BIOLOGY 152 Lecture Notes - Lecture 5: Allele Frequency, Directional Selection

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9 Feb 2017

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Biology Week 3
Hardy Weinberg Equilibrium
If nothing is happening to alleles to cause them to leave population, then
allele frequencies will remain the same from generation to generation
Test and if observed frequency matches expected frequency then there is
no evolution
good test if you want to know when evolution is occurring
Two ways to use Hardy Weinberg Equilibrium
If question tells you population is in equilibrium:
just plug numbers into the equation
this has very little use scientifically
remember the equation is p^2+2pq+q^2
If question asks if it meets the conditions for Hardy Weinberg Equilibrium
calculate allele frequency from observed genotype frequency
use this to determine your expected value
If they match then you have Hardy Weinberg equilibrium, if not evolution
is occurring
If knocked out of equilibrium for some reason, a population will require one generation
of random mating and no evolutionary mechanisms acting on alleles to get back
can have new genotype frequency that conforms to the equilibrium
although the frequency is different that doesn't mean it doesn't conform
Positive and Negative Selection
can lead to allele fixation
Positive means allele is being selected for, all of population will have it,
usually leads to fixation
Negative means allele is being selected against
both of these mean it is not in Hardy Weinberg equilibrium
the allele no longer varies
Other Types of Selection
Stabilizing Selection
reinforces mean
find more resources at
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