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.