Evolution is the consequence of the following factors:
1. the potential for a species to increase its numbers
2. the genetic variability of offspring due to mutation and recombination of genes
3. a finite supply of the resources required for life
4. the ensuing selection by the environment of those offspring better able to survive and leave offspring.Some characteristics give individuals an advantage over others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. The proportion of individuals that have advantageous characteristics will increase.
An Example of Variation Driving Natural Selection
Natural selection favors longer necks better chance to get higher leaves. Favored character passed on to next generation. Original group exhibits variation in neck length. After many generations, the group is still variable, but shows a general increase in neck length.The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions.

The great diversity of organisms is the result of billions of years of selection for favorable variations that has filled available niches of our planet with life forms.