There's a large element of "chance" in all biological systems. Whether it's a biochemical process within a cell, the movement of cells throughout an organism, or the evolution of the those organisms, stochasticity plays a large a part in biology. Unfortunately, this is often missed by most students of biology -- either because they fail to grasp the concept or it is never even presented to them.
Yann points us to an article in PLoS Biology on teaching students how to appreciate the importance of randomness (I prefer to call it stochasticity) in biology:
Klymkowsky MW, Garvin-Doxas K (2008) Recognizing Student Misconceptions through Ed's Tools and the Biology Concept Inventory. PLoS Biol 6(1): e3 doi:10.1371/journal.pbio.0060003
The article presents some approaches that can be used by instructors when teaching about various aspects of biology to enforce the importance that stochasticity plays in all of biology.
Check out this video: Scientists for Better PCR