Biological evolution is driven by changes in DNA produced by mutation, bacterial recombination, and symbiogenesis. Symbiogenesis is the process whereby, long ago, bacteria formed such inextricable associations with each other that they created whole new life forms. Every cell in every plant and every animal on earth contains myriad independently reproducing mitochondria, each with its own DNA and RNA, that are the living descendants of these symbiotic relationships.
Species tend to emerge to fill empty eco-niches.
Generally this follows catastrophic events, such as asteroid collisions or the oxygen crisis. Occasionally, it is the result of new environments being created. An example of noncatastrophic opportunism is the existence of some 170 species of fish of the same genus found only in Lake Victoria in East Africa. They evolved from a river-dwelling ancestor when earth movement suddenly created one of the largest bodies of fresh water on the planet. James Grier Miller provides a functional view of a living system, which includes 19 distinct subsystems.