BIO1M03 - Week 3 Lecture Notes

Lecture 1 Content:

Evolutionary Processes:

  • Change in allele frequencies in population over generations

    1. Natural Selection: increase frequency of alleles that contribute to reproductive success in particular environment

    2. Genetic Drift: allele frequencies change randomly

    3. Gene Flow: individuals leave one population, join another & breed

    4. Mutation: modify allele frequencies - introducing new alleles


HARDY-WEINBERG

  • Hardy-Weinberg Principle: Mathematical null-hypothesis for study of evolutionary processes

    • Gene Pool: All alleles from all gametes in each generation in single group

    • Calculated what would happen if pairs of gametes picked randomly many times & produced offspring

    • Predicts genotypes of offspring that population wuld produce (frequency of genotype)

  • 2 Alleles, A1 = p, A2 = q

    • p + q = 1

  • 3 Genotypes: A1A1, A1A2, A2A2:

    • A1A1 = p²

    • A1A2 = 2pq

    • A2A2 = q²

  • Hardy Weinberg Equation: p² + 2pq + q²

    • Allele frequencies should NOT change w/ meiosis & random combination of gametes

    • Therefore evolution has NOT occured

IMPORTANT ASSUMPTIONS:

  1. No natural selection at gene (all members contribute equal numbers of gamete to gene pool)

  2. No genetic drift (random allele frequency changes)

    • Assumes population is larged

  3. No gene flow (no new alleles added/lost through emigration)

  4. No mutaiton (no new alleles introduced)

  5. Random mating w.r.t (no sexual selection)

CONCLUSIONS OF HW

  1. Allele frequencies in population will not change throughout generations

  2. Allele frequencies in population given by p & q, can calculate genotype frequencies w/ p², 2pq, and q²

Null Hypothesis of Evolution

  • Natural selection, genetic drift, gene flow, mutation changes allele frequencies over time

  • Null hypothesis: Specifies what should be seen if hypothesis being tested is incorrect


Case Study 1: Are blood-type alleles in HWE

  • M & N alleles

    • MM, MN, NN

  1. Estimate observed genotype frequencies: total # of individuals w/ each genptyle/total # of individuals

  2. Calculate observed allele frequencies:

    • Frequency of M allele = freq of MM + ½ MN

  3. Calculate genotype expected: HWE

  4. Statistically compare observed vs expected values

  • Results: Observed = expected

    • genotypes are in HWE

    • not beign affected by any four evolutionary processing

    • mating was random w.r.t gene

    • Null hypothesis not rejected

Natural Selection:

  • Individuals w/ certain phenotypes produce more surving offspring than individuals w/ others

  • Certain alleles associated w/ favoured phenotypes

    • increase in frequency, others decrease

  • Result = evolution (HWE rejected)

  • Genetic Variation: # & relative frequency of alleles present in population

    • maintaining genetic variation:

      • Selection only occurs if heritable variation exists

      • lack of variation = population less able to respond successfully to changes in environment & average fitness will decline

      • environmental change is severe —> population may become extinct

  1. Directional Selection: changes average phenotypes in population in one direction

    • Reducing genetic diversity of populations

    • Fixed: favoured allele frequency approaches 1

    • Lost: allele frequencies reach 0.0

    • Purifying Selection: disadvantageous alleles decline

  2. Stabilizing Selection: Reduces genetic variation in trait

    • Reduces both extremes in population

    • no change in average value of trait over time

  3. Disruptive Selection

    • Intermediate phenotypes selected against

    • extreme phenotypes favoured

    • opposite of stabilizing selection

    • increases variation in a trait

    • Can cause speciation*

    • Speciation: formation of new species

      • If individuals of one extreme mate w/ individuals of the same extreme —> two distinct populations, new species

  4. Balancing Selection: no single allele has distinct advantage

    • balance among several alleles in terms of fitness & frequency

    • Heterozygote Advantage: heterozygous individuals higher fitness

      • Selection on different alleles balanced

      • genetic variation maintained in population

    • certain alleles favoured at different times/different places

      • Allele frequencies change locally, overall genetic variation in population maintained

    • Frequency-dependent Selection: Certain alleles favoured when rare, but not when common

Lecture 2 Content:

Genetic Drift:

  • Genetic Drift: Change in allele frequency in population due to chance (Sampling Error)

    • Allele Frequencies drift randomly over time

    • prevalent in small populations

    • random w.r.t fitness (changes not adaptive)

    • Can lead to random loss/fixation of alleles —> decrease in genetic variation in population

    • increases genetic differences BETWEEN populations

  • Genetic Markers: Specific alleles that cause distinctive phenotype

Causes of Genetic Drift in Natural Populations:

  • caused by any event/process involving sampling error

    1. Founder Effects: change in allele freq.

      • Small group of individuals establish new population in new area

      • allele freq differ from souce population if new population small enough

      • Eg. Iguanas of Anguilla

    2. Bottlenecks:

      • Population bottleneck: sudden decrease in population size in large population

        • Leads to genetic bottlenecks - sudden reduction in number of alleles in population

