Evolution of Populations & Population Genetics - PDF

Evolution of Populations & Population Genetics Ch. 19 Be able to: Contrast our modern understanding of genetics with Darwin’s assumptions about inheritance of characteristics. Describe the types and causes of phenotypic & genetic variation within a species. Describ...

Evolution of Populations & Population Genetics Ch. 19 Be able to: Contrast our modern understanding of genetics with Darwin’s assumptions about inheritance of characteristics. Describe the types and causes of phenotypic & genetic variation within a species. Describe a population in terms of its genotypic and allelic frequencies. Find solutions to simple population genetics problems, correctly applying the Hardy-Weinberg equation as appropriate. Explain how the Hardy-Weinberg Principle relates to microevolution. Name and describe the mechanisms of microevolution and their expected outcomes. Describe and explain various forms of selection and the limitations of evolution by natural selection. 2 EVOLUTION OF POPULATIONS Genes, Mutations, & Inheritance Hardy-Weinberg & Microevolution Mechanisms of Microevolution Understanding Natural Selection Genes, Mutations, and Inheritance  Genes, Mutations, and Inheritance (What Darwin Never Knew) Darwin (1859) Mendel (1866) DeVries (1890-1900) continuous variation in discrete genetic factors in species individuals accumulation of no blending; no “accumulation” differences in offspring importance of mutations Sutton-Boveri (1902) chromosome theory of inheritance Morgan (1910’s) mutations & modern genetics  Genes, Mutations, and Inheritance – Phenotypic variation is mostly genetic but environment can influence expression, creating non-heritable variation Larvae fed on oak flowers. Larvae fed on oak leaves. Both forms produce offspring with both possible variations. Expression is environmentally influenced!  Genes, Mutations, and Inheritance discrete genetic variation single gene locus –2 or more alleles continuous variation –phenotypes produced by combined effects of 2 or more genes What gives rise to genetic diversity among offspring? 8  Genes, Mutations, and Inheritance – Sexual recombination produces genetic diversity among offspring: crossovers independent assortment random fertilization  Genes, Mutations, and Inheritance – New alleles arise from mutations in DNA in cells that ultimately make gametes point mutations –1 to 3 base-pairs  Genes, Mutations, and Inheritance chromosomal alterations deletions, duplications, translocations  Genes, Mutations, and Inheritance gene duplications expand size of genome –duplicates can mutate into new alleles Over time, there is potential for more complexity  Genes, Mutations, and Inheritance – Most new alleles are harmful (“deleterious”) but harmful effects may be “hidden” in heterozygotes – Some new alleles may be neutral with regard to selection –new phenotype does not affect likelihood of leaving offspring – If environment changes, harmful or neutral alleles may become adaptive –new phenotype increases likelihood of leaving offspring  Genes, Mutations, and Inheritance – However, most DNA variability does NOT affect phenotype not a new allele because protein translation/gene expression is not affected This is the ONLY mutation that created a new allele (w/ potential for phenotypic variation) Hardy-Weinberg & Microevolution The Population The Hardy-Weinberg Principle  Hardy-Weinberg & Microevolution The Population population = a group of interbreeding individuals in the same area, somewhat isolated from other groups occasionally, a centipede crosses river  Hardy-Weinberg & Microevolution two populations of caribou, since interbreeding between the two herds is rare  Hardy-Weinberg & Microevolution – Populations differ in genetic makeup gene pool = all the alleles of all the genes in a population – many genes have “fixed” alleles (homozygous in all individuals) – other genes: 2 or more alleles genotypic frequency: = % (proportion) of each genotype in the population %AA, %Aa & %aa allelic frequency: = % of each allele in the population %A allele and %a allele  Hardy-Weinberg & Microevolution Population #1 3 phenotypes 4/25 = 16% = 0.16 AA genotypic 12/25 = 48% = 0.48 Aa frequency allele 9/25 = 36% = 0.36 aa frequency 20/50 = 40% = 0.4 A 30/50 = 60% = 0.6 a  Hardy-Weinberg & Microevolution Population #2 3 phenotypes 28% AA same allele frequency, 24% Aa 48% aa but genotypic freq. is different 40% A 60% a  Hardy-Weinberg & Microevolution Population #3 2 phenotypes, aa never lives AA AA 64% AA 36% Aa both allele frequency, 0% aa and genotypic freq. are different 82% A 18% a Notice that the general appearance of the population is more green/yellow with no blue.  