THE “JANUS CONCEPT” OF PALEONTOLOGICAL SPECIES: ARE FOSSIL SPECIES CRITICALLY IMPORTANT OR ARE WE KIDDING OURSELVES?

For more than a century, paleontological views of species have occupied a spectrum between two very different perspectives. At one end of this spectrum is the view that species are either completely impossible to recognize in the fossil record, or completely subjective, and therefore not comparable units to what neontologists call species. This view is in part reflected today in the common practice of using genera and families for diversity analyses in the fossil record instead of species. The other view is that species are the basic units of macroevolution and can be recognized and studied in the fossil record. This view has roots in pre-Synthesis ideas, but was first formalized in Simpson’s “evolutionary species definition”, and became a central concept of post-Synthesis paleobiological ideas like punctuated equilibrium. Modern paleontological views of species persist in this strangely Janus-like tradition: we use species but we don’t uniformly respect them. We constantly discuss species evolution, but are simultaneously a bit queasy about taking them seriously. Neontologists famously cannot agree on “what is a species” either, but they at least (mostly) agree that it is an important question that deserves serious attention. This situation is problematic because much of macroevolutionary theory – at least in its current form – is based on and can only be tested with concepts of species. If paleontology cannot articulate an explicit concept of species as biological entities represented by fossils, we risk losing many of the hard-won gains of the past 40 years toward the goal of paleontologists and neontologists talking and thinking about some of the same data, questions, and theories in macroevolution. Such a species concept appears to be possible, and most elements of it are already available in the literature. It may not be uniformly applicable to all taxa at all times and places, but its widespread application – or at least serious discussion – will almost certainly yield improved macroevolutionary understanding.