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Evolution and Natural Selection Natural Selection in Populations Population – group of organisms of the same species living together in a given region Natural Selection – process whereby organisms with favorable variations survive and produce more offspring than less well-adapted organisms • • Proposed by Darwin Key mechanism of evolution Scientists who came before Darwin Lamarck’s theory of evolution • Inheritance of acquired characteristics – the characteristics an organism develops over time can be passed on to offspring – Mouse gets its tail cut off, offspring should have no tails – Giraffe stretches its neck out to reach tall leaves, offspring should have longer necks • Incorrect, not supported by scientific evidence • Stimulated thought about evolution Natural Selection in Populations No individual is capable of evolving • Natural selection produces changes in populations, not individual organisms • Organism may be born with a favorable mutation – Example: dark-fur for pocket mouse living on lava rock • Acquired traits are not passed on to offspring Natural Selection in Moth Population • Random mutations - color variations in moth population – Allows the moths to blend into tree trunks – Harder for birds to locate “camouflaged” moths • Natural selection occurs – Moths with mutation survive and reproduce – Favorable coloring is passed on to offspring Image by Gilles San Martin (Own work) [CC-BY-SA-3.0] Natural Selection Natural selection leads to development of • Adaptations • Genetic diversity What are Adaptations? Adaptation – trait that helps an organism to survive and/or reproduce in its unique environment • Natural selection favors variations of traits that increase organism’s ability to survive and reproduce Images by L. Shyamal (Own work) [CC-BY-SA-2.5] Development of Adaptations Variations in traits occur naturally in a population Individuals with favorable variations produce more offspring • Example: – Darwin’s Finches – Rock Pocket mouse Frequency of favorable variation increases in population Variation becomes an adaptation of the population over time What is Genetic Diversity? • Genetic diversity – differences in genetic characteristics among organisms within the same species or among different species • Natural selection can increase or decrease genetic diversity Image by Graphodatsky et al. [CC-BY-2.0] Monday in Biology • Rock Pocket Mouse QUIZ – You will have 15 minutes to complete the quiz over the Rock Pocket mouse – Two grades One lab and one quiz grade (both major grades) • Take notes over Evolution • Vimeo Video: Selection, Gene Flow, Genetic Drift – Handouts and questions Types of Natural Selection and other mechanisms of evolution Genetic Diversity Within a Species 1. Directional 2. Diversifying 3. Stabilizing Frequency • Three major types of natural selection can affect the diversity within a species Frequency Frequency Frequency Phenotypes Phenotypes Phenotypes Phenotypes Directional Diversifying Stabilizing Directional Selection • Directional selection – a single variation of a trait that was not previously favored is now favored in a species Frequency – Result of migration or environmental changes Frequency Phenotypes Phenotypes • May increase or decrease diversity within a species • Diversifying selection – multiple variations of a trait are favored in a single species Frequency Diversifying Selection (Disruptive) Phenotypes Frequency – Increases diversity Phenotypes – Decreases diversity Phenotypes Frequency • Stabilizing selection – a single variation of a trait is favored in a species Frequency Stabilizing Selection Phenotypes Mechanisms of Evolution 1. 2. 3. 4. Natural Selection “survival of the fittest” Mutations create new alleles Migration moves alleles into and out of a population Genetic drift happens when random events change the allele frequencies of small populations 5. Sexual Selection non-random mating • Giraffe video • https://vimeo.com/19797501 Tuesday/Thursday Species • A group of organisms that can interbreed in nature and produce fertile offspring Speciation How Do New Species Form? • Must be isolated from other populations for a long period of time. • Different selection pressures cause different traits to be selected for or against. Geographic Isolation • Species are separated from each other by a physical barrier Geographic Isolation • The Kaibab and Albert squirrels are separated by the Grand Canyon Behavioral Isolation • Species cannot interbreed because they have different mating rituals or behavior Temporal Isolation • Species mate at different times of the day or the year Adaptive Radiation • A single species evolves into many species • Occurs as new opportunities for survival arise Co-evolution • When two species evolve together in response to changes in each other • Example: Bees and Flowers Evidences of Evolution Homologous Structures • Similar body structures that are adapted in different ways for different organisms • Provide evidence that related organisms descended, with modifications, from a common ancestor Vestigial Structures • The structures of organisms that remain from a common ancestor, but may no longer affect its ability to survive and reproduce • Example: Appendix, Tail Bone Analogous Structures • Unrelated organisms in similar environments develop similar adaptations. • Example: Flight of Insects vs. Birds vs. Bats Fossil Record • The fossilized remains of organisms found in the layers of rock and soil • Shows that the diversity of organisms has changed over time Fossil Record • https://vimeo.com/19796909 Embryology • Organisms with a common ancestor share similar stages of development. DNA Analysis • All organisms use the same four bases for their DNA (genetic code). • The more similar two sequences of DNA are, the more recent their common ancestor, or the more closely related the organisms are. DNA Analysis • Modern biologists compare DNA sequences to determine which organisms are most-closely related to each other