This area, Variation, Natural Selection and Evolution, can be seen as the central organising principle that biologists use to understand the world. Variation is at the core of evolution and selection. Selection of dogs for specialised tasks would not be possible if all dogs were genetically identical. Fear about the reduction of diversity in many plants and animal species is related to the knowledge that as variability in a species is reduced then the ability of that species to respond to changes in the environment is also reduced. This will become clearer to you if you consider the usual steps considered in natural selection.
Variation exists among members of a species.
In adverse conditions in the environment, some individuals may die but others will survive. Genetically controlled characteristics favourable for survival can be passed to the next generation by survivors. Hence allele frequencies can change from generation to generation and the frequency of particular phenotypes may change from generation to generation. Selection for or against particular phenotypes impacts on allele frequencies in the next generation. Ongoing change over generations may eventually lead to the evolution of new species of organisms.