An interesting blog post on
Pleiotropy about fitness landscapes and deleterious mutations. The blog's author, Bjrn ‘stman, uses a simple simulation to look at hill climbing for an organism with 2 traits. There are four fitness peaks and the simulation makes it clear that in order to reach more optimal peaks the populations first need to move
away from their current fitness peak, which obviously requires deleterious mutations.
The basic idea is very simple but the video of the simulation very nicely illustrates the point.
TTFN,
WK