The kinds of selection. That is a particular size or weight or level of enzyme activity which yields the best survival rate. If one end or the other of the spectrum is favored, we have Directional Selection. Here maize was selected for high and low oil production over 80 generations. Note that selection can be either in the positive or negative direction.
Selection can only work if there is variation on which to work.
It can't work on pure lines as has been shown by selection of pure lines of beans. Community selection or the Red-Queen Hypothesis is possible since as one species increases its fitness with respect to other species with which it comes in contact, a selection pressure is put on the other species to increase their fitness as well. In this way, different species can become more and more dependent on one another. These two process do not act separately in nature, but in tandem. Holes in side of container represent selection removing the mutations.