Every cell of the organism has the same DNA sequence and same genes. However, not all genes are expressed in every single cell at the same time. Only those genes necessary for a specialised function of a specialised cell are expressed in the specialised cell. For example, skin cells have some different proteins than nerve cells. Genes responsible for those "skin proteins" are activated in skin cells and are turned off in nerve cells.
<span>Large changes in allele frequencies can occur in a short time - any changes in frequency, occurring as a result of random chance, would have a proportionally larger effect in a small population. </span>
