Bellringer – Bunny Lab

• What happened to the number of F alleles?

• What happened to the number of f alleles?

• What happened to the frequency of F alleles?

• What happened to the frequency of f alleles?

• EXPLAIN these results.

What is Evolution?

•Genetic Change in a population over time

Population Genetics

• Individuals do NOT evolve, populations do!• Natural selection acts on a range of phenotypes in

a population• Evolution occurs as a population’s genes change

over time• Gene pool = all the genes (alleles) of a population• Allelic frequency = % of any one allele• Think about “Breeding Bunnies”

                                                                                                                                                      

  

Evolution does not work this way.

Genetic Equilibrium

• Genetic Equilibrium = a population where the allelic frequency is the same over generations = NOT evolving!

• If a population is NOT in genetic If a population is NOT in genetic equilibrium, it is EVOLVINGequilibrium, it is EVOLVING!!!

• So, how do you disrupt genetic equilibrium and have evolution???

Disrupting Genetic Equilibrium

(change the gene pool)• Mutation

– Change DNA = change genes!– Some lethal, some useful– Radiation, chemicals, spontaneous, etc.

• Migration = Gene flow– Movement into or out of a population = different gene pool– Immigration (into) and emigration (out of)

• Genetic Drift– Alter frequency by chance events (Ex: catastrophe, habitat

destruction)– Occurs in small populations that are isolated – only those

genes are passed– Cheetahs were inbred because they were endangered

• Cheetahs had small gene pool with little variation so they were less likely to adapt

Types of Selection(will cause change in allelic frequencies in gene pool!)

• Stabilizing Selection = favors the average individuals; extremes die off ; reduces variation– Ex: Plant height, spider size

• Directional Selection = favors one extreme variation– Ex: Giraffe neck, woodpecker beak

• Disruptive Selection = favors both extremes; average dies off, eliminates intermediate phenotype– Ex: limpet shell

A species is a group of individuals that can interbreed and produce fertile offspring.

For example, these happy face spiders look different, but since they can interbreed, they are considered the same species:

Theridion grallator.

                                                                                                      

               

Speciation (evolution of a new species)

• Geographic isolation – a physical barrier divides a population and each population adapts to its own environment and develops its own gene pool

• Reproductive isolation = can’t interbreed anymore = new species

The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...

                                                                                                            

Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.

                                                                                        

                    

•The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

                                                                                  

                          

•So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.

                                                                                                      

      

In our fruit-flies-in-rotten-bananas-in-a-hurricane example, a geographic barrier kicked off the speciation process, but different selection pressures on the island caused the island population to diverge genetically from the mainland population.

                                                                                           

             

Geographic isolation can instigate a speciation event — but genetic changes are necessary to complete the process.

What might have caused that to happen? Perhaps, different fruits were abundant on the island. The island population was selected to specialize on a particular type of fruit and evolved a different food preference from the mainland flies.

                                                                                           

             

Differing selection pressures on the two islands can complete the differentiation of the new species.

Could this small difference be a barrier to gene flow with the mainland flies? Yes, if the flies find mates by hanging out on preferred foods, then if they return to the mainland, they will

not end up mating with mainland flies because of this different food preference. Gene flow would be greatly reduced; and

once gene flow between the two species is stopped or reduced, larger genetic differences between the species can accumulate.

                                                                                                 

           

Patterns of Evolution

• Divergent Evolution– Species once similar

to an ancestor diverge and become distinct

– Adaptive Radiation = ancestral species evolves into many species to fit diverse habitats

• Ex: finches

• Convergent Evolution– Unrelated organisms

evolve similar traits

– Why? Unrelated species may occupy similar environments in different parts of the world

• Punctualism– Evolution happens in

brief intense bursts with long periods of inactivity in between (represented by a stepped line on a graph)

• Gradualism– Evolution proceeds at a

slow steady pace (represented by a strait raising line on a graph)