newborn, my way of understanding evolution is to think of symbiosis - I find I can relate to evolution much better like this. We see symbiosis all around us particularly in the plant world and it is an important catapult of evolution. For example the chloroplasts that enable plants to synthesize sugars were once free living bacteria. Nowadays they cannot live apart from their hosts, the plants that cannot survive without them.
Wiki has some interesting detail
http://en.wikipedia.org/wiki/Symbiosis
In a symbiotic commensalism, the clownfish feeds on small invertebrates which otherwise potentially could harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone