Ch 17 Microevolution.docx

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University of Toronto Scarborough
Biological Sciences
Ivana Stehlik

Ch 17 Microevolution: Genetic changes within populations 17.1 Variation in Natural Populations (Pg 374)  Microevolution: heritable change in the genetics of a population o change in the genetic makeup of a population from generation to generation due to natural Selection o leads to variation in genetic makeup of population over time  Population: group of individuals belonging to the same species living at the same time and in the same area (individuals can interact)  a new era of biology began in 1859 when Charles Darwin published On the Origin of Species by Means of Natural Selection o focused biologists’ attention on great diversity of organisms o Darwin made two major points:  presented evidence that present-day species are descendants of ancestral species that were different o proposed a mechanism for evolutionary processes = Natural Selection  Differential Survival in Nature (= Natural Selection): o populations in nature do not grow exponentially indefinitely o reach a stable level (carrying capacity) determined by limiting resources o leads to struggle for existence o only a portion of offspring survive and reproduce o most offspring die (80% is typical in wild populations)  Variation: differences between individuals in a population  Heredity: - some variations are passed on to offspring o individuals with some variations survive better than others with none o traits that increase survival are passed on to offspring  Result: - differential ability to survive and reproduce will lead to gradual change in population – with accumulation of favourable traits selected over generations (by nature)  Favourable traits that survive lead to genetic variation among individuals in populations - contribute to evolution of species  common misconception about evolution is that individual organisms evolve during their lifetimes  Natural Selection acts on individuals, but populations evolve  Phenotypic variation: differences in appearance or function that are passed from generation to generation  evolutionary biologists describe and quantify phenotypic variation due to: o heritable variation in appearance and/or function  genetic cause - mutations, recombination through crossing over, independent assortment and random fertilization) o phenotypic plasticity (environmental cause): the ability of an organism to change its phenotype in response to changes in the environment  Quantitative variation: individuals differ in small, incremental ways. o Characteristics with a range of variation o Controlled by multiple genes   Qualitative variation: they exist in two or more discrete states and intermediate forms are often absent o Characteristics with distinct states o Polymorphism: distinct variants of character  The occurrence of something in different forms, in particular. o Describe by percentage or frequency of each  Causes of phenotypic variation o Genetic differences between individuals o Diff in env. Factors that individuals experience o Interaction between genetics and the environment  Genetic and phenotypic variations may not be perfectly correlated.  Diff genotypes can have the same phenotype  Same genotype can have diff phenotype. Eg: acidity of soil changes flower color  Only genetically based variation is subject to evolutionary change  Phenotype is product of interaction btween genotype and its env. o Test for env cause of variation: same genotype, change one env variable and measure effects. o Breeding experiments can demonstrate the genetic basis of phenotypic variation  Traits that vary quantitatively will respond to artificial selection only if variation has genetic basis. o Genetic causes of variation can also be identified by analyzing genealogical pedigrees.  Genealogical: Of or relating to the study or tracing of lines of family descent  What generates genetic variation o 2 potential sources  Production of new alleles  Arise from small scale mutations in DNA  Rearrangement of existing alleles  Arise from large scale changes in chromosome structure or number  several forms of genetic recombination o crossing over between homologous chromosomes during meiosis o independent assortment of nonhomologous chromosomes during meiosis o random fertilizations between genetically different sperm and eggs o More than 10 60combinations of alleles are possible in human gametes.  1960s, gel electrophoresis of proteins.  Identification of protein polymorphism allows researchers to infer genetic variation at the locus coding for that protein. o Problems: same size but diff amino acids – can’t distinguish. o Every locus exhibits some variability in its nucleotide sequence. 17.2 Population Genetics (Pg 377)  study of how populations change genetically over time  reconciled ideas of Darwin and Mendel  focuses on populations as units of evolution  for a population to evolve, its members must have heritable genetic variation (= raw material on which agents of evolution act)  Phenotype = the physical / physiological expressions of all of the genes of an individual organism  Genotype = genetic make-up of all of the genes of an individual organism  A population evolves when individuals with different genotypes survive or reproduce at different rates.  heritable variation determined in genotype  heritable variation studied by following changes in the phenotype of a population  features of a phenotype = characters (e.g. ear attachment) o encoded in genotype as genes  specific forms of a character = traits (e.g. attached / detached) o encoded by different versions of a gene = alleles  A single individual has only some of the alleles found in the population to which it belongs.  