Gene Flow

  • Gene Flow: Movement of alleles between populations

    • Leave one population, join another & breed

    • equalizes allele frequencies between source & recipient populations

      • Homogenizes alelle frequencies among populations

  • Habitat fragmentation: isolating events between wild populations

  • Effects of accidental/purposeful gene flow between captive-bred populations & wild populations:

    • DNA tests to study fitness:

      • 2 wild parents

      • 1 wild 1 captive bred

      • 2 captive bred parents

    • Results:

      • 1 captive bred —> 16% lower fitness

      • 2 captive bred —> 38% lower fitness

      • Reduces fitness of wild population

        • Efforts to augment wild populations w/ captive bred individuals may lead to decreases in population sizes

  • Does not always reduce fitness

    • Can replenish alleles in population that lost alleles due to genetic drift

    • increases genetic diversity

  • Increase/decrease fitness depending on situation

    • Movement of alleles between populations always reduces genetic differences

Lecture 3 Content:

Mutation:

  • Restores genetic diversity

    • Creates new alleles

  • Random w.r.t fitness of individuals

  • Most organisms well adapted to habitat, random changes in genes may result in products that don’t work as well

    • Deleterious alleles: alleles that lower fitness

    • Purifying Selection: how deleterious alleles are eliminated

  • Produces beneficial alleles (that increase fitness) on rare occasions

    • Would increase in fitness in population -- natural selection

  • Can produce natural allele w/ no effect on fitness

    • Does not change amino acid sequence

  • not often enough to make important factor in changing allele frequencies

  • more significant in species w/ short generation times

  • Slowest evolutionary process

  1. Point Mutations: Change in single base pair in DNA

    • Different amino acid in polypeptide

    • change regulation of expression of other genes

  2. Chromosome-level mutations

    • change in number/composition of chromosomes

    • gene duplication: diversify w/ subsequent point mutations

      • lose function, or create new alleles

  3. Lateral gene transfer (horizontal gene transfer)

    • transfer of genes from one species to another

    • more important source of heritable genetic variation

  • Experiment w/ Ecoli

    • Relative fitness increased dramatically over time

    • relative fitness of populations increased in jumps

      • Novel mutations arising & conferring in fitness benefit under selection

    • Beneficial mutation occured, fitness stabilized until another beneficial mutation occured


  1. Mutation is ultimate source of genetic variation

  2. W/o mutations, evolution would stop by maintaining genetic variation

  3. Mutation alone inconsequential in changing allele freq

Summary of Evolutionary mechanisms

  • All result in biological diversity

  • All violate HWE

  • All have diff consequences on allele freq

  1. Genetic drift:

    • Random changes in allele freq

    • important in small pop

    • reduces genetic diversity

    • Founder effects, population bottlenecks

  2. Gene flow:

    • equalize allele freq among populations

    • introduce new alleles from one population to another

    • beneficial, neutral, or deleterious

  3. Mutation:

    • creates new alleles

    • beneficial, neutral, deleterious

    • loss-of-function allele can be adaptive

    • chance

    • infrequent, important with natural selection, genetic drift and gene flow

Nonrandom Mating

  • Mating not random w.r.t particular gene:

    • Violates HWE

  1. Inbreeding

    • relateives likely share alleles

    • self-fertilization

    • homozygous parents —> homozycous offspring

    • heterozygous self fertilize —> homozygous and heterozygous (1:2:1 ratio)

    • Increases homozygosity & decreases Heterozygosity

    • Does not cause evolution

    • allele frequencies do not change in population as a whole

    • changes genotype frequencies

    • Speed rate of evolutionary change

      • Increases rate at which recessive deleterious alleles are eliminated

    • Results in lower fitness

    • Inbreeding Depression: Decline in average fitness, takes place when homozygosity increases & heterozygosity decreases

      1. Many recessive alleles —> loss-of-function mutations

        • Normally homozygote recessive individuals are rare in population

          • Loss-of-function alleles exist in heterozygotes (little/no effects)

          • increases frequency of homozygous recessive —> higher freq of individuals expressing mutation

      2. Many genes under intense selection for heterozygote advantage

        • If homozygous, fitness declines

  2. Assortative Mating

    • Positive assortment: individuals choose mates that share a particular phenotypic trait

    • Negative assortment: individuals choose mates that differ in specific phenotypic trait

  3. Sexual Selection

    • Individuals w/in population differ in ability to attract mates

    • favours individuals w/ heritable traits that enhance ability to obtain mates

    • leads to changes in allele freq, increases fitness

    • Intersexual Selection: Mate choise

    • Intrasexual Selection: Compete to obtain mates

Theroy: Fundamental Asymmetry of Sex

  • Why extravagant traits in males, not females

  • Pattern: traits attract opposite sex more elaborate in males

  • Process: eggs more energetically costly than sperm

  • Females invest more into their offspring than males

    1. Female fitness limited by ability to gain resources needed to produce more eggs & healthier offspring

      • Choosy about mates since invest more in each egg

    2. Male fitness limited by number of females they can mate w/, can make frequently

      • Males compete w/ each other for mates

      • alleles that increase attractiveness increases in population

      • sexual selection acts more strongly on men

  • Good Alleles:

    1. Colourful feathers (carotenoids)

    2. Carotenoids —> healthy

    3. eat more plants to obtain more —> able to source more food/resources