Population Genetics

Phenotypic variation Mendelian traits: some phenotypes (what you see/expressed) either on/off -> B or b, some are gradations (2+ genes) Genetic variation Sexual reproduction: recombination (doesn’t create new alleles) Mutations are source of new alleles; alterations in chromosomes; rapid asexual reproduction = lots of mutations Population and allele frequencies Allele: alternate versions of gene Fixed alleles: all individuals either WW or ww (no variation; only pass down W or w), no natural selection Gene pool: all alleles of all genes in population Allele frequency: proportion of specific allele (W/w) in population 0.5 population alleles W and 0.5 w, frequency of each is 0.5 -> f(W) = f(w) = .5 = 50% Could be 0.5 birds WW and 0.5 ww (or all Ww) and alleles are reshuffled Example: allele frequency over generations have both parents: Ww and children: WW, Ww, ww ( f(W) = .5 and f(w) = .5; 8 alleles total among children and have = freq. As parents) Hardy-Weinberg equilibrium (population not evolving, baseline to study evolutionary change) Each generation has = allele % and process of inheritance doesn’t lead to genetic change Hardy-Weinberg equation: frequency of dominant allele (p) and frequency of recessive allele (q) Probability of WW offspring: p^2 / Ww offspring: 2pq / ww offspring: q^2 2 formulas to calculate expected freq. Of phenotypes when known p^2 + 2pq + q^2 =1 (frequency of organisms) P + q = 1 (frequency of alleles) -> 100% of alleles Ex: freq. W = 0.7 and freq. W = 0.3; p + q = ? .7^2 = .49 (WW) and 2(.7)(.3) = .42 (Ww) and .3^2 = .09 (ww) Five conditions must be met to maintain HW equilibrium No mutations, random mating, no natural selection, very large population, no gene flow between population (if not true, allele frequencies are changing) Mechanisms of evolution: mutations Changes of seq. Of DNA heritable, source of gen. Variation New genes/alleles can arise Only mutations in cells producing gametes can be passed to offspring Silent: doesn’t alter function Non-random mating Random mating: mate choice has nothing to do with genotype Non-random: some choice/performance Consanguineous mating: more closely related to mate, > chance of sharing recessive alleles Natural selection Major mechanism of evolution Adaptation, differential survival (traits favored for survival) Acts on phenotype: change gene frequency and increase adaptation -> phen. Var. results from different alleles Can only be evolution if there is gen. Var. Genetic drift Random evolutionary changes in small breeding population Alleles lost, random events -> drive who survives/breeds 2 ways population can end up small Bottleneck effect: many individuals die, few survivors limits # alleles in next generation Founder effect: few individuals become isolated from larger population and establish own population Few colonists = limits # alleles in next generation (less variety) Gene flow Gene flow: movement of alleles between 2+ population (movement of fertile individuals--birds/gametes--pollen) Tends to decrease differences between populations over time No gene flow: isolated populations -> differences accumulate Inheritance doesn’t alter alleles Allele frequencies in gene pool: constant until altered by outside force Microevolution: shifts in gene freq. (alleles) Over generations, small gradual changes Macroevolution: larger-scale changes, results in changes large enough to place as different taxonomic groups