Presentation on theme: "Lecture 2: Evolution of Populations"— Presentation transcript:
1 Lecture 2: Evolution of Populations Campbell & Reece chapters:Chapter 23Microevolution – evolution at the population level = change in allele frequencies over generations
2 Genetics= science dealing with inheritance or heredity, the transmission of acquired traits
3 Ultimate source of heritable variation is change in DNA Change in DNA caused by: 1) Mutation 2) Genetic Recombination
4 Mutations = change in genotype other than by recombination. Three types:1) Point Mutations 2) Chromosome Mutations 3) Change in Chromosome Number
5 1) Point Mutation Change in a single DNA Nucleotide. Point mutation rate per gene = ~1 in 100,000 gametes. In humans:= 1 mutation/gene x (~25,000 genes) 100,000 gametes= ~0.25 point mutations/gamete
6 E.g., human hemoglobin: 1 in 2,000 people have mutant hemoglobin gene. 2 alpha chains (141 amino acids)2 beta chains (146 amino acids)1973 sampling of population (thousands): 169 mutation types recorded:62 substitutions in alpha99 substitutions in beta1 deletion in alpha7 deletions in beta1 in 2,000 people have mutant hemoglobin gene.hemoglobin
7 2) Chromosome Mutations Rearrangements (including losses and gains) of large pieces of DNA. E.g., inversion:A B C D E F G A B F E D C GRe-attaches here and here[3% of pop. of Edinburgh, Scotland have inversion in Chromosome #1][Humans differ from chimps by 6 inversions, from gorillas by 8 (also difference in chromosome number)]
8 3) Change in Chromosome No. a) Aneuploidy - change in chromosome number of less than an entire genome.Horse (2n = 64) versus donkey (2n = 62)Humans (2n = 46) versus chimp or gorilla (2n = 48)Some Genetic DiseasesTrisomy (addition of a chromosome to the original diploid pair) of chromosome 21 in humans = Down's syndrome.Extra or one sex chromosomes ( e. g., XYY, XXY, X).
9 b) PolyploidyEvolution of chromosome number which is a multiple of some ancestral set.Has been a major mechanism of evolution in plants.
12 Genetic Recombination (in sexual reproduction) = Natural, shuffling of existing genes, occurring with meiosis and sexual reproductionTwo types:Independent AssortmentCrossing over
13 Independent assortment Sorting of homologous chromosomes independently of one another during meiosisE. g., (where A,B,&C genes are unlinked)AaBBcc X AabbCC ---> AaBbCc (one of many possibilities)
14 Independent assortment Results in great variation of gametes, and therefore progeny. [E. g., one human: 223 = 8,388,608 possible types of gametes (each with different combination of alleles).]
15 Crossing overExchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase).Greatly increases variability of gametes and, therefore, of progeny.
16 Genetic VariationGenetic recombination - source of most variation (in sexual organisms), via new allele combinations.Mutation - ultimate source of variation, source of new alleles and genes.
17 Fitness= measure of the relative contribution of a given genotype to the next generationCan measure for individual or population.
18 Fitness= allele/genotype freq. in future generation allele/genotype freq. in prev. generationE. g., 1st gen. 25%AA : 50%Aa : 25%aa [freq. A = 25% + .5(50%) = 50%]2nd gen.: 36%AA : 48%Aa : 16%aa [freq. A = 36% + .5(48%) = 60%]Fitness of A allele is 60/50 = 1.2; a is 40/50 = 0.8Fitness of AA genotype is 36/25 = 1.44 , etc.
19 Hardy-Weinberg Equilibrium (1908) The frequency of a gene / allele does not change over time (given certain conditions). A,a = alleles of one gene, combine as AA, Aa, or aaGeneration 1: p = freq. A q = freq. a p + q = 1 (100%)pA qa pA p2AA pqAa qa pqAa q2aa}=gene frequencies in generation 1p2AA + 2pqAa + q2aa = 1
21 Hardy-Weinberg Equilibrium (1908) The frequency of a gene / allele does not change over time (given certain conditions). What will be the frequency of alleles in the second generation?p2AA + 2pqAa + q2aa = 1freq. A (generation 2) = (p2 + pq) / (p2 + 2pq + q2)= p(p + q) / (p + q)2 = p / (p + q) = pTherefore, freq. A = p; freq. a = q, same as in generation 1.}=gene frequencies in generation 1
22 Hardy-Weinberg Equilibrium Maintained only if:1) No mutation Mutations rare, but do occur (1 new mutation in 10, ,000,000 genes per individual per generation)
23 Hardy-Weinberg Equilibrium 2) No migration (no gene flow into or out of population) But, can occur . . .
24 Hardy-Weinberg Equilibrium 3) Population size largeTwo things can disrupt:a) Population bottleneck (large pop. gets very small)b) Founder effect (one or a few individuals dispersed from a large pop.)
25 Hardy-Weinberg Equilibrium 4) Mating is randomBut, most animals mate selectively, e.g.,1) harem breeding (e. g., elephant seals);2) assortative mating (like mates with like)3) sexual selection
26 Hardy-Weinberg Equilibrium 5) All genotypes equally adaptive (i.e., no selection)But, selection does occur . . .
27 If any conditions of Hardy-Weinberg not met: Genotype frequencies changeEvolution occurs!Evolution = change in gene frequency of a population over time.
28 Selective Pressure= agent or causative force that results in selection.E. g., for dark skin, selective pressure = UV radiation (UV increases sunburn and skin cancer in lighter skinned individuals)E. g., for light skin, selective pressure = Vitamin D synthesis
29 Genetic Drift = change in genotype solely by chance effects random! promoted by:Population Bottleneck -drastic reduction in population sizeFounder Effect - isolated colonies founded by small no. individuals
30 Population Bottleneck Fig. 23-9FigRangeof greaterprairiechickenPre-bottleneck(Illinois, 1820)Post-bottleneck(Illinois, 1993)(a)OriginalpopulationBottleneckingeventSurvivingFigure 23.9 The bottleneck effect
31 Summary: Evolution can occur by two major mechanisms: Natural Selection (non-random)Genetic Drift (random)
32 Pepper Moth: Biston betularia Selective pressure=predation by birds Single gene: AA/Aa = dark aa = lightCamoflague selected for!
33 Result: Balanced polymorphism E.g., Sickle Cell Anemia: Mutation = single amino acid subst. in beta chain of hemoglobin --> single a.a. difference.Sickle blood cellsNormal blood cells
34 Sickle Cell AnemiaHomozygotes for sickle mutation (HsHs): lethal
35 Sickle Cell AnemiaHeterozygotes (HsHn): resistant to malaria,selected for in malaria-infested regions,selected against where malaria not present.
36 General Principle: Selection dependent on the environment! If environmental conditions change, selective pressure can change!!
37 Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans, clutch size in birds)
38 Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects antibiotic resistance in bacteria)
39 Disruptive selection - selection for the two extremes in a population, against the average forms (e.g., limpets w/ 2 color forms: light & dark in mosaic environment; flies on two hosts: apple & hawthorn)
40 Sexual Selection- selection resulting in greater reproductive fitness in certain individuals of one sex
41 Sexual SelectionIntrasexual selection – within one sex; competition between members of one sex (usually males)
42 Sexual SelectionIntersexual selection – between two sexes; preference by one sex for features of the other sex. Usu. female choice.