MORPHOLOGY AND MOLECULES AT THE SPECIES LEVEL: SOLVING A SIX-POINT PROBLEM

The relationship between morphological variation (skeletal phenotype) and molecular differences (shared genetic code) is fundamental to establishing and/ or reconciling species concepts between fossils as morphotaxa and biological species, classically defined by reproductive isolation. Biological species in practice, however, are increasingly being defined by similarities and variation in their molecules (mostly mitochondrial DNA). Results from relationships between morphology and molecules at the species-level have cascading implication for the modeling and calibration of molecular clocks, models for rates and processes of speciation and all subsequent data generated from these methods and applied to other studies.

Using extant, sister taxa with fossil records, two 2-dimensional theoretical spaces can be defined by 1) morphology vs. time and 2) molecules vs. time, with four data points in each (morphological difference vs. time) and (molecular difference vs. time). Because two of the data points are shared between theoretical spaces (modern morphology and molecular composition) and the third axis (time) is defined independently in each space, data in two spaces can be scaled and superimposed. By modeling rates of change in these independent 2-D spaces that intersect in two shared data points, relationships between morphology and molecules and any resulting influences on fossil and biological species concepts are more clearly defined and consequences for practical species identification can be evaluated, e.g. presence of cryptic species.