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Chapter 16: pp. 283 - 298 BIOLOGY 10th Edition How Populations Evolve Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10% of population natural disaster kills five green frogs 20% of population PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display 1 DARWINIAN EVOLUTION MEETS GENETICS Darwin = no genetics Today = The modern synthesis connects Darwin’s theory of natural selection with population genetics INHERITANCE 2 genes inherited different genes may be same or different alleles alleles determine genotype genotype and environment determines phenotype natural selection acts on phenotypes POPULATION GENETICS In 1930s population geneticists described variations in a population in terms of alleles Microevolution pertains to evolutionary changes within a population. Various alleles at all the gene loci in all individuals make up the gene pool of the population. Gene pool of a population: Genotype Allele frequencies Industrial Melanism and Microevolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early observation 36% dark-colored phenotype Later observation 64% dark-colored phenotype 10 Frequency of Gametes Calculation From genotype frequencies, the allele and gamete frequencies can be calculated Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. genotypes DD frequency From ofgenotype frequencies, genotypes Dd dd the allele and0.64 0.32 ingamete the population frequencies can be calculated 0.04 frequency of alleles and gametes in the population 0.04 + 0.16 0.16 + 0.64 0.20 D 0.80 d 7 Hardy-Weinberg Equilibrium Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F1 generation Genotypes: DD Dd dd Genotype frequencies: 0.04 0.32 0.64 Allele and gamete frequencies: D = 0.20 d = 0.80 eggs F2 generation 0.20 D 0.80 d sperm 0.20 D 0.04 DD 0.16 Dd 0.16 Dd 0.64 dd 0.80 d Offspring Genotype frequencies: 0.04 DD + 0.32 Dd + 0.64 dd = 1 p + 2pq + q = 1 2 2 p2 = frequency of DD genotype (dark-colored) = (0.20)2 = 0.04 2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80) = 0.32 q2 = frequency of dd genotype (light-colored) = (0.80)2 = 0.64 1.00 9 Hardy-Weinberg Theorem describes and predicts genotype and allele frequencies in a non-evolving population (equilibrium) Evolution = change in allele frequencies in a population – hypothetical: what conditions would NOT 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) So, what is the point of discussing Hardy Weinberg? • The Hardy-Weinberg formulas allow scientists to determine whether evolution has occurred. • Any changes in the gene frequencies in the population over time can be detected. • It allows us to compare how drastic or not changes are compared to what it could have been without evolution • Then compare to other organisms Hardy-Weinberg Equation p2 + 2pq + q2 = 1 and p + q = 1 •p = frequency of the dominant allele in the population q = frequency of the recessive allele in the population •p2 = percentage of homozygous dominant individuals q2 = percentage of homozygous recessive individuals 2pq = percentage of heterozygous individuals G.H. Hardy mathematician W. Weinberg physician Using Hardy-Weinberg equation q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 population: 100 cats 84 black, 16 white How many of each genotype? p2=.36 BB 2pq=.48 Bb q2=.16 bb Must assume population is in H-W equilibrium! What are the genotype frequencies? 5 Causes of evolutionary change Genetic Drift Gene Flow Mutation Non-random mating Selection 1. GENETIC DRIFT • Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability) • types = bottleneck effect and founder effect GENETIC DRIFT 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 Ex: Tay Sachs Jewish Middle Ages GENETIC DRIFT Founder Effect Ex: Polydactyly in Amish • 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: Genetic Drift Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10% of population natural disaster kills five green frogs 20% of population 17 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 3. NON RANDOM MATING • Individuals choose their mates • Sexual selection • inbreeding and assortive mating (both shift frequencies of different genotypes) 4. MUTATIONS • Mutation creates variation = a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) 5. NATURAL SELECTION Fitness depends on •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 Outcomes of natural selection Frequency of individuals Original population Original population Evolved population Stabilizing selection Phenotypes (fur color) Directional selection Diversifying selection Figure 13.19 Effects of Selection DIRECTIONAL SELECTION giraffe neck STABILIZING SELECTION human birth weight DISRUPTIVE SELECTION weeds Disruptive Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Forested areas Low-lying vegetatio © Bob Evans/Peter Arnold, Inc. 29 Directional Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. No predation All guppies are drab and small Amount of Color Low predation above waterfall High predation 0 below waterfall Experimental site 4 Months 8 12 Result © Helen Rodd 26 Natural Selection • Selection acts on any trait that affects survival or reproduction – predation selection – physiological selection – sexual selection 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 Sources of Variation in a Population Mutations Sex Geographic Variation Diploidy Polymorphism Heterozygote Superiority Frequency Dependent Selection Evolutionary Neutral Traits Population Variation – random changes to DNA Beak depth • Mutation Wet year Dry year Dry year • errors in mitosis & meiosis 1977 Dry year 1980 1982 1984 • environmental damage • Sex – mixing of alleles • recombination of alleles • new combinations = new phenotypes – spreads variation • offspring inherit traits from parent Beak depth of offspring (mm) 11 10 9 8 Medium ground finch 8 9 10 11 Mean beak depth of parents (mm) Independent Assortment Key Maternal set of chromosomesPossibility 1 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1Combination 2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Combination 3Combination 4 Population variation • Geographical variation: differences in location causing differences in individuals partly due to environment • Diploidy 2nd set of chromosomes hides variation in the heterozygote Population Variation • Prevention of natural selection’s reduction of variation Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population (light and dark crabs) • 1- heterozygote advantage – AA = normal hemo; malaria – Aa = normal hemo; malaria resistant – aa = sickle cell 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host) Population Variation • Fingerprints - Not all genetic variation may be subject to natural selection • Some variations may be neutral, providing no apparent advantage or disadvantage – Example: human fingerprints Figure 13.16 Sexual Selection Female Choice Choice of a mate is serious consideration Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival. Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance. Male Competition Can father many offspring because they continuously produce sperm in great quantity. Compete to inseminate as many females as possible. 30 Sexual Selection Sexual selection adaptive changes in males and females to increase ability to secure a mate. Males - ability to compete Females choose to select a male with the best fitness (ability to produce surviving offspring). 31 Sexual Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The drab females tend to choose flamboyant males as mates. 32 Sexual Selection: Competition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images 33 Sexual Selection in Humans Study shows that female choice and male competition apply to humans too Women must invest more in having a child than men. Men, need only contribute sperm Generally more available for mating than are women. More Men men = competition Also Have a Choice Prefer women who are most likely to 35 Natural selection cannot fashion perfect organisms • This is due to: – historical constraints (birds can’t have 4 legs + 2 wings)…descent with modification (not randomly making new ones) – adaptive compromises (flexible limbs in humans sprains)….structure over flexibility – chance events (natural disasters) – selection can only edit existing variations (fittest phenotypes currently available survive)…are they necessarily the best traits though?