Evolution is a change in frequency of occurence of alleles in a population.

Why does the frequency of alleles in a population change?

• The population of progeny always differs slightly from its parental population because of genetic drift. The drift is caused by random changes in allele frequency in subsequent generations. The smaller the population, the faster the drift.

(more about genetic drift, Game)

• If the number of individuals in a population changes suddenly and sizably, it is unlikely that the distribution of the alleles will remain the same after a flood, fire or an epidemic. In such a small population the gene pool can change appreciably due to the genetic drift.
  
  example

• If a few organisms settle within a new territory, they form a new population with different frequency of the alleles. Three alleles of blood types IA, IB, and i0 produce four different phenotypes with blood types A, B, AB, and 0.
  
  

It is likely that among the small number of the ancestors of Australian Aborigines there was nobody with the IB allele. Hence the indigenous population of Australia has no blood type AB or B (11).

• When flow of genes takes place. Individuals moving from one population to another take their genes with them, changing the distribution of the alleles in both populations. Extensive migration causes the two populations to become genetically similar, which counteracts the results of genetic drift and natural selection.
  
  example

• Genetic drift is a result of completely random changes of the allele frequency. The flow of genes can act in a certain direction, but these changes are not of adaptive nature. The only force making populations adapt to the environmental conditions is natural selection.