The evolution of Bolbites onitoides (Coleoptera: Scarabaeidae: Phanaeini): its phylogenetic significance, geographical polychromatism and the subspecies problem

Abstract

Abstract The distribution of the iridescent dung beetle Bolbites onitoides can be divided, based on dorsal colouration, into a blue western-half and a red eastern-half. This has raised the question, in 1959, of whether the two colour variants could represent distinct subspecies. Having examined ~1200 specimens and evaluated the proposal under both an ontological and an operational subspecies concept, we conclude that B. onitoides should continue to be treated as a single monotypic species because: (1) two kinds of colour intermediates were found living among populations of the two main variants; (2) the distribution of the variants overlaps; (3) no other characters were seen to vary consistently in accordance with the colour variants; (4) the overall geographical pattern can be explained by phenomena other than (incipient) speciation, such as phenotypic plasticity and distinct selective regimes; and (5) colour has been extensively shown not to be a reliable indicator of speciation processes among dung beetles. By comparing our findings with other cases of polychromatism among scarabaeines, we discuss publications proposing subspecies taxa relying uniquely upon colour variants. We contend that, due to the often continuous, populational, polytopic and, several times, clinal nature of the intraspecific geographical variation, subspecies classification schemes should not be established simply to classify variation across a species range without a commitment to the reality as evolutionary entities of the taxa being proposed. The alternative stance championed by many contemporary authors to give trinomina to conspecific (meta)population lineages, in turn, may eventually prove to be adequate, but we express some of our concerns as to the feasibility of this practice. Whether these intraspecific taxa should be ranked in a Linnaean hierarchy – e.g. as subspecies – is equally an open question. We also elaborate on an evolutionary scenario where the role of the iridescence in sexual selection, as hypothesized in a previous work, may be an exaptation, not an adaptation, among the horned Phanaeina.