I was interested to see that over at The Tree of Life Jonathan Eisen discussed a few different metaphors for common descent, which include:
The tree of life
The coral of life
The watershed of life
Blood vessels of life
Lungs of life
Shrub of life
I think metaphors are useful for teaching the idea of common descent and tree thinking, and so I try to think of new metaphors from time to time, since I teach a course on Macroevolution and also talk a lot about phylogenies in an Invertebrate Zoology course.
One that I think is really powerful, is found in Richard Dawkins' book Ancestor's Tale. The idea there is to reverse time in common descent, and imagine humans walking on a path to join our ancestors. First chimps join the band, later other apes, then other primates, other mammals, etc. This analogy is good for understanding the history of a particular entity, like the human species. However, since it has a specific starting point, it tends to re-enforce the way people conceive of evolution as a linear series of events. This is true if we have one specific endpoint in mind (like humans), but evolution does not have one specific endpoint.
So, this morning, this was going around in my head, and a new metaphor sprung into my brain: A choose your own adventure book. These books have a section of the story, and then the reader must choose between two (or more) different paths, each with a different outcome to the story.
Congratulations, your character has just achieved bilateral symmetry! If you would like to proceed down the path of protostomy, go to page 150. If you would like to proceed down the path of deuterostomy, go to page 376. It might be fun to write an actual evolution book this way!