1. Evolution How Natural Selection Shapes Populations Chapter 17 Miller Levine Honors Biology NNHS 2015

2.  Genotype  Phenotype  Alleles  Describe how natural selection affects genotypes by acting on phenotypes. (Hint- THINK BUNNIES!) Evolution & Natural Selection Work via Genetics & Heredity

3. Definitions of Evolution  Gradual process (over millions of years!) by which modern organisms have descended from ancient ancestors  Evolution, in genetic terms, involves a change in the frequency of alleles in a population over time.

4.  Mutation – random changes in the DNA sequence  Gen Shuffling- during sexual reproduction  Creating haploid cells  Crossing over Sources of genetic variation

5.  Can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies  Think of those Naked Bunnies! Natural selection acting on a single gene trait

6. Polygenic traits

7. Polygenetic Trait- distribution of phenotypes

8.  Natural selection can act on traits produced by multiple genes in  Directional selection  Stabilizing selection  Disruptive selection Natural selection acting on a polygenic trait

10. Directional Selection  Changes the trait in one direction or the other – bigger or smaller, darker colored or lighter colored, faster or smaller, etc.

11. 3 Ways Natural Selection Affects Genotypes 1. Directional Selection-  favors individuals at one end of the phenotypic range.  Most common during times of environmental change or when moving to new habitats.  Example: Greyhounds

12. 3 Ways Natural Selection Affects Genotypes Disruptive Selection  Favors extreme over intermediate phenotypes.  Occurs when environmental change favors an extreme phenotype.  If pressures are too strong and last long enough they can cause the curve to split.

13. 3 Ways Natural Selection Affects Genotypes 3. Stabilizing Selection  favors intermediate over extreme phenotypes.  Reduces variation and maintains the current average.  Example : Siberian Husky

14. Genetic Drift  Natural selection is NOT the only source of evolutionary change.  Smaller populations do not always follow laws of probability.  Individuals may leave more descendants than other individuals by chance  A number of chance occurrences can cause an allele to become common in a population.

15. Genetic drift  Bottlenecks  Founder effect

16.  Genetic equilibrium – if a population is NOT experiencing natural selection then the allele frequencies in a population will remain the same  IF nothing is disturbing equilibrium then: p 2 + 2pq +q 2 = 1.0  5 conditions that can disturb genetic equilibrium:  Nonrandom mating (sexual selection)  Small population size (genetic drift)  Immigration or emigration  Mutations  Natural selection The Hardy-Weinberg Principle

17. Applying Hardy-Weinberg to Alleles Example: In a certain breed of fish, blue is dominant to yellow Blue fish can have the following genotype(s): BB Bb Yellow fish can have the following genotype(s): bb Genetic Variables: p= the frequency of dominant allele B (blue) (frequency = total # of B alleles/total number of alleles) q= the frequency of recessive allele b (yellow) (frequency = total # of b alleles/total number of alleles) Since there are only two alleles in the population and everyone in the population has one of the two alleles, the frequencies of the two must add up to one. p+ q = 1 (remember 1 --> 100%)

18. Hardy-Weinberg Equation Heterozygous fish Ex: The equation: 1.0 = p 2 + 2pq + q 2 Where: p 2 = frequency of BB genotype 2pq = frequency of Bb genotype q 2 = frequency of bb genotype p p q q p2p2 pq q2q2 B B b b BBBb bbBb

19. Let’s Watch a Video: http://ed.ted.com/lessons/five-fingers-of- evolutionhttp://ed.ted.com/lessons/five-fingers-of- evolution  The Five Causes of Evolution (take notes!) 1.Natural Selection 1.Small Populations or GENETIC DRIFT 1.Non-Random Mating 2.Mutation 3.Gene Flow (Immigration and Emmigration)

20. Break save for next time…

21. What is a species?  A species is a population or group of populations that can interbreed and produce fertile offspring  Speciation is the formation of a new species.  Occurs thanks to  Natural Selection  Chance Events  How does this happen?

22. Isolating Mechanisms  The gene pool of two populations become separated.  Reproductive Isolation  When members of two species cannot  Interbreed  Produce fertile offspring  These two populations will respond to natural selection and genetic drift as separate units

23. Types of Reproductive Isolation  Behavioral isolation  Geographic isolation  Temporal isolation