Many of us are familiar with the biological species concept, which defines a species as a group of individuals that actually or potentially interbreed in nature. That definition of a species might seem cut and dried — and for many organisms (e.g., mammals), it works well — but in many other cases, this definition is difficult to apply. For example, many bacteria reproduce mainly asexually. How can the biological species concept be applied to them?
Most of evolutionary biology deals with how life changed after its origin.
Regardless of how life started, afterwards it branched and diversified, and most studies of evolution are focused on those processes.
Some important mechanisms of evolution are non-random and these make the overall process non-random.
Evolution occurs slowly and gradually, but it can also occur rapidly.
Since humans often cause major changes in the environment, we are frequently the instigators of evolution in other organisms.
(All the above distilled from: http://evolution.berkeley.edu/evolibrary/misconceptions_teacherfaq.php)