(Note: All previous parts in the Complexity Explained series by Dr. Vinod Wadhawan can be accessed through the ‘Related Posts’ listed below the article.)
In any evolutionary process, what evolves is complexity. Chemical complexity evolved till some of it became indistinguishable from biological complexity. 
Evolution of biological complexity is determined by two main factors: natural selection (made famous by Charles Darwin), and self-organization. I focus on the natural-selection aspect of biological evolution in this article.
13.1 Darwinian Evolution
The greatest single contribution to the subject of complexity was made (unwittingly, perhaps) by Charles Darwin. The year 2009 marked the second birth centenary of Darwin, as also 150 years of the publication of his celebrated book On the Origin of Species by Means of Natural Selection.
Living organisms are open systems, i.e. they are constantly exchanging matter and energy with the environment. There is a fair amount of dynamic equilibrium between a living organism and its surroundings. The organism cannot survive if this equilibrium is disturbed too much, or for too long. The fact that an organism survives implies that, in its present form, it has been able to adapt itself to the environment. If the environment changes slowly enough, living entities can evolve (over a long enough time period) a new set of capabilities or features which enable them to survive even under the changed conditions. Over long periods of such evolutionary change, creatures may even develop into new species. This was the message of Charles Darwin’s (1859) bold theory of evolution through cumulative natural selection. He demonstrated that adaptation to the environment was a necessary outcome of the exchange processes going on between organisms and their surroundings. A consequence of his theory was that all living organisms are the descendants of one or a few simple ancestral forms. Read the full story
