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BIOL 1001 (162)
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York University
BIOL 1001
Tamara Kelly

Chapter 17: Microevolution Genetic changes within populations 17.1 Variation in natural Populations Most individuals of a species look similar to each other however none are identical Ie, differences in mass, length, physiology, internal anatomy these are examples of phenotypic variation differences in appearance or function that are passed from generation to generation 17.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation Micro evolutionary studies often begin with assessing phenotypic variation within populations Most characters exhibit quantitative variation individuals differ in small incremental ways o Displayed in a bar graph (if large enough a curve) o The width of the curve is proportional to the amount of variation o The mean describes the average value of the character or its variability within the pop. Other characters exhibit qualitative variation existing in two or more discrete states where intermediate forms are often absent o Example: snow geese are either blue or white o Polymorphism the existence of discrete variants of a character (ie. Blood type) o Phenotypic polymorphisms are described quantitatively by calculating the percentage(frequency) of each trait 17.1 b Phenotypic variation can have genetic and environmental causes phenotypic variation within populations may be causes by o genetic differences between individuals o differences in the environmental factors the individual experiences o interaction between genetics and the environment result is genetic and phenotypic variances may not be perfectly correlated different genotypes can exhibit the same phenotype same genotypes can exhibit diff. phenotypes (ie. flower colour determined by soil) knowing why phenotypic variation is caused is important because o only genetically based variation is subject to evolutionary change how can we determine this? o Test for environmental cause experimentally Traits that vary quantitatively will respond to artificial selection (breeding) only if the variation has some genetic basis 17.1 c Several Processes Generate Genetic Variation Genetic variation has two possible sources o Production of new alleles Most arise from small scale mutation in DNA o Rearrangement of existing alleles Result from larger scale changes in chromosome structure/number and several forms of genetic recombination (ie. Crossing over, independent assortment of non-homologous chromosomes and random fertilization) 17.1 d Populations Often contain Substantial Genetic Variation identification of biochemical polymorphisms in diverse orgamisms o separates two or more forms of a given proteins if they differ significantly in shape, mass, electrical charge o allows variation at the locus coding for that protein to be confirmed with new technology we can see that every locus exhibits some variability in its nucleotide sequence 17.2 Population Genetics predicting how certain factors influence genetic variation o Describe genetic structure of population o Hypothesis in form of mathematical equations o Test predictions using models 17.2a All Populations Have a Genetic Structure Populations made up of individuals o Each with their own genotype Diploid organisms (pairs of homologous chromosomes) Genotype includes 2 alleles at every gene locus Sum of all alleles in all gene loci of all individuals is called populations gene pool Describing the structure of a gene pool o Identify genotypes in a representative sample o Calculate genotype frequencies (% of individual that has each genotype) o In diploid organisms you can calculate the allele frequencies and the relative abundances of each allele Allele frequencies represent the commonness or rarity of each allele in the gene pool For gene locus with two alleles there are o 3 genotype frequencies and only 2 allele frequencieso the sum of like frequencies must equal 1 17.2 The Hardy Weinberg equation The Hardy Weinberg equation is a null model that defines how evolution does not occur In observational data their in often no suitable control . Null models are created to predict what you would see if a factor has not effect Hardy Weinberg specifies conditions under which a population of diploid organisms achieves genetic equilibrium o Point at which neither allele nor genotype frequencies change in succeeding generations, and dominant alleles do not replace recessive Genetic equilibrium is possible if o No mutations occur o Pop. Is closed to migration from other pops. o Pop. Is infinite in size o All genotypes in pop. Survive and reproduce equally o Mating is random if these conditions are met microevolution will not occur by determining which conditions are not met you can understand why and how the gene pool is changing 17.3a Mutations Create New Genetic Variations mutation- hereditable change in DNA
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