Presentation on theme: "Lecture 2: Evolution of Populations Campbell & Reece chapters: Chapter 23 Microevolution – evolution at the population level = change in allele frequencies."— Presentation transcript:
Lecture 2: Evolution of Populations Campbell & Reece chapters: Chapter 23 Microevolution – evolution at the population level = change in allele frequencies over generations
Genetics = science dealing with inheritance or heredity, the transmission of acquired traits
Ultimate source of heritable variation is change in DNA Change in DNA caused by: 1) Mutation 2) Genetic Recombination
Mutations = change in genotype other than by recombination. Three types: 1) Point Mutations 2) Chromosome Mutations 3) Change in Chromosome Number
1) Point Mutation Change in a single DNA Nucleotide. 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
E.g., human hemoglobin: 2 alpha chains (141 amino acids) 2 beta chains (146 amino acids) 1973 sampling of population (thousands): 169 mutation types recorded: 62 substitutions in alpha 99 substitutions in beta 1 deletion in alpha 7 deletions in beta 1 in 2,000 people have mutant hemoglobin gene. hemoglobin
2) Chromosome Mutations Rearrangements (including losses and gains) of large pieces of DNA. E.g., inversion: Re-attaches here and here A B C D E F G A B F E D C G [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)]
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 Diseases Trisomy (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).
b) Polyploidy Evolution of chromosome number which is a multiple of some ancestral set. Has been a major mechanism of evolution in plants.
Two ways polyploidy can occur:
Polyploid evolution of wheat
Genetic Recombination (in sexual reproduction) = Natural, shuffling of existing genes, occurring with meiosis and sexual reproduction Two types: –Independent Assortment –Crossing over
Independent assortment Sorting of homologous chromosomes independently of one another during meiosis E. g., (where A,B,&C genes are unlinked) AaBBcc X AabbCC ---> AaBbCc (one of many possibilities)
Results in great variation of gametes, and therefore progeny. [E. g., one human: 2 23 = 8,388,608 possible types of gametes (each with different combination of alleles).] Independent assortment
Crossing over Exchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase). Greatly increases variability of gametes and, therefore, of progeny.
Genetic Variation Genetic recombination - source of most variation (in sexual organisms), via new allele combinations. Mutation - ultimate source of variation, source of new alleles and genes.
Fitness = measure of the relative contribution of a given genotype to the next generation Can measure for individual or population.
Fitness = allele/genotype freq. in future generation allele/genotype freq. in prev. generation E. 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 = 0.8 Fitness of AA genotype is 36/25 = 1.44, etc.
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 aa Generation 1: p = freq. A q = freq. a p + q = 1 (100%) pAqa pAp 2 AApqAa qapqAaq 2 aa } =gene frequencies in generation 1 p 2 AA + 2pqAa + q 2 aa = 1
Hardy-Weinberg Equilibrium (1908) Example: Generation 1: p = 0.4 q = 0.6 p + q = 1 (100%) 0.4A0.6a 0.4A0.16AA0.24Aa 0.6a0.24Aa0.36aa } =gene frequencies in generation 1 p 2 AA + 2pqAa + q 2 aa = = 1
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? p 2 AA + 2pqAa + q 2 aa = 1 freq. A (generation 2) = (p 2 + pq) / (p 2 + 2pq + q 2 ) = p(p + q) / (p + q) 2 = p / (p + q) = p Therefore, freq. A = p; freq. a = q, same as in generation 1. } =gene frequencies in generation 1
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)
2) No migration (no gene flow into or out of population) But, can occur... Hardy-Weinberg Equilibrium
3) Population size large Two things can disrupt: –a) Population bottleneck (large pop. gets very small) –b) Founder effect (one or a few individuals dispersed from a large pop.) Hardy-Weinberg Equilibrium
4) Mating is random But, most animals mate selectively, e.g., –1) harem breeding (e. g., elephant seals); –2) assortative mating (like mates with like) –3) sexual selection Hardy-Weinberg Equilibrium
5) All genotypes equally adaptive (i.e., no selection) But, selection does occur... Hardy-Weinberg Equilibrium
If any conditions of Hardy-Weinberg not met: Genotype frequencies change Evolution occurs! Evolution = change in gene frequency of a population over time.
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
Genetic Drift = change in genotype solely by chance effects random! promoted by: Population Bottleneck -drastic reduction in population size Founder Effect - isolated colonies founded by small no. individuals
Fig Original population Bottlenecking event Surviving population Population Bottleneck Fig Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a)
Summary: Evolution can occur by two major mechanisms: Natural Selection (non-random) Genetic Drift (random)
Pepper Moth: Biston betularia Selective pressure=predation by birds Single gene: AA/Aa = dark aa = light Camoflague selected for!
Result: Balanced polymorphism E.g., Sickle Cell Anemia: Mutation = single amino acid subst. in beta chain of hemoglobin --> single a.a. difference. Sickle blood cells Normal blood cells
Homozygotes for sickle mutation (HsHs): lethal Sickle Cell Anemia
Heterozygotes (HsHn): resistant to malaria, selected for in malaria- infested regions, selected against where malaria not present.
General Principle: Selection dependent on the environment! If environmental conditions change, selective pressure can change!!
Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans, clutch size in birds)
Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects antibiotic resistance in bacteria)
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)
Sexual Selection - selection resulting in greater reproductive fitness in certain individuals of one sex
Sexual Selection Intrasexual selection – within one sex; competition between members of one sex (usually males)
Sexual Selection Intersexual selection – between two sexes; preference by one sex for features of the other sex. Usu. female choice.
Balance between survivorship (decreased) reproductive potential (increased)