1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Some properties of Genetic Drift
Suppose we run a simulation of genetic drift with an initial allele frequency of 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 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

9. 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

10. 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.