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Ernst Mayr, causal understanding, and systematics: an example using sabelliform polychaetes

Fitzhugh, Kirk
Invertebrate biology 2016 v.135 no.4 pp. 302-313
Sabellidae, cilia, histology, larvae, ontogeny, peristome, synapomorphy
Ernst Mayr suggested that understanding the features of organisms involves the study of what he called “proximate” and “ultimate” causes. Proximate causes of characters occur during the life of the organism; ultimate causes occur prior to the life of the organism, as part of the evolutionary history leading to organisms in the present. Mayr also pointed out that descriptive biology is important because it provides the basis for pursuing understanding by way of proximate and ultimate causes. Systematics encompasses each of these components. We routinely describe organisms (not taxa), and often infer proximate hypotheses (e.g., ontogenetic, functional, and physiological causes) to explain particular characters. Characters are also explained by ultimate causes in the form of intraspecific, specific, and phylogenetic hypotheses. Success at attaining ultimate understanding is distinctly limited, however, since these hypotheses are rarely tested. The consequence is that we often use specific and phylogenetic hypotheses to move toward the study of descriptive aspects and proximate understanding, where increases in causal understanding are more successful. An example of interactions between descriptive, proximate, and ultimate understanding are presented from research on the sabelliform groups Fabriciidae and Sabellidae. Members of both families have what is interpreted as two peristomial rings, anterior and posterior. A distinct mid‐ventral patch of cilia occurs on the anterior peristomial ring among Fabriciidae, and the posterior peristomial ring among Sabellidae. Phylogenetic hypotheses suggest these different cilia patches are synapomorphies for the respective families. Since these phylogenetic hypotheses cannot be tested, it will be more productive to investigate whether or not these patches of cilia are homologous, which might be established through ontogenetic and histological studies of proximate causes: are these cilia derived from the larval prototroch or metatroch? Are anterior and posterior peristomial rings really peristomial? Studies directed at these proximate causes offer valuable increased understanding not possible for ultimate causes in systematics.