Well, one of the things that a hypothesis of phylogeny allows us to do is to look at the direction of evolutionary change that takes place within a lineage.
Basically, if we construct this phylogenetic tree and determine that this whole group is what we call a monophyletic or natural group that includes a common ancestor and all of the descendants of that common ancestor. This group then provides a good basis to test various other hypothesis that we want to find out something about. In the case of coloration, if we look at this group of slugs, we find that all of the ones in the lower branches of the tree, have one adaptation of body color that's also found in the out groups of that particular monophyletic or natural group. Other members of this lineage have evolved a different adaptation. Let me show you exactly what I mean.
This is one of the more basal members of this phylogenetic tree. You can see that it has a green body color. All of the members of the immediate outgroups of this group of slugs have a green body color and almost all of them are very cryptic on their natural food. This species is unique in terms of all of the members of this monophyletic group, rather than being closely associated with food items they sort of crawl around out in the open and are less cryptic and less intimately associated with their food. So there's a major behavioral change within this monophyletic group but all of the basal members still have a green body color
Well, what do we find in the derived members of this group? A very different situation. All of them are distasteful but some members of this group, as there is an increasing tendency to spend more time out in the open, advertise their presence in a far more flamboyant manner. And you can see there are many different variations on the general themes of color patterns. So, in this case, by having a phylogeny, we are able to determine the direction of evolutionary change. If you didn't know anything about this group other than that some of them were green and some of them had far more elaborate and vibrant colors, you wouldn't be able to tell anything about the direction of evolutionary change.
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