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Published byMyles Shaw Modified over 9 years ago
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How Evolution Occurs Genetically
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Evolution occurs in _populations_ not _individuals. Why? Within the _lifespan_ of one individual, new features cannot evolve in response to _natural_ selection. Only as traits are inherited, can the _environment select for them
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_gene_ _pool_ is the entire collection of genes in a population.
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If you know the genotypes of a population, you can calculate the _allelic _frequencies, or the percentage of an allele in a population. When the frequency of alleles is not changing from generation to generation the population is in _genetic_ _equilibrium. When allelic frequencies are changing, the population is evolving.
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Hardy-Weinberg postulated that populations WILL NOT change allelic frequenices if these five conditions are met: A. No Mutation B. Large population C. No migration D. Random mating E. No natural selection
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_Mutation___ is any change in DNA. While most often _harmful in some cases it can be _beneficial_. Causes: a. _chemicals____ b. _radiation___ c. _DNA replication errors__
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2._genetic _drift is random change of allelic frequencies due to _chance. More important in _small_ _populations_ than large ones. For example, there is village in Spain that is polydactyl, that recessive gene shows up in a much larger percentage_ than normal due to chance. Continued inbreeding in the same population _concentrates_ the trait.
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3. _Migration___ Individuals _enter_ or _leave_ a population either adding or subtracting alleles from a population.
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4. Sexual selection is non-randoming mating, which can result in dramatic evolution of certain traits.
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5. Natural selection will always cause allelic change due to survival differences.
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Variations tend to go over a _range. Ex – Height of people. Selection occurs when _one area__ of traits are favored
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1. Stabilizing __selection__ is when average individuals in a population are favored. This reduces __variation_ in a population
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2. directional _selection is when one extreme form of the trait is favored. This can lead to rapid _evolution in a population.
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3. disruptive _selection_ is when selection favors both the extreme forms of a variation. May lead to new _species_.
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A _species are organisms that basically look similar and can have fertile offspring. _speciation_ is the evolution of a new species.
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1. geographic isolation is when a physical Barrier separates two groups leading to _unique gene pools. _Australia__ is considered a good example of this
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2. _reproductive_ isolation is when Formerly interbreeding organisms no longer produce viable offspring. Happens in two ways: _reproductive, genetic differences lead to death of embryo. seasonal, animals start breeding at different times of the year based on their environment.
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3. polyploidy is when, during meiosis, extra chromosomes are added to the genome. This is seen in plants. Perhaps _half_ of all flowers_ and _crops_ are polyploid. It leads to new species instantly.
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1.._gradualism is the idea that species develop through the slow buildup of adaptations. This was _Darwin’s_ view. However, many species have been amazingly stable over time.
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punctuated equilibrium says that living Things have long periods of stability with speciation that occurs in rapid bursts. This is caused by changes in the environment or introduction of new alleles.
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. convergent evolution is when two species evolve similar adaptations in different locations. This happens with geographic isolation Ex. Rabbits and patagonian hares_
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4. divergent evolution is when two species grow less alike over time due to reproductive isolation. Ex. African and Asian elephant
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