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 = 0.16 + 0.48 + 0.36 = 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 - 1,000,000 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. 23-9 Original population Bottlenecking event Surviving population Population Bottleneck Fig. 23-10 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)