Presentation on theme: "Lecture 2: Evolution of Populations"— Presentation transcript:
1Lecture 2: Evolution of Populations Campbell & Reece chapters:Chapter 23Microevolution – evolution at the population level = change in allele frequencies over generations
2Genetics= science dealing with inheritance or heredity, the transmission of acquired traits
3Ultimate source of heritable variation is change in DNA Change in DNA caused by: 1) Mutation 2) Genetic Recombination
4Mutations = change in genotype other than by recombination. Three types:1) Point Mutations 2) Chromosome Mutations 3) Change in Chromosome Number
51) 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
6E.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
72) 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)]
83) 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).
9b) PolyploidyEvolution of chromosome number which is a multiple of some ancestral set.Has been a major mechanism of evolution in plants.
12Genetic Recombination (in sexual reproduction) = Natural, shuffling of existing genes, occurring with meiosis and sexual reproductionTwo types:Independent AssortmentCrossing over
13Independent 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)
14Independent 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).]
15Crossing overExchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase).Greatly increases variability of gametes and, therefore, of progeny.
16Genetic VariationGenetic recombination - source of most variation (in sexual organisms), via new allele combinations.Mutation - ultimate source of variation, source of new alleles and genes.
17Fitness= measure of the relative contribution of a given genotype to the next generationCan measure for individual or population.
18Fitness= 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.
19Hardy-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
21Hardy-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
22Hardy-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)
23Hardy-Weinberg Equilibrium 2) No migration (no gene flow into or out of population) But, can occur . . .
24Hardy-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.)
25Hardy-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
26Hardy-Weinberg Equilibrium 5) All genotypes equally adaptive (i.e., no selection)But, selection does occur . . .
27If any conditions of Hardy-Weinberg not met: Genotype frequencies changeEvolution occurs!Evolution = change in gene frequency of a population over time.
28Selective 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
29Genetic 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
31Summary: Evolution can occur by two major mechanisms: Natural Selection (non-random)Genetic Drift (random)
32Pepper Moth: Biston betularia Selective pressure=predation by birds Single gene: AA/Aa = dark aa = lightCamoflague selected for!
33Result: 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
34Sickle Cell AnemiaHomozygotes for sickle mutation (HsHs): lethal
35Sickle Cell AnemiaHeterozygotes (HsHn): resistant to malaria,selected for in malaria-infested regions,selected against where malaria not present.
36General Principle: Selection dependent on the environment! If environmental conditions change, selective pressure can change!!
37Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans, clutch size in birds)
38Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects antibiotic resistance in bacteria)
39Disruptive 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)
40Sexual Selection- selection resulting in greater reproductive fitness in certain individuals of one sex
41Sexual SelectionIntrasexual selection – within one sex; competition between members of one sex (usually males)
42Sexual SelectionIntersexual selection – between two sexes; preference by one sex for features of the other sex. Usu. female choice.