THE QUATERNARY RECORD FOR EXTINCTION AND SPECIATION IN THE COLEOPTERA

One of the great paradoxes of the Quaternary fossil record is that the Coleoptera, the most diverse of all living organisms, are represented by exceptionally low levels of extinction and speciation. In North America, only two species of dung beetles, Copris pristinus Pierce and Onthophagous everestae Pierce from the Rancho La Brea deposits, are reported to have become extinct. The extinction of megafauna and reduction in dung were considered to be contributing factors. The only record for speciation is that the weevil species, Lepidophorus lineaticollis Kirby, might have evolved during the early Pleistocene.

Beetles are ectotherms and are dependent on environmental temperatures for lifespan duration, diapause, dispersal, mortality, and genetic adaptation. Theoretically, the fragmentation and isolation of populations resulting during Quaternary climatic changes should have resulted in elevated rates of extinction and speciation. Why then are there so few examples of extinction and speciation compared to other invertebrates and vertebrates. Is it possible that new and extinct species go undetected? The fossil record represents only a small percentage of the fauna and preservation favors certain ecological types, especially those of mesic habitats. Even so, a large number of species have been reported as fossils and under representation does not explain why some percentage of those should not be extinct or new species.

Coope addressed part of the paradox by suggesting that the lack of speciation might be linked to the instability of environments. He suggested that the ‘migrations’ of species caused by climate change did not allow time for genetic differentiation to develop. This would suggest that levels of genetic differentiation in modern populations might be relatively low. This is not the observation, however, and for the ground beetle, Amara alpina, genetically differentiated populations maintained morphological stasis through at least one glacial-interglacial cycle. Fossil evidence indicates species survived climatic changes by ‘migration’, sometimes involving continental-scale movements. The reasons for the lack of speciation and extinction remain unknown but some combination of population dynamics and ease of dispersal are more likely to be involved than a mechanism involving genetic flux.