5/18/2017. Genotypic, phenotypic or allelic frequencies each sum to 1. Changes in allele frequencies determine gene pool composition over generations

Transcription

1 Topics How to track evolution allele frequencies Hardy Weinberg principle applications Requirements for genetic equilibrium Types of natural selection Population genetic polymorphism in populations, pp Population gene pool - consists of different combinations of alleles genetic variation - the raw material for evolution Frequencies of genotype, phenotype and alleles in a population help understand evolution Changes in allele/genotype frequencies over generations in a population - microevolution (usually over few generations thus relatively minor changes) in populations, p Genotypic, phenotypic or allelic frequencies each sum to 1 Changes in allele frequencies determine gene pool composition over generations If genotypic or allelic frequencies do not change in a population over time - genetic equilibrium - no evolution in that locus 1

2 in populations, p The conventional view - dominant alleles would eventually come to dominate the gene pool, and the recessives disappear Allele frequencies change only when influenced by external factors Stability of populations over time is explained by the Hardy-Weinberg Principle in populations, p Hardy-Weinberg Principle Hardy-Weinberg Principle in populations, p

3 in populations, pp p 2 + 2pq + q 2 = 1 (sum of genotype frequencies) When frequency of homozygous recessive genotype (q 2 ) is known, we can calculate all the frequencies (genotype, phenotype and allele) using H-W equation Because, p + q = 1 (sum of allele frequencies) When genotype frequencies show stability based on H-W equation - genetic equilibrium - population is not evolving for that gene/allele in populations, p Requirements for Hardy-Weinberg equilibrium No selective mating - random mating No net mutations - no change in DNA No genetic drift - large population size No gene flow - no migration No natural selection - all phenotypes equally adaptive If these conditions are met - Genetic Equilibrium If not in populations, p Selective/nonrandom mating - 1. Mate with neighbors Inbreeding: Mating with genetically similar individuals (neighbors - self-fertilization is an extreme) - increases homozygosity - causes inbreeding depression (reduced fitness) in some populations 3

4 2. Assortative mating: for mating by phenotype Positive - Increases homozygosity Negative - Decreases homozygosity in populations, pp Affects the genotype frequency only at loci of genes that are involved in mate-choice unlike in inbreeding Mutations 1. Changes in base pairs of genes 2. Gene reposition/rearrangement 3. Change in chromosome structure Somatic cell vs. reproductive cell Introns vs Exons in populations, p Cause small deviations in allele frequencies in populations, pp Mutations do not decide direction of (or force) evolutionary change, and are not dominant evolutionary forces, but are the source of genetic variability for natural selection to work on Genetic drift If population is small Random events (chance) in small breeding populations can cause changes in allelic frequencies decreasing genetic diversity within population 4

5 in populations, pp A major evolutionary force in bottlenecks - reduction in population size - due to predation, disease, physical environmental changes etc. Founder effect - a small part of a large population breaks away and colonizes a new area - genetic drift is important Gene flow In Migrations Migration between populations - allele movement Usually increases genetic variability in recipient population in populations, p Eventually reduce variation between populations - tends to counteract effects of natural selection and genetic drift 50 yrs in populations, pp Natural selection - Operates on phenotype Changes allele frequencies toward increased adaptation - adaptive evolutionary change - therefore different from other four Many phenotypes are polygenic - variation within character - normal distribution 5

6 in populations, pp Natural Artificial Ecological Sexual Group selection individual fitness depends on group structure or group behavior Categories depending on the fate of mean in populations, pp Genetic Variation in Population Genetic polymorphism SNPs and CNVs Balanced polymorphism heterozygote advantage and frequency-dependent may help long term stability of certain alleles Neutral variation Clinal variation 6