Mendelian vs. Polygenic Traits Modern Synthesis A Current Definition of Evolution. Processes of evolution - Factors That Produce and Redistribute Variation Mutation Gene Flow Genetic Drift Founder Effect Recombination Natural selection provides directional change in allele frequency relative to specific environmental factors. Kinds of Selection: Directional Kinds of Selection: Stabilizing Kinds of Selection: Disruptive A case of biocultural evolution Sickle-cell Polymorphism Mitochondrial Inheritance All cells contain mitochondria that convert energy into a form that can be used by the cell. Each mitochondrion contains several copies of a ring-shaped DNA molecule, or chromosome. Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers. All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time. The Modern Synthesis Evolution is a two-stage process: The production and redistribution of variation (inherited differences between individuals). Natural selection acting on this variation (whereby inherited differences, or variation, among individuals differentially affect their ability to reproduce successfully). Sewall Wright Ronald Fisher J.B.S.Haldane Mitochondrial Inheritance All cells contain mitochondria that convert energy into a form that can be used by the cell. Each mitochondrion contains several copies of a ring-shaped DNA molecule, or chromosome. Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers. All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time. Genetic and Environmental Factors Phenotypes are not completely determined by genotypes. The genotype sets limits and potentials for development. It also interacts with the environment. Genetic-environment interaction influences phenotype expression. A Current Definition Of Evolution From a modern genetic perspective, we define evolution as a change in allele frequency from one generation to the next. Allele frequencies are indicators of the genetic makeup of an interbreeding group of individuals known as a population. Mutation Mutation is a molecular alteration in genetic material: For a mutation to have evolutionary significance it must occur in a gamete (sex cell). Such mutations will be carried on one of the individual's chromosomes. During meiosis the chromosome carrying the mutation will assort giving a 50% chance of passing the allele to an offspring. Processes of Evolution Gene Flow Gene flow is the exchange of genes between populations. If individuals move temporarily and mate in the new population (leaving a genetic contribution), they don?t necessarily remain in the population. Example: The offspring of U.S. soldiers and Vietnamese women represent gene flow, even though the fathers returned to their native population. Processes of Evolution Genetic Drift Genetic drift is directly related to population size. Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations. Drift may also occur solely because the population is small: Alleles with low frequencies may simply not be passed on to offspring, so they eventually disappear from the population. Processes of Evolution Founder Effect Genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations. A new population will be established, and as long as mates are chosen only within this population, all the members will be descended from the founders. An allele that was rare in the founders? parent population but is carried by even one of the founders can eventually become common. Processes of Evolution Recombination In sexually reproducing species both parents contribute genes to offspring. The genetic information is reshuffled every generation. Recombination doesn?t change allele frequencies, however, it does produce different combinations of genes that natural selection may be able to act on. Processes of Evolution Natural Selection Natural selection provides directional change in allele frequency relative to specific environmental factors. If the environment changes, selection pressures also change. If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation. Processes of Evolution Kinds of Selection: Directional Occurs when selection favors a single trait Kinds of Selection: Stabilizing Stabilizing selection favors those values of a particular trait that are intermediate between the extremes in a distribution curve. Kinds of Selection: Disruptive Disruptive selection favors both extreme values for a particular trait in a distribution curve. Natural Selection in Humans: Abnormal Hemoglobins and Resistance to Malaria -Sickle-cell Polymorphism Two Alleles involved: A = normal hemoglobin S = sickle trait Three possible genotypes: AA = homozygous dominant AS = heterozygous SS = homozygous recessive The Hemoglobin S gene causes sickle-cell anemia in humans. Individuals with the SS genotype suffer from sickle-cell anemia, an illness fatal without medical intervention. The Geography of Sickle-Cell Anemia and a Possible Association with Malaria Twenty to 30 percent of people living in equatorial Africa have the S gene. Frequencies overlap areas where malaria is endemic (constantly present). A relationship has been documented between possession of one S gene and higher survival when exposed to malaria. Sickle-cell Distribution in the Old World Malaria Distribution in the Old World The Biology of Sickle-Cell Anemia and Malarial Infection Heterozygotic individuals (those with the AS genotype) do not suffer from sickling crises that the SS genotype causes. They do possess somewhat lower oxygen levels in their hemoglobin, which is where the malaria parasite finishes its life cycle. The parasite generally cannot survive and reproduce. Sickle Cell Disease and Malaria Distribution of malaria parasite in the Old-World tropics. Frequency of HbS in human populations in Africa and Asia. History of Sickle-Cell Anemia and Malaria Sickle-cell tied to spread of Bantu people, who carried the S mutation into equatorial Africa. The Bantu introduced agriculture into the region; large, cleared areas were ideal environments for mosquitoes carrying malaria. Natural Selection and the Sickle Cell Trait Sickle-cell anemia is caused when children inherit two defective copies of a gene involved in making hemoglobin. But a single copy of this allele can give some protection against malaria. Natural selection finds a balance between the reproductive disadvantage of being born with two copies of the allele and the advantage of having one. Genetic disorders such as sickle-cell anemia are actually the agonizing byproduct of natural selection acting on our ancestors. Levels of Organization in the Evolutionary Process Mutation Mutation Evolutionary Factor A vehicle for packaging and transmitting DNA Chromosomes Storage of genetic information; ability to replicate; influences phenotype by production of proteins DNA Evolutionary Process Level Levels of Organization in the Evolutionary Process Changes in allele frequencies between generations Population Drift, gene flow Natural selection Recombination (sex cells only) Evolutionary Factor The unit that reproduces and which we observe for phenotypic traits Organism Basic unit of life, contains chromosomes, divides for growth and production of sex cells Cell Evolutionary Process Level Models of Evolutionary Change Divergent Differing Rates Convergent Parallel
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