2. AP Biology Lecture #42 Population Genetics

3. The Evolution of Populations

4. Population genetics Population: a localized group of individuals belonging to the same species Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations Modern synthesis/neo-Darwinism “Individuals are selected, but populations evolve.”

5. Hardy-Weinberg Theorem Serves as a model for the genetic structure of a nonevolving population (equilibrium) Evolution = change in allele frequencies in a population – hypothetical: what conditions not would cause allele frequencies to change? – non-evolving population REMOVE all agents of evolutionary change 1.very large population size (no genetic drift) 2.no migration (no gene flow in or out) 3.no mutation (no genetic change) 4.random mating (no sexual selection) 5.no natural selection (everyone is equally fit)

6. Hardy-Weinberg Equation p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p) p2=frequency of AA genotype; 2pq=frequency of Aa genotype; q2=frequency of aa genotype; frequencies of all individuals must add to 1 (100%), so: p 2 + 2pq + q 2 = 1 G.H. Hardy mathematician W. Weinberg physician

7. What are the genotype frequencies? Using Hardy-Weinberg equation q 2 (bb): 16/100 =.16 0.4 q (b): √.16 = 0.4 0.6 p (B): 1 - 0.4 = 0.6 q 2 (bb): 16/100 =.16 0.4 q (b): √.16 = 0.4 0.6 p (B): 1 - 0.4 = 0.6 population: 100 cats 84 black, 16 white How many of each genotype? population: 100 cats 84 black, 16 white How many of each genotype? bbBbBB p 2 =.36 2pq=.48 q 2 =.16 Must assume population is in H-W equilibrium!

8. 5 Agents of evolutionary change Mutation Gene Flow Genetic Drift Selection Non-random mating

9. Microevolution, I A change in the gene pool of a population over a succession of generations 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

10. GENETIC DRIFT Refers to changes in allele frequencies, usually in small populations Occurs when founders start a new population or after a bottleneck

11. Microevolution, II: type of genetic drift The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

12. Bottleneck Effect – caused by a severe reduction in population, reduces overall diversity. Ex Cheetah

13. Conservation issues Bottlenecking is an important concept in conservation biology of endangered species – loss of alleles from gene pool – reduces variation – reduces adaptability Breeding programs must consciously outcross Peregrine Falcon Golden Lion Tamarin

14. Microevolution, III type of genetic drift Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population – just by chance some rare alleles may be at high frequency; others may be missing – skew the gene pool of new population human populations that started from small group of colonists example: colonization of New World

15. FOUNDER EFFECT The founder effect is an example of genetic drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population. Dwarfism in Amish communities Due to few German founders

16. Microevolution, IV 2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) seed & pollen distribution by wind & insect migration of animals

17. Microevolution, V 3- Mutations: Mutation creates variation a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

18. Microevolution, VI 4- Nonrandom mating: Sexual selection inbreeding and assortive mating (both shift frequencies of different genotypes)

19. Sexual selection It’s FEMALE CHOICE, baby!

20. TYPES OF SELECTION Directional Selection One phenotype is favored over another Causes a shift in the overall appearance of the species Ex: horses get larger

21. STABILIZING SELECTION occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored.

22. Human babies have an average size Too big and they can't get through birth canal Too small and they have low survivability

23. Microevolution, VII 5.NaturalSelection differential success in reproduction; climate change food source availability predators, parasites, diseases toxins only form of microevolution that adapts a population to its environment combinations of alleles that provide “fitness” increase in the population

24. DISRUPTIVE SELECTION -- occurs when extreme phenotypes are favored and can lead to more than one distinct form. CAN LEAD TO SPECIATION

25. Types of Selection

26. Natural Selection Selection acts on any trait that affects survival or reproduction – predation selection – physiological selection – sexual selection

27. Variation & natural selection Variation is the raw material for natural selection – there have to be differences within population – some individuals must be more fit than others

28. Mean beak depth of parents (mm) Medium ground finch 8 891011 9 10 11 1977198019821984 Dry year Wet year Beak depth Beak depth of offspring (mm) Where does Variation come from? Mutation – random changes to DNA errors in mitosis & meiosis environmental damage Sex – mixing of alleles recombination of alleles – new arrangements in every offspring new combinations = new phenotypes – spreads variation offspring inherit traits from parent

29. Population variation Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation: differences in genetic structure between populations (cline)

30. Variation preservation Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle- cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

31. Natural selection Fitness: contribution an individual makes to the gene pool of the next generation 3 types: A. Directional B. Diversifying C. Stabilizing

32. Effects of Selection Changes in the average trait of a population DIRECTIONAL SELECTION STABILIZING SELECTION DISRUPTIVE SELECTION giraffe neck horse size human birth weightrock pocket mice

33. Sexual selection Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

34. Any Questions??