I.Evolve - Population Evolution On the Origin of Species…...

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Unformatted text preview: Population Evolution On the Origin of Species… WHAT IS A SPECIES? MICROEVOLUTION • Individuals in one or more populations • Potential to interbreed • Produce fertile offspring WHAT IS A POPULATION? • Group of interacting individuals belonging to one species and living in the same geographic area • A population is the smallest unit that can evolve. EVOLUTION OF POPULATIONS The Synthetic Theory of Evolution – Applying population genetics to the Darwinian model • • • • Genetics & Variability Non-Adaptive Evolution Fitness & Natural Selection Sexual Selection Figure 23.6 Genetic variation: how do new forms arise? Genes & Alleles • Genes code for morphology, physiology, or behavior. • Gene for hair color or leg length. • Alleles are alternative forms genes can have: red hair, brown hair, etc. • Point mutations: change code, change protein. DNA RNA protein chain of amino acids Heyer 1 Population Evolution Genetic variation: how do new forms arise? Mutation Events Are Random • Chromosomal mutations of base sequences. Genetic variation: how do new forms arise? • Mutations are destructive alterations to previously existing complex systems. • Most mutations are neutral or detrimental — very few are beneficial. • The need for a mutation to arise does not increase its probability. Individual Variation • Crossing over between maternal & paternal chromosome makes new genotypes. • Individuals vary in bell-curve of phenotypes – phenotype = genotype + environment. • Due to different environments or genotypes – But only gene differences are passed on. # of ind. in popn. leg length amount of saliva produced rate of eye blinking Non-genetic Phenotypic Variation Alleles to Gene Pools • Environmentally induced phenotypic variations within the same genotype (a) Map butterflies that emerge in spring: orange and brown (b) Map butterflies that emerge in late summer: black and white Alleles in an individual Alleles in all the individuals of the population Figure 23.9 Heyer 2 Population Evolution GENE POOL & FREQUENCY OF ALLELES IN THE POPULATION • Gene pool is the total collection of genes and their variations (alleles) in a population • Reservoir of variations from which the next generation derives its genes • Polymorphic CW CW genotype CRCR genotype Plants mate Generation 2 All CRCW (all pink flowers) 50% CR gametes 50% CW gametes Come together at random Generation 3 25% CR CR 50% CR CW 50% CR gametes 25% CW CW 50% CW gametes Come together at random Allelic frequency= 49% AA + 42% Aa + 9% aa = 70% A + 30% a The Hardy-Weinberg Theorem Generation 1 • If gametes contribute to the next generation randomly, Mendelian segregation and recombination of alleles preserves genetic variation in a population \ the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation Generation 4 25% CR CR 50% CR CW 25% CW CW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions Figure 23.4 A population in A population in Hardy-Weinberg equilibrium • • p = frequency of occurrence of the CR allele in the population q = frequency of occurrence of the Cw allele in the population Hardy-Weinberg equilibrium • • p = frequency of occurrence of the CR allele in the population q = frequency of occurrence of the Cw allele in the population Then: 64% CRCR, 32% CR CW, and 4% CW CW Gametes for each generation are drawn at random from the gene pool of the previous generation: CR (80%) Sperm 16% CRCW CR (80%) p2 CW (20%) CR (80%) pq p2 64% CRCR q2 4% CWCW If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% CRCR, 32% CR CW, and 4% CW CW CW (20%) qp 20% (q = 0.2) Sperm 16% CRCW Gametes of the next generation: 64% CR from 16% CR from + CR CR homozygotes CR CW homozygotes R = 0.8 = p = 80% C CW (20%) 4% CW from 16% CW from + CW CW homozygotes CR CW heterozygotes = 20% CW = 0.2 = q pq 16% CRCW Eggs 16% CRCW Eggs CW (20%) 80% (p = 0.8) 20% CW (q = 0.2) CW CR (80%) 80% CR (p = 0.8) CR qp 64% CRCR q2 With random mating, these gametes will result in the same mix of plants in the next generation: 4% CWCW 