gene pool : sum of all alleles at all gene loci in all individuals o includes all of the alleles at all of the gene loci in all of the individuals in the population at any 1 time o contains all of the variation (= different alleles) that produces differing phenotypes on which the agents of evolution act   Genotype frequency: percentages of individuals possessing each genotype  Allele frequencies: measure genetic variation in the genotypes in a population by determining the relative proportions of all alleles in a population  mmeasurements of allele frequencies range from 0 to 1  sum of all allele frequencies at a given locus is 1 and sum of genotype frequency in a population is also equal to 1  an allele’s frequency (p) is calculated by dividing the number of copies of the allele in a population by the total number of alleles  if only two alleles (A and a) for a given locus - may form three different genotypes: AA, Aa, and aa  total number of alleles in a population is 2N because each individual is diploid (e.g. 3 genotypes - AA, Aa or aa)  p = the frequency of allele A  q = the frequency of allele a  allele frequency for each population: p + q = 1   Studying Population Genetics using Null Models  conceptual models that serve as theoretical reference points to observations made on populations  use Hardy-Weinburg Principle: a mathematical model that describes how genotype frequencies are established in sexually reproducing organisms. o It is a null model for evaluating the circumstances under which evolution may occur  describes conditions where a population of diploid organisms show genetic equilibrium : point at which neither allele frequencies nor genotype frequencies change in succeeding generations o if conditions met, microevolution does not occur o if observations do not match null, microevolution occurring  Their work showed that dominant alleles need not replace recessive ones and that the shuffling of genes in sexual reproduction does not in itself cause the gene pool to change.   population is in Hardy-Weinberg Equilibrium = no evolution   p + q = 1 where p and q remain unchanged  genotype frequencies remain in constant proportions 2 2  p + 2pq + q = 1 where p + q = 1  p and q represent frequencies of the homozygous genotypes  and 2pq represents the frequency of the heterozygous genotype  the Hardy-Weinberg Equilibrium Model describes a model situation in which allele frequencies do not change  if the followiconditions are met, genetic structure of a population does not change over time o no mutation (or mutations can be ignored) o no gene flow - no migration into or out of the population (population is closed) o population size is infinite - large populations are not affected by genetic drift o Mating is random o no selection - natural selection does not affect survival of any genotypes  - all genotypes in the population survive and reproduce equally well.  most important message of the Hardy–Weinberg Equilibrium is that allele frequencies remain unchanged from generation to generation unless some agent acts to change them  can determine whether an agent of evolution is acting on a population by comparing a population’s observed genotype frequencies with a population’s ‘expected’ genotype frequencies predicted from Hardy–Weinberg Equilibrium  populations in nature never fit conditions for Hardy-Weinberg Equilibrium Model  it is useful in predicting genotype frequencies from allele frequencies because Model describes conditions that would result if no evolution occurred  any patterns of deviation from the Model help identify specific mechanisms of evolution 17.3 The Agents of Microevolution (Pg 379)  Evolutionary Agents o mutation o gene flow o random genetic drift o non-random (assortative) mating o natural selection  cause changes in allele / genotype frequencies in a population  are observed as deviations from the expected frequencies predicted from Hardy–Weinberg Equilibrium Model 1)Mutation  major source of genetic variation  occurs at a low but variable rate o btw 1 gamete in 100,000 to 1 in 1M will include a new mutation at a particular gene locus o infrequent- exert little or no immediate effect on allele frequencies in most populations o but over evolutionary time scales, their numbers are significant o mutations accumulating in biological lineages for billions of years  heritable change in DNA. o Mutations in the germ line (cell lineage that produces gametes) are heritable o Others have no direct effect on next generation o In plants, mutations may occur in meristem cells which eventually produce flowers as well as non reproductive structures- so mutations may be passed to the next generation and ultimately influence the gene pool  Deleterious mutations: alter an individual’s structure, function or beh in harmful ways.  Lethal mutations: death of organisms carrying them o if death b4 reproduction, mutations eliminated from populations  Neutral mutations : neither harmful nor helpful o Cuz of diff codons for same amino acids, some substitutions wont affect proteins o Mutations at third position persist longer in population than those in first two positions. o May be beneficial later if env changes  Other mutations may change an organism’s phenotype without influencing its survival and reproduction  Advantageous mutation: confers some benefit on an individual that carries it. o Natural selection may preserve the new allele and even inc its frequency over time. o Once the mutation is passed to new generation, other agent of microevolution determine its long term fate.  an irreversible change in DNA or RNA (stable)  random  spontaneous  types: gene (point) or chromosomal  significant over long time scales 2) Gene flow  Organisms or gametes sometimes move from one population to another.  If reproduce, they introduce novel(new) alleles into the population: Gene flow o Violates hardy Weinberg requirement that populations must be closed to migration  may add new alleles to the gene pool of the new population  may change the frequencies of alleles already present  Dispersal agents: most population: pollen-carrying wind or seed carrying animals o Enhance gene flow  Hard to document; researchers can use phenotypic or genetic marks but must also show they reproduced and contributed to the gene pool  Evolutionary importance depends on the degree of genetic differentiation btw population and rate of gene flow o If two gene pools are very different, little gene flow may inc genetic variability within population that receives immigrants and it will make the two populations more similar. o If two populations already similar, lot of gene flow will have little effect  life history and behaviour may enhance gene flow Genetic Drift  def: Ch
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