Polytomy (1 of 3)
Often, one sees phylogenies that include polytomies, nodes with more than two descendent lineages, creating a "pitchfork."
Polytomy (2 of 3) Lack of knowledge
A polytomy may mean that we don't have enough data to figure out how the lineages are related. There are six possible solutions to this polytomy. Often, gathering more data can resolve a polytomy.