Hardy-Weinberg & Microevolution Population #4 4 phenotypes AA’ 56% AA Here, a random mutation changes 32% Aa allele & genotype ratios & produces 8% aa a new phenotype. 4% AA’ This gene now has three alleles in this 74% A 24% a 2% A’ population.  Hardy-Weinberg & Microevolution – Any change in population allelic or genotypic frequency over time = microevolution smallest (fundamental) unit of evolution Natural selection or chance? Galapagos Medium Ground Finch  Hardy-Weinberg & Microevolution The Hardy-Weinberg Principle – The H-W equilibrium: IF a large population reproduces sexually at random, THEN the genetic frequencies should not change in next generation (remains in equilibrium) same frequency of alleles & genotypes in next generation Generation 1 Generation 2  Hardy-Weinberg & Microevolution The H-W conditions: 1. no mutations 2. mating is random 3. no selection (equal survival) 4. very large population size 5. no gene flow in or out  Hardy-Weinberg & Microevolution Example: population of 500 flowers – 320 20 160  Hardy-Weinberg & Microevolution Example: population of 500 flowers – phenotype genotype # indiv. #CR #CW red CRCR 320 ___ 640 ___ 0 white CWCW 20 ___ 0 ___ 40 pink CRCW 160 ___ 160 ___ 160 TOTALS: 500 800 200 genotypic freq: CRCR 320/500 = 64% or 0.64 CWCW 20/500 = 4% or 0.04 CRCW 160/500 = 32% or 0.32 allele freq: CR: 800/1000 = 80% or 0.8 CW: 200/1000 = 20% or 0.2 Hardy-Weinberg & Microevolution IF this population meets H-W random conditions, then every generation over time will have same ratios! 64% 4% 32% How is that possible??  Hardy-Weinberg & Microevolution note that p + q = 1  Hardy-Weinberg & Microevolution 0.8 0.2 0.16 0.8 0.64 0.04 0.16 0.2  Hardy-Weinberg & Microevolution 0.8 0.2 expected genotypic ratio of next 0.16 gen. same as 0.8 previous 0.64 generation 0.16 0.04 0.2  Hardy-Weinberg & Microevolution Note that allelic freq. (p & q) the same too! For every 100 plants: » 64 are CRCR » 32 are CRCW » 4 are CWCW p = (128+32)/200 = 0.8 and q = (32+8)/200 = 0.2 3rd gen – The H-W equation (population at equilibrium): if p = frequency of dominant allele and q = frequency of recessive allele and p + q = 1, then in any generation: p2 + 2pq + q2 = 1 where p2 = freq. of homozygous dominant genotype 2pq = freq. of heterozygous genotype q2 = freq. of homozygous recessive genotype  Hardy-Weinberg & Microevolution – Using the H-W equation: If you KNOW or CAN ASSUME a HW equilibrium, then use the equation to determine population genetic makeup. Example: An island population of butterflies is in H-W equilibrium. 64% have black stripes, a trait due to an autosomal dominant allele B. What are the allele frequencies? What % of the population is homozygous dominant? 37  Hardy-Weinberg & Microevolution Example: An island population of butterflies is in H- W equilibrium. 64% have black stripes, a trait due to an autosomal dominant allele B. What are the allele frequencies? p2 + 2pq + q2 = 1 If 64% dominant phenotype, then 36% are homozygous recessive thus, q2 = 0.36 q = √0.36 = 0.6 or 60% b p = 1-q = 0.4 or 40% B What % of the population is homozygous dominant? %BB = p2 = (0.4)2 = 0.16 = 16%  Hardy-Weinberg & Microevolution Example: In an isolated human population (assume H-W), the frequency of the Tay Sach’s allele t is 0.02. What is the chance that any one individual is a carrier? Carriers are Tt P(Tt) = % heterozygotes = 2pq q = 0.02 p+q = 1.0 p = 0.98 2pq= 2(0.98)(0.02) = 0.04 or 4%  Hardy-Weinberg & Microevolution – H-W also lets us detect microevolution: H-W equilibrium is “null hypothesis” –If actual (observed) ratios ≠ expected H-W ratios, then the population is evolving –Microevolution: an evolving population is one that is showing genetic change over generations.  