Much of this diversity is concentrated in tropical habitats. The patterns of diversity we see today result from the history of life on earth. Life evolved at least 3.85 billion years ago. Evolution is one of the most important organizing principles in biology. The unity and diversity of living organisms reflects the evolutionary history of life on earth.
Microevolution involves processes that lead to evolutionary changes within species.
These mechanisms include mutation, natural selection, genetic drift and gene flow. Chapter 5 Introduction to Evolutionary Biology - an excellent introduction to evolution,which covers the Talk. Origins Archive - an extensive collection of background material for and discussion of topics in evolution. Warning, this is not easy reading by today's standards. New species are being identified every day.