Download Random Genetic Drift

Evolution
The two most important mechanisms of
evolution are
natural selection and
genetic drift
The source of variation these mechanisms
act on come from two main sources
random mating and random assortment of
alleles
mutation
Causes of Variation that alter
gene frequency
Natural selection
removes unfit alleles
Mutation
adds new alleles
Non-random mating or mate selection
Not so random assortment of alleles
Causes of Variation that alter
gene frequency
Migration - or movement between
populations
Immigration – involves organisms migrating
into a population. This will effect allele
frequency
Emigration – involves organisms migrating
out of a population. This will also effect allele
frequency
Causes of Variation that alter
gene frequency
Non-random mating: When members of a
species prefer to mate with others that
have desirable characteristics
The founder effect: when a small group or
a pregnant female of a population is
isolated (or migrates) to some new habitat
The bottleneck effect - result of disaster
Genetic drift: see below
The Founder Effect
When members of a species colonize a
new environment they do not usually
represent the allele frequencies of the
original population. This is called the
founder effect.
Eg pregnant yellow type female Drosophila
blown to desert island - hot with little food
Random Genetic Drift
The allele frequency in a population
fluctuates due to sampling error - that is
due to random chance - this is called
genetic drift
Genetic Drift
The smaller the population the more significant
the change
Eg. This can be modelled with coins – there is a
fifty- fifty chance of getting a head or a tail but if
you only throw a coin ten times it is unlikely you
will get five heads and five tails
 The gene frequency will change over time
 Changes accumulate with time
 The population size determines the rate of
change caused by genetic drift
Some properties of Genetic Drift
Suppose we run a simulation of genetic drift with
an initial allele frequency of 0.5. Then:
When the allele frequencies of a large
population are averaged over a number of
generations, there is almost no change from the
initial allele frequencies of 0.5 - DRIFT HAS NO
DIRECTION
The chances of any small sub-population
changing from 0.5 and the size of the change
increases with each generation. - DRIFT
ACCUMULATES WITH TIME
With increasing time, more and more
populations become fixed for one allele.
Eventually all populations will become fixed for
one allele. DRIFT CAUSES THE LOSS OF
GENETIC VARIABILITY WITHIN A
POPULATION
All populations started out with identical gene
pools, but with time, the gene frequencies will
change. The populations will become different
from each other over time. DRIFT CAUSES AN
INCREASE OF GENETIC VARIABILITY
BETWEEN POPULATIONS
Some conclusions on Genetic Drift
Genetic drift is an evolutionary force that can
alter populations through time
 The amount of evolutionary change is inversely
related to population size: the larger the
population the less the allele frequency will
change
 Because deviations in allele frequent occur
above and below the current frequency (eg 0.5)
drift has no direction
 Genetic drift only occurs if there is genetic
variability present.