Hardy-Weinberg & Microevolution Example: A population of 1000 ducks has: Is this 500 AA, 200 Aa, 300 aa population freq a = (300 x 2 + 200)/2000 = q = 0.4 evolving? p = 1- 0.4 = 0.6 Expected H-W ratios: Actual (observed) ratios: p2 = (0.6)2 = 0.36 (360) 0.50 (500/1000) 2 pq = 2(0.6)(0.4)= 0.48 (480) 0.20 (200/1000) q2 = (0.4)2 = 0.16 (160) 0.30 (300/1000) Since observed ratios ≠ expected, this population is evolving. Mechanisms of Microevolution 1. Natural Selection That is, what CAUSES 2. Genetic Drift microevolution 3. Gene Flow to happen?  Mechanisms of Microevolution 1. Natural Selection: – Acts non-randomly on phenotypes of individuals – Changes allelic & genotypic frequencies of populations non-randomly – Always leads to adaptation of population to current environment  Mechanisms of Microevolution Ex: resistance to DDT nonrandom selection of phenotype of individuals  Mechanisms of Microevolution Ex: resistance to DDT Allele frequencies changed! Population is now more resistant to DDT (adaptation of the population)  Mechanisms of Microevolution Ex: resistance to DDT before DDT after DDT (1930) (1960+) microevolution Did exposure DDT-R freq. to DDT itself DDT-R freq. = 0% create DDT- = 37% R allele? ? Mechanisms of Microevolution 1. Natural Selection That is, what CAUSES 2. Genetic Drift microevolution 3. Gene Flow to happen?  Mechanisms of Microevolution 2. Genetic Drift = genetic frequency changes due to random events – Often occurs in small populations (like “sampling errors” in statistics) Mechanisms of Microevolution Mechanisms of Microevolution Mechanisms of Microevolution  Mechanisms of Microevolution – Outcomes of genetic drift: random changes in allele frequency in either direction often reduces diversity one allele may become “fixed” (all other alleles lost)  Mechanisms of Microevolution Two places genetic drift is important in microevolution: 1. The founder effect & genetic drift a few founders start new isolated population – founder gene pool differs from original source – small population size leads to more drift – better alleles may be lost! random, less diverse founder population diverse original population more genetic drift; some adaptive alleles are lost  Mechanisms of Microevolution Ex: high rate of inherited blindness on Tristan da Cunha – maladaptive allele frequency increased! » Retinitis pigmentosa—autosomal recessive ~250 descendants from 15 settlers  Mechanisms of Microevolution 2. The bottleneck effect & genetic drift an event drastically cuts population size gene pool of survivors is random; some alleles are lost more genetic drift  Mechanisms of Microevolution Example: overhunting of elephant seals & cheetahs reduced genetic variation  Mechanisms of Microevolution Example: prairie chicken habitat loss  Mechanisms of Microevolution Example: prairie chicken habitat loss infer that harmful alleles increased “bottleneck” reduced genetic variation  Mechanisms of Microevolution 3. Gene Flow = alleles move in/out of population – Includes: migration of adults dispersal of gametes, seeds, larvae  Mechanisms of Microevolution Example: Africanized Honey Bees 62  Mechanisms of Microevolution – Results of gene flow: tends to add diversity to population tends to reduce differences between populations Few differences between populations where gene flow is higher Understanding Natural Selection Relative Fitness Forms of Natural Selection Sexual Selection How is Genetic Variation Maintained? Limitations of Natural Selection  Understanding Natural Selection Relative Fitness – Fitness is relative to other individuals in the population “fittest” = best reproductive success Relative fitness is ZERO because he and his children all die!  Understanding Natural Selection fitness includes: survival, finding mates, & the # healthy, fertile offspring Who has the greater relative fitness?  