Chapter 18: Evolutionary Change in Populations

Remember: Basis of evolution is GENETIC We study POPULATIONS (not in an individual lifetime) Variations are due to ENVIRONMENT HEREDITY

Populations genetics Study of genetic variability within a population and of the forces that act on it

5 factors responsible for evolutionary change: Nonrandom mating Mutation Genetic drift Gene flow Natural selection

Gene pool All the alleles for all genes present in the population Genotype frequency Phenotype frequency Allele frequency

Hardy-Weinberg Genetic equilibrium – allele and genotypes frequencies do not change between generations Ideal situation – rare Provides model Equation:

The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01 The frequency of normal alleles is p = 1 – q = 1 – 0.01 = 0.99 The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 or approximately 2% of the U.S. population

Conditions for Hardy-Weinberg 1.) Random Matings 2.) No net mutations 3.) Large population size 4. ) No migration 5.) No natural selection

Hardy-Weinberg Problems Page 405 of textbook Review Questions (# 1-10) Check answers in Appendix Bring questions to class

Microevolution Generation-to-generation changes in allele or genotype frequencies within a population 5 processes at work: Nonrandom mating Inbreeding Assortative breeding Mutation Genetic drift Bottleneck effect Founder effect Gene flow Natural selection

Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 Fig. 23-8-3 CR CR CR CR CW CW CR CR CR CR CR CW CR CW CR CR CR CR CW CW CR CR CR CR CW CW CR CR CR CR CR CW CR CW CR CR CR CR CR CR CR CR CR CW CW CW CR CR Figure 23.8 Genetic drift CR CR CR CR CR CR CR CW CR CW CR CW Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 0.5 p = 1.0 q (frequency of CW ) = 0.3 q = 0.5 q = 0.0

Original population Bottlenecking event Surviving population Fig. 23-9 Figure 23.9 The bottleneck effect Original population Bottlenecking event Surviving population

Fig. 23-11 Figure 23.11 Gene flow and human evolution

Natural Selection – 3 types Stabilizing selection – intermediate phenotypes Directional selection – 1 extreme phenotype Disruptive selection – favors phenotypic extremes

Frequency of individuals Fig. 23-13 Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Figure 23.13 Modes of selection (a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Genetic Variation is Necessary for Natural Selection Sources of Variation Mutation Sexual reproduction Meiosis Crossing over Independent assortment Random union of gametes

Genetic Polymorphism Presence in a population of 2+ alleles for a give locus Variation Low frequency of alleles

Balanced polymorphism Type of genetic polymorphism 2+ alleles persist in a population over many generations because of natural selection 2 types: Heterozygote advantage Sickle cell Frequency-dependent selection More value when rare

Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% Fig. 23-17 Frequencies of the sickle-cell allele 0–2.5% Figure 23.17 Mapping malaria and the sickle-cell allele 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% >12.5%

“left-mouthed” individuals Fig. 23-18 “Right-mouthed” 1.0 “Left-mouthed” “left-mouthed” individuals Frequency of 0.5 Figure 23.18 Frequency-dependent selection in scale-eating fish (Perissodus microlepis) 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year

Neutral Variation Variation that does not alter the ability of an individual to survive and reproduce so is not adaptive