64% CRCR, 32% CR CW, and 4% CW CW Figure 23.5 A population in Hardy-Weinberg equilibrium • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then 2 2 q p + 2pq + q = 1 64% CRCR, 32% CR CW and 4% CW CW plants again! Figure 23.5 BIOLOGICAL EVOLUTION • Change in a population’s gene pool over time as a result of a change in frequency of an allele • p2 = frequency of the genotype homozygous for the first allele • q2 = frequency of the genotype homozygous for the first allele • 2pq = frequency of the heterozygous genotype 70% A + 30% a Heyer 50% A + 50% a 3 Population Evolution BIOLOGICAL EVOLUTION • Change in a population ’s gene pool over time as a result of a change in frequency of an allele • But according to Hardy- Weinburg Equilibrium: if mating is random, the frequency of alleles in a population remains constant over time. BIOLOGICAL EVOLUTION • Change in a population’s gene pool over time as a result of a change in frequency of an allele • But according to Hardy-Weinburg Equilibrium: if mating is random, the frequency of alleles in a population remains constant over time. • Therefore, population evolution is a product of non-random mating. 70% A + 30% a 70% A + 30% a A population in BIOLOGICAL EVOLUTION Hardy-Weinberg equilibrium • The five conditions for Hardy-Weinberg equilibrium: ¸ Large population size ¸ No significant gene flow ¸ Mutation rate is trivial compared to recombination ¸ Random mating ¸ No significant natural selection • If any/several of these conditions are not met, changes in allele frequency may occur ÿ non-equilibrium = evolution EVOLUTION OF POPULATIONS • • • • Heyer Genetics & Variability Non-Adaptive Evolution Fitness & Natural Selection Sexual Selection Remember! — • Natural selection works on phenotype – But only genotype is inherited • Natural selection works on individuals – But only populations evolve Non-Adaptive Evolution: Most Likely in Small Populations • • • • • Genetic Drift Genetic Bottleneck Founder Effect Gene Flow Assortative Mating 4 Population Evolution Genetic Drift • Random changes in gene frequencies. Non-Adaptive Evolution – it can lead to homozygosity. • • • • • Genetic Bottleneck Genetic Drift Genetic Bottleneck Founder Effect Gene Flow Assortative Mating VARIATION IN CHEETAH POPULATIONS • Few survive a population crash – cheetahs – elephant seals (~20) Non-Adaptive Evolution • • • • • Founder Effect Genetic Drift Genetic Bottleneck Founder Effect Gene Flow Assortative Mating • A few individuals disperse to new place – ABO frequencies of Dunkers in PA vs. Germany. Heyer 5 Population Evolution Non-Adaptive Evolution • • • • • Genetic Drift Genetic Bottleneck Founder Effect Gene Flow Assortative Mating • Breeding w/ outsiders – Spaniards in Central America Non-Adaptive Evolution: Significant only in small populations. * Genetic Drift: random changes in gene freq. * Genetic Bottleneck: few survive pop. crash ÿ cheetahs & elephant seals * Founder Effect: few disperse to new place ÿ ABO Gene Flow Non-adaptive Evolution: Nonrandom mating • Assortative Mating: with those who look most like you. • Sexual Selection: coming soon! frequencies of Dunkers in PA vs. Germany. * Gene Flow: breeding w/ outsiders ÿ Spaniards in Central America * Æ↑ homozygosity. * ÆØ homozygosity. EVOLUTION OF POPULATIONS • Genetics & Variability • Non-Adaptive Evolution • Fitness & Natural Selection – Modes of Selection • Sexual Selection Terms used in Natural Selection • Fitness: measure of how many genes you pass on to future generations. • Differential representation of genes in future generations due to differential survival to reproductive maturity. – Requires heritable (genetic) variation among individuals. • If differential survival is based upon expressed genotypic differences – it may lead to changes in population gene frequency. Heyer 6 Population Evolution “Survival of the fittest” Fitness and Selection • Darwinian fitness is a relative measure – how many offspring does one individual leave relative to others in the population. “Reproduction of the fittest” • Inheritance acts upon genotype • Selection acts upon phenotype – morphology, physiology, or