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16-2 Evolution as Genetic Change
16.2 – Evolution as Genetic Change
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16-2 Evolution as Genetic Change
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16-2 Evolution as Genetic Change
§ If an individual dies without reproducing, it does not contribute to the gene pool.
§ If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.
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16-2 Evolution as Genetic Change
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Natural Selection on Single-Gene Traits
§ Natural selection on single-gene traits can lead to changes in allele frequencies – and thus to evolution.
§ Ex: hypothetical lizard population
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16-2 Evolution as Genetic Change
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Natural Selection on Single-Gene Traits
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16-2 Evolution as Genetic Change
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Natural Selection on Polygenic Traits
§ On polygenic traits, natural selection can affect distribution of phenotypes in 3 ways:
① directional selection
② stabilizing selection
③ disruptive selection
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16-2 Evolution as Genetic Change
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Directional Selection
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Directional Selection
§ Directional selection – when individuals at one end of the curve have higher fitness (than the rest)
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Stabilizing Selection
§ Stabilizing selection – when individuals near the center of the curve have higher fitness than individuals at either end of the curve
§ Ex: average birth weight
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Natural Selection on Polygenic Traits
§ Disruptive selection – when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
§ Ex: average-sized seeds become scarce…
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Genetic Drift
§ Natural selection gives “direction” to changes in population
§ “adaptive” change
§ Genetic drift – a change in allele frequency due to random events
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Natural Selection on Single-Gene Traits
Bozeman on Genetic Drift http://www.youtube.com/watch?
feature=player_embedded&v=mjQ_yN5znyk
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Bottleneck
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Bottleneck
§ Bottleneck – an event in which a population’s size is greatly reduced (like squeezing through a bottleneck)
§ may have a substantial effect on the population. Why?
§ many genes may be lost from the population, reducing the population’s genetic variation
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Bottleneck
§ Founder Effect – when allele frequencies change due to migration of a small subgroup of a population
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Evolution Versus Genetic Equilibrium
§ The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.
§ When allele frequencies remain constant it is called genetic equilibrium.
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Evolution Versus Genetic Equilibrium
Five conditions are required to maintain genetic (Hardy-Weinberg) equilibrium ① random mating ② large population ③ no movement into or out of the population ④ no mutations ⑤ no natural selection
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Evolution Versus Genetic Equilibrium
§ Random mating ensures that each individual has an equal chance of passing on its alleles to offspring.
§ Genetic drift has less effect on large populations than on small ones.
§ Immigration or emigration can bring alleles in or out of the gene pool.
§ Mutations will introduce new alleles into the gene pool.
§ All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another.
