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Biological Sciences
Mark Fitzpatrick

BIOA01H3 – Lecture 25 th Chapter 16 Microevolution: is a heritable change in the genetic makeup of a population. Population: includes all individuals of a single species that live together in the same place and time. 16.1 Variation in Natural Populations Phenotypic variation: differences in appearance or function that – if based on heritable information – are passed from generation to generation. 16.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation  Microevolutionary studies often begin by assessing phenotypic variation within populations  Most characters exhibit quantitative variation  individuals differ in small, incremental ways  Other characters exhibit qualitative variation  they exist in two or more discrete states, and intermediate forms are often absent Polymorphism: existence of discrete variants of a character. 16.1b Phenotypic Variation Can Have Genetic and Environmental Causes  Phenotypic variation within populations may be caused by genetic differences between individuals, by differences in the environmental factors, and a combination of both  Therefore genetic and phenotypic variations may not be perfectly correlated 16.1c Several Processes Generate Genetic Variation Genetic variation has two potential sources: 1. The production of new alleles 2. The rearrangement of existing alleles  Most new alleles arises from small-scale mutations in DNA  Rearrangement of existing alleles into new combos result from larger-scale changes in chromosome structure or number and from several forms of genetic recombination 16.1d Populations Often Contain Substantial Genetic Variation  Identify biochemical polymorphisms in diverse organisms using gel electrophoresis 1  Technique separates two or more forms of a given protein if differ significantly in shape, mass, or net electrical charge  Identification of protein polymorphism allows researchers to infer genetic variation at locus coding for that protein 16.2a All Populations Have a Genetic Structure  Populations are made up of individuals of the same species, each w/ its own genotype  In diploid organisms an individual’s genotype includes two alleles at every gene locus  Sum of all alleles at all gene loci in all individuals called population’s gene pool  To describe structure of gene pool, scientists first identify genotypes in representative sample and calculate genotype frequencies, the percentage of individuals possessing each genotype  Knowing that each diploid organism has two alleles (either same alleles or diff alleles) at each gene locus, scientist can then calculate allele frequencies, the relative abundances of the different alleles  For locus w/ two alleles, use symbol p to identify frequency of one allele, and q to identify frequency of other allele 16.2b The Hardy-Weinberg Principle Is a Null Model That Defines How Evolution Does Not Occur Hardy-Weinberg Principle: specifies the conditions under which a population of diploid organisms achieves genetic equilibrium  the point at which neither allele frequencies nor genotype frequencies change in succeeding generations.  Also showed that dominant alleles needn’t replace recessive ones, and shuffling of genes in sexual reproduction doesn’t itself cause gene pool to change According to this model, genetic equilibrium only possible if all following conditions met: 1. No mutations are occurring. 2. The population is closed to migration from other populations. 3. The population is infinite in size. 4. All genotypes in the populations survive and reproduce equally well. 5. Individuals in the population mate randomly with respect to genotypes.  If these conditions are met, allele frequencies of population for identified gene locus will never change, and genotype frequencies will stop changing after one generation  microevolution will not occur 2 16.3a Mutations Create New Genetic Variations Mutations: heritable changes in DNA; may be neutral, deleterious, or beneficial.  Deleterious mutations example; several simple mutations in humans cause forms of Ehlers-Danlos syndrome, a disruption of collagen synthesis that may result in loose skin; weak joints; or sudden death from the rupture of major blood vessels, the colon, or the uterus. 16.3b Gene Flow Introduces Novel Genetic Variants into Populations Gene flow: immigration of organisms/gametes that reproduce and introduce novel alleles into populations they’ve joined. 16.3c Genetic Drift Reduces Genetic Variability within Populations Genetic drift: chances events causing allele frequencies in a population to change unpredictably.  Particularly common in small populations b/c only few individuals contribute to gene pool and b/c any given allele present in very few individuals  Leads to loss of alleles and reduced genetic variability Two general circumstances: 1. Population Bottlenecks 2. Founder Effect Population Bottlenecks  Stressful factor i.e. droughts/disease/starvation kills individuals & eliminates some alleles from population, producing population bottleneck Founder Effect  When few individuals colonize distant locality and start new population, carry only small sample of parent population’s genetic variation  By chance, some alleles may be totally missing from new population, whereas alleles that were rare back home, might occur at relatively high frequencies now  Change in gene pool called founder effect Conservation Implications  Genetic drift has improtatn implications for conservation of biology  By definition, endangered species experience severe population bottlenecks, resulting in loss of genetic variability 3  Small number of individuals available for captive breeding programs may not fully represent species’ genetic diversity; w/o variation, population more subjective to diseases & environmental changes no
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