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Readings 12 notes

Biological Sciences
Course Code
Maydianne Andrade

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Readings 12
Pg338-350 (refer to Galen example on pg342-343)
-to measure strength of selection, one must first note who survives and reproduces and who
fails to do so, then the difference is quantified
-the first measure of selection is called the selection differential (S)
-the second measure of selection is called the selection gradient, which is calculated by:
x Assign absolute fitness to individuals in the population (survival to reproductive age)
x }vÀ}oµ(]v}o]À(]vÇoµo]vPZ]v]À]µo[o]À
fitness by dividing absolute fitness from mean fitness
x Make a scatterplot and the slope of the line of best fit is the selection gradient
-the selection gradient is equal to the selection differential divided by the variance in tail length
-selection gradient can be calculated for any measure of fitness and not just survival
2A^lÀ]v where 2 is the selection gradient and S is the selection differential
-evolutionary response to selection can be calculated from the heritability and selection
R = h2S where R is the predicted response to selection, h2 is the heritability, and S is
the selection differential
-the slope of a graph of evolutionary response to selection measures the heritability
-in directional selection, fitness consistently increases or decreases with the value of a trait
-directional selection on a continuous trait changes the average value of the trait in the
population and slight reduces variation
-in stabilizing selection, individuals with intermediate values of a trait have highest fitness
-]o]Ì]vPo]}v}v}v]}µ]}v[oZÀPÀoµ}(Zit in the
population but reduces variation dramatically
-in disruptive selection, individuals with extreme values of the trait have the highest fitness
population and increases variation
-all three modes of selection increase the mean fitness of the population
(refer to examples on pg348)
-directional selection and stabilizing selection are common whereas disruptive selection is rare
-disruptive selection and similar patterns of selection such as frequency dependent selection
may be more common than generally recognized, thereby increasing genetic variation in fitness
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