behavior • Agents of selection – physical environment – biological environment • con- or hetero-specifics. Modes of Selection Modes of Selection • • • • Stabilizing Selection: Human Birth-weights Fitness and Selection Stabilizing Selection Directional Selection Diversifying Selection Heterozygote Advantage and the Sickle Cell Gene • HbS is a recessive allele for the hemoglobin gene • 1/11 African Americans are carriers • 1/500 are homozygous recessive (sickle cell disease) Heyer 7 Population Evolution MALARIA AND SICKLE CELL DISEASE Modes of Selection • • • • Heterozygote Advantage: Malaria and Sickle Cell Anemia Directional Selection: The Pepper Moth Biston betularia Fitness and Selection Stabilizing Selection Directional Selection Diversifying Selection Industrial melanism in early 1900’s Directional Selection: The Pepper Moth Biston betularia Heyer EVOLUTION OF PEST RESISTANCE • Red gene confers resistance to pesticide • Insecticide application • Only individuals carrying red gene survive • Red gene increases in population 8 Population Evolution MALARIA TODAY MALARIAL RESISTANCE • Mostly under control in 1947 • Common today in tropical countries • Kills 2.7 million per year • Mostly children Modes of Selection • • • • Diversifying Selection: African Seedcrackers Fitness and Selection Stabilizing Selection Directional Selection Diversifying Selection – a.k.a. disruptive selection These birds feed on seeds of two sedge species. Diversifying Selection: African Seedcrackers Diversifying Selection: African Seedcrackers “Balanced polymorphism ” Heyer 9 Population Evolution Diversifying Selection Diversifying Selection: Different phenotypes favored in different parts of the population ’s range E.g., a cline: graded change in a trait along a geographic axis CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences. Experimental group sample Sierra Nevada Range Great Basin Plateau 0.06 0.05 0.04 Frequencyindependent control 0.03 Seed collection sites 0.02 Figure 23.11 EVOLUTION OF POPULATIONS • Genetics & Variability • Non-Adaptive Evolution • Adaptive Evolution: Natural Selection – Modes of Selection • Sexual Selection Sexual Selection • Observed sexual dimorphism – sexes differ in size, color, or behavior • Some differences don’t aid survival – dimorphic feature makes animal more obvious animal Heyer On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity). Parental population sample Phenotypic diversity RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. – Predators learn to focus on most common phenotype – Minor alternate phenotypes escape notice Figure 23.14 Mean height (cm) EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants. Heights of yarrow plants grown in common garden Atitude (m) • • • Frequency-dependent selection 0 Plain background Patterned background 20 40 60 80 Generation number 100 Sexual Selection • Natural Selection (NS): differential reproduction due to differential survival. • Sexual Selection (SS): differential reproduction due to increased Reproductive Success (RS) despite possible decreased survival. Sexual Selection • Intrasexual Selection: – competition among members of one sex for access to members of the other sex. – a.k.a. Male-Male Competition. 10 Population Evolution Sexual Selection Female Choice • Intrasexual Selection • Intersexual Selection: – ability of one sex to woo the opposite sex. – a.k.a. Female Choice. Female Choice in New Guinea Birds of Paradise & Hills Tribes Female Choice • Bowerbirds: display is separate from bird. Why Females Choose and Males Fight: Parental Investment & Sexual Selection • Sex w/ most invested has most to loose: Reversed Dimorphism Where the female is the pursuer because she invests less. • Phalarope females are bigger and brighter. – Eggs more “expensive” than sperm – Females must be selective • Female RS limited by # of young they raise. • Male RS limited by # of females they mate. • Females lay a clutch every 10-12 days • Male clutch care takes 3 months • Females will destroy eggs to free up a male Ala lions, primates, mice Heyer 11 ...
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