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Genetic Variation I. Evolution as Genetic Change
Evolution is dependent upon genetic variation within a population. A. Background Information: 1. Gene Pool: Consists of all genes, including all the different types of alleles, that are present within a population. 2. Relative Frequency: number of times that a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.
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Genetic Variation I. Evolution as Genetic Change
B. Sources of Variation: 1. Mutations: Change in the DNA sequence. Some can affect an organisms fitness or ability to survive and others have no effect. 2. Gene Shuffling: During Meiosis crossing over occurs and mixes up the alleles. You don’t look exactly like your siblings, different alleles were passed on to each of you.
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Natural Selection II. Distribution of Phenotypes Evolution is a change over time. So, how does a population change over time in relation to their gene pool. A. 3 ways in which natural selection can affect the distribution of phenotypes: 1. Directional Selection 2. Stabilizing Selection 3. Disruptive Selection
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Directional selection: Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end of the curve. Ex: Birds with larger beaks are more successful than those with medium or small sized beak due to some change in the environment.
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2. Stabilizing Selection: Individuals near the center of the curve have higher fitness than individuals at either end of the curve. Ex: Human babies tend to be fairly close in weight Being born too small or too large was selected against. Therefore the curve moves towards the middle.
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3. Disruptive selection: Individuals at either end of the curve have higher fitness than those in the middle. Ex: Bird with smaller and larger beaks are more successful than those with medium sized beaks. Due to some change in the environment. THIS LEADS TO NEW SPECIES!
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Evolution in Process III. Evolution Changes in a population, over time, in response to environmental pressures. IV. Patterns of Evolution A. Coevolution: Evolution of two species that interact together. Ex: A prey changes, so the predator must also change. (Knute and Snake!) B. Convergent Evolution: Unrelated organisms develop similar characteristics due to similar environments. (Ex: dolphins and sharks)
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Evolution in Process C. Divergent Evolution: Previously related organisms become less alike. Two types: 1. Adaptive Radiation Many related species evolve from a single ancestral species Ex: Different beak shapes for different food sources
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What food source are these beak shapes most likely adapted for?
INSECTS CRACKING SEEDS
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Evolution in Process C. Divergent Evolution: Previously related organisms become less alike. Two types: 1. Adaptive Radiation Many related species evolve from a single ancestral species Ex: Different beak shapes for different food sources 2. Artificial Selection Divergent evolution done artificially. Ex: Breeding dogs, pollinating flowers
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Evolution in Process V. Mechanisms for Evolution Evolution, or change in a population, occurs at irregular rates throughout history (punctuated equillibrium). What causes these changes? A. Background Information: 1. Species: A population of interbreeding organisms capable of producing fertile offspring. 2. Speciation: Formation of a new species.
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Evolution in Process V. Mechanisms for Evolution B. Migration: Changes occur as individuals move into or out of a population. 1. Immigration: Movement into a population 2. Emigration: Movement out of a population
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Evolution in Process V. Mechanisms for Evolution C. Isolation: Separation of populations into groups that no longer interact. 1. Geographic Isolation: Physical separation Ex: Squirrels on either side of the Grand Canyon 2. Genetic/Reproductive Isolation: Inability for populations to interbreed Ex: A horse and a donkey can produce a mule, but that mule cannot reproduce
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3. Temporal Isolation: Reproduction occurs at different times
Ex: Frogs: one species mates during the day and the other at night 4. Behavioral Isolation: Different courtship rituals or reproductive strategies Ex: Eastern and Western Meadowlarks have different songs to attract mates
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V. Mechanisms for Evolution
D. Punctuated Equilibrium: Long stable periods of populations interrupted by brief periods of more rapid change. 1. Gradual change: Fossil records confirm that generally populations change very slowly over time. Ex: Many fish fossils show slow changes. 2. Rapid Change: Something happens to a populations equilibrium to cause sudden changes in the gene pool. Ex: Geographic Isolation of two groups within a population Migration and Immigration.
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