Time is vertical, green bars represent DNA-based hosts, red bars represent ideas being copied inside their hosts' brains. Social learning is not represented in the diagram - for the sake of simplicity - so the red learned variants are destroyed when their associated hosts die, but the reader can imagine what social-transmission would look like, if it was illustrated on this diagram.
The learned variation alters the environment the hosts are selected in - and so influences their evolution. There is no attempt to illustrate this in the diagram, though.
The basic point in this post is to illustrate the similarity between this diagram, and plant roots:
The diagrams look similar because both represent evolutionary tress. In both cases, the ancestors are neared to the root than the descendants.
Indeed, the resemblance is closer than it may at first appear - since the plant roots are hosts to mycorrhizal fungi. So, in both cases, there's a host and small, rapidly-reproducing symbionts. The symbionts explore the surrounding space, and help to determine the path that their host takes through that space.