Each three nucleotides code for a specific amino acid. DNA can reproduce itself. It is this combination of its ability to code for specific proteins and to reproduce itself that makes DNA so essential to life as we know it. Mutation and recombination result in genetic variability in populations. When an organism reproduces, it passes its DNA to its offspring.
Genetic variation is the basis for natural selection.
Some genetic variants will be better suited to their particular environment than others. And consequently the genes they possess will increase in frequency in the population. A species is a group of interbreeding populations that is reproductively isolated from other such groups. The importance of reproductive isolation is that it insures that different species are different evolutionary lineages. New traits arising in one species can spread through that species, but can't spread into other species unless they arise by independent mutation.