Pre existing genetic variability is available from wild relatives of the crop.Collection and preservation of all diff wild varieties,species and relatives of cultivated species is a pre requisite for effectiveuse of natural genes available in the population.Explain.

Pre existing genetic variability means the desired variations or characters already present in the gene pool of the species or populations. For example, there are different wild varieties of plants which have genes for different characters like disease resistance, pest resistance, high yield, resistance to environmental stress etc.
 
These all varieties of plants should be collected and preserved, so that their genes can be further used or introduced in other plants to create a hybrid variety that posses all the desired characters of these wild type plants.