (next slide)Without these, a population will reach Hardy-Hardy-

Weinberg equilibriumWeinberg equilibrium (meaning the percent of each allele will stay the same) and evolution will not occur.

Genetic driftGenetic driftThis occurs when you have a small

population and the genes in the gene pool become skewed as a result.

Example one: founder effect Example one: founder effect – when a small population moves to an isolated area and interbreeds within itself (like Darwin’s finches)

Example two: bottleneck effect Example two: bottleneck effect – when species numbers dwindle down to a low amount and increase again (like cheetahs)

Natural Selection typesNatural Selection typesStabilizing selection Stabilizing selection (common) – the

average form of an organism is selected(ex: birth size)

Directional selection Directional selection – one of the two extremes is selected (ex: pepper moths)

Disruptive selection Disruptive selection – both extremes are selected – the average selected against (ex: Northern water snake)

SpeciationSpeciationQ: How do you know when you have a new Q: How do you know when you have a new

species?species?A: When some members of a population can no A: When some members of a population can no

longer breed and produce fertile offspring with longer breed and produce fertile offspring with members of the original population.members of the original population.

The sexual isolation that arises in this process can be prezygoticprezygotic (before fertilization) or postzygoticpostzygotic (after). This would lead to sympatric sympatric speciation speciation (new species arise within a population)

Speciation can also be caused by geographic boundaries where the organisms can’t breed (allopatric speciationallopatric speciation)

Evolution typesDivergent evolution Divergent evolution – many new species

arise from a common ancestor due to unique adaptations (i.e. adaptive radiation).

CoevolutionCoevolution – two or more species evolving in close relationship to one another.

Convergent evolution Convergent evolution – species evolving similarly, despite being separated geographically.