Download Genetic Drift

By
Bryce Perry and Cecil Brown
Genetic Drift
vs.
Natural Selection
 How does natural selection work?
 Adaptation
 Selection of new beneficial traits according to selective
pressures at the time
 Natural selection produces adaptation of an organism
to its environment, genetic drift only will cause a
change in the allele frequencies
Genetic Drift
vs.
Natural selection (cont.)
 So, how does genetic drift work?
 First off, we have to violate the Hardy-Weinberg
equilibrium model

Limited population sizes
 The other important point is that of sampling error
Sampling Error
 Sampling Error is defined as a random discrepancy
between theoretical expectations and the actual
results.
 Occurs because the populations are finite and small
Sampling Error (cont.)
 Selection is differential reproductive success that
happens for a reason, genetic drift is differential
reproductive success that just happens.
 Genetic drift is evolution that simply happens by
chance due to sampling error.
RANDOM FIXATION
 Allele frequencies can drift between 0 and 1.
 At 0 that frequency is lost
 At 1 the allele is said to be fixed. Why is that?
 Fixation is the elimination from a population all the
alleles at a locus but one.
 WHAT IS HETEROZYGOSITY?
 WHAT EFFECT DOES GENETIC
DRIFT HAVE ON
HETEROZYGOSITY?
Population Evolutionary Paths
 Why it is said that every population follows a unique
evolutionary path?
Population Size
 What happens with a large population?
 Lots of variation
 Genetic drift is less powerful
 What happens with a small population?
 Genetic drift is very powerful
 Loss of variation
WHAT IS THE FOUNDER
EFFECT?
 Founder hypothesis, is the hypothesis that many
speciation events begin when small population
colonize new geographic areas.
 Founder event is the establishment of a new
population usually by a small number of individuals.
WHAT IS THE BOTTLE NECK
EFFECT?
SEWALL WRIGHT EQUATION
 The probability of one allele drifting to fixation is
given by the formula
Number of copies of allele
Total number of gene copies