1. Chapter 23 Chapter 23

2. Population genetics Population: a localized group of individuals belonging to the same species 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 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 Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations Population genetics: the study of genetic changes in populations Modern synthesis/neo-Darwinism Modern synthesis/neo-Darwinism “Individuals are selected, but populations evolve.” “Individuals are selected, but populations evolve.”

3. Hardy-Weinberg Theorem Serves as a model for the genetic structure of a nonevolving population (equilibrium) Serves as a model for the genetic structure of a nonevolving population (equilibrium) 5 conditions: 5 conditions: 1- Very large population size; 1- Very large population size; 2- No migration; 2- No migration; 3- No net mutations; 3- No net mutations; 4- Random mating; 4- Random mating; 5- No natural selection 5- No natural selection

4. 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) p=frequency of one allele (A); q=frequency of the other allele (a); p + q=1.0 (p=1-q & q=1-p) P 2 =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q 2 =frequency of aa genotype; p 2 + 2pq + q 2 = 1.0 P 2 =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q 2 =frequency of aa genotype; p 2 + 2pq + q 2 = 1.0

5. Microevolution, I New definition of Evolution at the population level. New definition of Evolution at the population level. Evolution is a generation to generation change in a population’ s frequencies of alleles. Evolution is a generation to generation change in a population’ s frequencies of alleles. This also can be called microevolution: A change in the gene pool of a population over a succession of generations This also can be called microevolution: 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) 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

6. Figure 23.4 Genetic drift

7. Microevolution, II 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 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

8. Microevolution, III Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population

9. Microevolution, IV 2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) 2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

10. Microevolution, V 3- Mutations: a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) 3- Mutations: a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) 4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes) 4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes) 5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment 5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

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

12. Figure 23.8 Clinal variation in a plant

13. Two Random Processes that generate genetic variation Mutation – new alleles originate only by mutation. Rare and random events and usually occur in somatic cells and are not passed on to the offspring. Mutation – new alleles originate only by mutation. Rare and random events and usually occur in somatic cells and are not passed on to the offspring. Sexual Recombination combines old alleles with new and fresh assortments every generation. Sexual Recombination combines old alleles with new and fresh assortments every generation.

14. Variation preservation Prevention of natural selection’s reduction of variation Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote 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) 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)

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

16. Figure 23.12 Modes of selection

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

18. 18 16.3 Maintenance of Diversity Genetic Variability Populations with limited variation may not be able to adapt to new conditions Maintenance of variability is advantageous to the population Only exposed alleles are subject to natural selection

19. 19 Maintenance of Diversity Natural selection causes imperfect adaptations Depends on evolutionary history Imperfections are common because of necessary compromises The environment plays a role in maintaining diversity Disruptive selection due to environmental differences promotes polymorphisms in a population If a population occupies a wide range, it may have several subpopulations designated as subspecies The environment includes selecting agents that help maintain diversity

20. 20 Subspecies Help Maintain Diversity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.; (E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited Pantheropsis obsoleta lindheimeri Pantheropsis obsoleta rossalleni Pantheropsis obsoleta spiloides Pantheropsis obsoleta quadrivittata Pantheropsis obsoleta obsoleta

21. 21 Maintenance of Diversity Recessive alleles: Heterozygotes shelter recessive alleles from selection Heterozygotes allow even lethal alleles to remain in the population at low frequencies virtually forever Sometimes recessive alleles confer an advantage to heterozygotes The sickle-cell anemia allele is detrimental in homozygote However, heterozygotes are more likely to survive malaria The sickle-cell allele occurs at a higher frequency in malaria prone areas

22. 22 Maintenance of Diversity Heterozygote Advantage Assists the maintenance of genetic, and therefore phenotypic, variations in future generations. In sickle cell disease heterozygous individuals don’t die from sickle-cell disease, and they don’t die from malaria.