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York University
BIOL 1500

Chapter 8 Biology Notes: Evolution and Natural Selection Population: group of organisms of the same species living in a particular geographic region Evolution: genetic change in the population Charles Darwin • grew up in the 19th century in a biblical world • influenced by many people • went on a round-the-world voyage 1. noticed the resemblance of island finch species to mainland finch species 2. resemblance of extinct species to living species in the same area • first published his ideas in 1858 • worldview after 1859: organisms change over time; some organisms have gone extinct; earth is more than 6,000 years old; geology of earth is not constant but always changing Evolution • evolution occurs when the allele frequencies in a population change; the altering of the population amongst organisms of different traits • even a 1% change over time will make up a larger proportion of the population after multiple generations • Causes of Evolution: Mutation, Genetic Drift, Migration, Natural Selection 1. Mutation: a direct change in the DNA of an individual which may change an allele's property; ultimate source of variation; causes can be due from complexities within cell division and the duplication of 3 billion base pairs, induced by environmental phenomena (mutagens), ultraviolet radiation; mutations are random because the environment does not control which genes are mutated; the change in the base-pair sequence of a person's DNA may cause the production of a gene product that has never existed before; mutations are fairly rare because our bodies are built to protect our sperm or egg-producing DNA with a variety of error correction mechanisms 2. Genetic Drift: random change in allele frequencies in a population; genetic drift's change in allele frequency does not affect the allele's influence on reproductive success (unlike natural selection); impact of genetic drift is much greater in small populations than large populations; fixation from genetic drift when an allele's frequency in a population reaches 100% and causes no more variability in the population for this gene; founder effect is when a small group of individuals leave a population and become founding members of a new isolated population, if the new population has different allele frequencies, there has been evolution because the new population will be dominated by the genetic features that happened to be in the founding population (Amish population and polydactyly); population bottlenecks may occur during a great death of large proportions, remaining members are essentially a random, small sample of the original population, new population will be dominated by genetic features present in the surviving members (no genetic variation in the cheetahs) 3. Migration (gene flow): movement of some individuals of a species from one population to another; movement from population to population within a species distinguishes migration from the founder effect in which individuals migrate to a new habitat previously unpopulated by that species; if migrating individuals can survive and reproduce in the new population and carry a different proportion of alleles than the individuals in their new home, then the recipient population experiences a change in allele frequencies and experiences evolution; because alleles are lost from the population that the migrants left, that population will experiences a change in its allele frequencies as well; influenced by the mobility of the organisms and other factors such as barriers and human activity 4. Natural Selection: three conditions are necessary and when these three are satisfied, evolution by natural selection is occurring 1. There must be variation for the particular trait within a population: not limited to physical features; can vary in physiological and biochemical ways too 2. Variation must be inheritable (it must be capable of being passed on from parents to their offspring): offspring must inherit the trait from their parents for natural selection to work; transmission of traits from parents to their children through genetic information inheritance or heritability 3. 3 Individuals with one version of the trait must produce more offspring than those with a different version of the trait; deduced from more organisms are born than can survive, organisms are continually struggling for existence, some organisms are more likely to win this struggle and survive and reproduce; differential reproductive success led Darwin to this third condition • natural selection can be turned on its head and viewed as the elimination of some heritable traits from a population • a mechanism of evolution that occurs when there is heritable variation for a trait and individuals with one version of the trait have greater reproductive success than do individuals with different versions of that treat; can be thought from the losing side as well Fitness: Measure of the relative amount of reproduction of an individual with a particular phenotype as compared with the reproductive output of individuals with alternative phenotypes; an organism's fitness depends on the environment in which it lives in; has everything to do with reproductive success 1. An individual's fitness is measured relative to other genotypes or phenotypes in the population 2. Fitness depends on the specific environment in which the organism lives 3. Fitness depends on an organism's reproductive success compared with other organisms in the population • organisms that possess traits that allow them to better exploit the environment in which they live will tend to produce more offspring than the organisms with alternative traits • a population will be made up of more fitter organisms as the generations pass • Adaptation: refers to both the process by which organisms become better matched to their environment & to specific features that make an organism more fit; bats have an extremely accura
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