Chapter 23: The Evolution of Populations What is a population? Localized group of individuals of the same species What is a species? Organisms that can mate & produce fertile offspring in nature What is a gene pool? All of the genes (both alleles) in a population at any time What is a fixed allele? An allele that is the same for all members of the population (rare) How can we determine if a population is evolving? Hardy-Weinberg Theorem….which actually describes non-evolving populations….population is in equilibrium Alleles & genotypes in a gene pool will remain constant unless acted on by agents other than sexual recombination p + q = 1 p = dominant allele q = recessive allele 1 = ALL alleles in the gene pool

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? Hardy-Weinberg Theorem….which actually describes non-evolving populations….population is in equilibrium Alleles & genotypes in a gene pool will remain constant unless acted on by agents other than sexual recombination p + q = 1 p = dominant allele q = recessive allele 1 = ALL alleles in the gene pool What if we want to know specific genotypes of individual in the population? (p + q)(p + q) = 1 p2 + 2pq + q2 = 1 p2 = AA – homozygous dominant q2 = aa – homozygous recessive 2pq = Aa - heterozygotes

Example p + q = 1 p2 + 2pq + q2 = 1 500 plants in a population A – pink flowers a – white flowers 20 white flowers (genotype = _____) How many pink flowers are there? _______ Given: 320 AA 160 Aa 20 aa How many flower color alleles in the population? ______ How many dominant (A) alleles? _____ How many recessive (a) alleles? _____ What is the frequency of the A allele? ________ a allele? ________ What is the genotypic frequency of AA flowers? ______ Aa flowers? ______ aa flowers? ______ aa 480 1000 = 500 x 2 800 = (320 x 2) + (160 x 1) 200 = (20 x 2) + (160 x 1) 0.8 = 800/1000 0.2 = 200/1000 0.64 = p2 = (0.8)2 0.32 = 2pq = 2(0.8)(0.2) 0.04 = q2 = (0.2)2

X X2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 } biggest issues 0.01 0.04 0.09 0.16 0.25 0.36 0.49 0.64 0.81

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? What if we want to know specific genotypes of individual in the population? What are the 5 requirements for a population to be in H-W equilibrium? Large population Isolated population No net mutations Random mating No natural selection What happens if any one of these criteria is not met? No more equilibrium & microevolution occurs What are the causes of microevolution? Genetic drift – gene pool changes of a small population due to chance

Figure 23.7 Genetic drift Generation 1 Generation 2 Generation 3 CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? What if we want to know specific genotypes of individual in the population? What are the 5 requirements for a population to be in H-W equilibrium? Large population Isolated population No net mutations Random mating No natural selection What happens if any one of these criteria is not met? No more equilibrium & microevolution occurs What are the causes of microevolution? Genetic drift – gene pool changes of a small population due to chance Bottleneck effect – a disaster reduces the population size so that the surviving population is NOT representative of the original population Founder effect – genetic drift in a new colony such as an island

Figure 23.8 The bottleneck effect Original population Bottlenecking event Surviving (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? What is we want to know specific genotypes of individual in the population? What are the 5 requirements for a population to be in H-W equilibrium? What happens if any one of these criteria is not met? What are the causes of microevolution? Genetic drift – gene pool changes of a small population due to chance Bottleneck effect – a disaster reduces the population size so that the surviving population is NOT representative of the original population Founder effect – genetic drift in a new colony such as an island Gene flow – gain or loss of alleles due to immigration or emigration Mutations – changes in an organism’s DNA can create new alleles Non-random (selective) mating – favored alleles are selected for Natural selection – variability within a population enables some organisms to be better suited for survival and reproduction

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? What is we want to know specific genotypes of individual in the population? What are the 5 requirements for a population to be in H-W equilibrium? What happens if any one of these criteria is not met? What are the causes of microevolution? What are polymorphisms? 2 or more discrete traits within a population ex. freckles, blood type How can variation be preserved? Diploidy – hides less favorable alleles in Aa until natural selection favors them as aa What is a heterozygote advantage? Aa genotype has a selective advantage Sickle-cell allele & malaria resistance

Figure 23.13 Mapping malaria and the sickle-cell allele Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

Chapter 23: The Evolution of Populations What is a population? What is a species? What is a gene pool? What is a fixed allele? How can we determine if a population is evolving? What is we want to know specific genotypes of individual in the population? What are the 5 requirements for a population to be in H-W equilibrium? What happens if any one of these criteria is not met? What are the causes of microevolution? What are polymorphisms? How can variation be preserved? 12. What is a heterozygote advantage? 13. How can natural selection change a population? - Directional selection - Disruptive selection - Stabilizing selection

Figure 23.12 Modes of selection (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Frequency of individuals Phenotypes (fur color) Original population Original population Evolved