The nucleus of a cell contains chromosomes , each of which consists of a single DNA molecule wrapped up in a coat of proteins. Almost all vertebrate organisms have a double set of such chromosomes, one received from the mother and the other from the father. Although the DNA molecules are extremely complex they are built from only four different building blocks. A gene is a section of the DNA molecule whose sequence, after decoding, is responsible for a protein. Most individual mutations, however, have no or only minor effects on the final product.
Mutations take place regularly.
These mutations are then added to the gene-pool of the species. A gene-pool is the total of all the DNA sequences of all the sexually reproducing individuals of an interbreeding population. It thus follows that not only species but also geographically isolated populations of a single species will have different gene-pools. The blue fish may no longer be recognised by its conspecifics and therefore be excluded from reproduction. This means that its genes are lost from the gene-pool.