Topics How to track evolution – allele frequencies Hardy – Weinberg principle – applications Requirements for genetic equilibrium Types of natural selection Population genetic polymorphism

Population gene pool - consists of different combinations Ch. 19 - Evolutionary change in populations, pp. 402-416. 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)

Genotypic, phenotypic or allelic frequencies each sum to 1 Ch. 19 - Evolutionary change in populations, p. 402-404. 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

Allele frequencies change only when influenced by external factors Ch. 19 - Evolutionary change in populations, p. 403-405. 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

Hardy-Weinberg Principle Ch. 19 - Evolutionary change in populations, p. 403-404. Hardy-Weinberg Principle

Hardy-Weinberg Principle Ch. 19 - Evolutionary change in populations, p. 403-404. Hardy-Weinberg Principle

p2 + 2pq + q2 = 1 (sum of genotype frequencies) Ch. 19 - Evolutionary change in populations, pp. 403-404. p2 + 2pq + q2 = 1 (sum of genotype frequencies) When frequency of homozygous recessive genotype (q2) 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

Requirements for Hardy-Weinberg equilibrium Ch. 19 - Evolutionary change in populations, p. 405-415. 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 …

Selective/nonrandom mating - Ch. 19 - Evolutionary change in populations, p. 406-407. 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

2. Assortative mating: Selection for mating by phenotype Ch. 19 - Evolutionary change in populations, pp. 406. 2. Assortative mating: Selection for mating by phenotype Positive - Increases homozygosity Negative - Decreases homozygosity Affects the genotype frequency only at loci of genes that are involved in mate-choice – unlike in inbreeding

1. Changes in base pairs of genes Ch. 19 - Evolutionary change in populations, p. 406-407. 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 Cause small deviations in allele frequencies

Genetic drift – If population is small Ch. 19 - Evolutionary change in populations, pp. 407. 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

A major evolutionary force in bottlenecks - Ch. 19 - Evolutionary change in populations, pp. 407-408. 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 Ch. 19 - Evolutionary change in populations, p. 408. Gene flow – In Migrations Migration between populations - allele movement Usually increases genetic variability in recipient population Eventually reduce variation between populations - tends to counteract effects of natural selection and genetic drift 50 yrs

Natural selection - Operates on phenotype Ch. 19 - Evolutionary change in populations, pp. 409. 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

Categories depending on the fate of mean Ch. 19 - Evolutionary change in populations, pp. 409-411. Selection Natural Selection Ecological Selection Sexual Selection Artificial Selection Group selection - individual fitness depends on group structure or group behavior Categories depending on the fate of mean

Genetic Variation in Population Ch. 19 - Evolutionary change in populations, pp. 411-415. 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