Final answer:
The term "dead clade walking" describes a clade that survived a mass extinction but is nonetheless on a path to inevitable extinction, lacking the diversity or adaptability to thrive post-event. This term emphasizes that short-term survival does not necessarily equate to long-term evolutionary success.
Step-by-step explanation:
The phrase "dead clade walking", coined by David Jablonski in 2002, refers to a clade, or a group of species sharing a common ancestor, that has survived a mass extinction event but is doomed to eventual extinction. The concept highlights that just because a clade makes it through a catastrophic event, it doesn't guarantee long-term survival. These clades may have been so severely depleted in numbers and diversity during the extinction event that they are unable to adapt effectively and diversify afterwards. As a result, their continued existence is only temporary, and they are gradually out-competed by other groups or clades which are more capable of adapting to the post-extinction environment.
Understanding clades involves recognizing that they are monophyletic, deriving from a single, common ancestor. Cladistics rests on several assumptions, such as common descent, bifurcation in speciation, and measurable character state changes, often analyzed using outgroups and ingroups to determine evolutionary relationships and trait polarity. The phrase dead clade walking captures the struggle and eventual demise of a clade that, despite having survived an extinction level event, can't overcome the long-term effects of that event and succumbs to extinction.