When thinking about evolutionary mechanisms I tend to picture two basic processes: selection within the variability of a given species and random mutations. One is smooth and continuous whereas the other is discrete and abrupt. Is this a fair characterization? And if so, what is the relative importance of each? Do they operate differently at different time scales?
Thanks a lot!

Random mutations take place all the time and may or may not get repaired, passed on between generations or lead to viable offspring but they're what cause variability which is then subject to natural selection (positive or negative) over time.

agreed so answer Daniel's question - no there aren't 2 separate processes. there is only random mutation that may confer a selective advantage or disadvantage that is then selected for or against depending on the new trait/phenotype and the specifics of what is driving the selection.