Understanding Natural Selection Forms of Natural Selection –Directional selection shifts character's mean value to one direction Understanding Natural Selection shift in avg beak length directional selection  Understanding Natural Selection –Disruptive selection (Diversifying selection) intermediates are less fit than extremes maintains diversity  Understanding Natural Selection Example: seedcracker finch: 2 size beaks –seeds available mostly in one of 2 sizes disruptive From Life, Purves et al. 7th ed. selection  Understanding Natural Selection –Stabilizing selection intermediate types more fit than extremes variation reduced  Understanding Natural Selection stabilizing higher death rate narrow peak selects against selection low birth size higher death rate selects against large birth size From Life, Purves et al. 7th ed.  Understanding Natural Selection Sexual Selection = success based on traits related to obtaining mates (not directly related to environment) – Leads to sexual dimorphism female choice male: red attracts females female: brown hides from predators Understanding Natural Selection Due to error fertilization or early development Has one ovary, one teste Occurs in birds, crustaceans,insects, NOT mammals Shirley Caldwell Gynandromorphic cardinal Understanding Natural Selection direct competition only males compete; only males have big “noses” Understanding Natural Selection dominant male females have no choice  Understanding Natural Selection How is Genetic Variation Maintained? 1. Through diploidy: less successful recessive alleles are hidden in heterozygotes 2. Through disruptive selection: surviving extreme phenotypes will carry different alleles disruptive selection due to patchy resources  Understanding Natural Selection Maintaining Genetic Variation 3. Through heterozygote advantage selection favors heterozygote over either homozygote, maintaining both alleles Example: sickle cell allele when malaria present  Understanding Natural Selection Phenotype of heterozygote is HbAHbA = normal RBC, fitter than either homozygote. but more likely dies from malaria HbAHbB = malaria resistant HbBHbB = sickle cell disease NOT an intermediate phenotype!  Understanding Natural Selection Maintaining Genetic Variation 4. Through frequency-dependent selection fitness of a phenotype decreases as its frequency increases in population nest-mates! one species in rare forms are less likely to be one field identified by visual predators  Understanding Natural Selection Maintaining Genetic Variation – Through frequency-dependent selection fitness of a phenotype decreases as its frequency increases in population the more common the therefore expect both phenotypes phenotype is, the less fit it is! to “balance” over time  Understanding Natural Selection rarer type gets to left-mouthed breeding eat more and leave success is less when more offspring! they are more common  Understanding Natural Selection Limitations of Natural Selection – It acts on phenotype of entire individual. an adaptation may be a “compromise” in form due to competing needs adaptations for standing still adaptations for running fast  Understanding Natural Selection Limitations of Natural Selection – It can act only on existing variation extinction happens when adaptation is impossible! form is constrained by ancestry https://cisr.ucr.edu/chytrid_fungus.html Center for Invasive Species Research  Understanding Natural Selection – Chance, environment & natural selection interact history matters Be able to: Contrast our modern understanding of genetics with Darwin’s assumptions about inheritance of characteristics. Describe the types and causes of phenotypic & genetic variation within a species. Describe a population in terms of its genotypic and allelic frequencies. Find solutions to simple population genetics problems, correctly applying the Hardy-Weinberg equation as appropriate. Explain how the Hardy-Weinberg Principle relates to microevolution. Name and describe the mechanisms of microevolution and their expected outcomes. Describe and explain various forms of selection and the limitations of evolution by natural selection. 86