One of the beetles analyzed in the study.For a long time it has been believed that subterranean species are an evolutionary dead-end. It has been assumed that each one of the species or group of them had an ancestor which lived on the surface and moved to subterranean habitats where it became an evolutionary “dead-end”.
This supposition is based on the fact that cave beetles are very specialized; they have adapted to live in confined areas with very strict conditions of temperature and humidity. Therefore, they cannot easily move and colonize other ecological environments.
Scientists at the Instituto de Biología Evolutiva (CSIC-UPF), at the Museo Nacional de Ciencias Naturales del CSIC, and at the Museo Nacional de Ciencias Naturales de Barcelona, have analysed a group of Leptoridini beetles in the Pyrenees. And they have demonstrated that some subterranean lineages highly specialized are not an evolutionary ‘cul-de-sac’. On the contrary, they can diversify and have a widespread radiation.
Ignacio Ribera, scientists at the Instituto de Biología Evolutiva (IBE), explains that these beetles have little mobility as they lost, in the past, eyes and pigmentation. They live in confined spaces with very specific environmental conditions, within the rockery gaps, at great depths. They never go to the surface and never move too far.
After comparing genetic sequences of 81 lineages, the scientists conclude that all these beetles have a single and common ancestor
Nevertheless, after comparing genetic sequences of 81 lineages, the scientists have demonstrated that all these beetles have a single and common ancestor. And contrarily to the previous idea, scientists showed that these beetles have moved geographically and have generated new species during millions of years, and have diversified at a similar rate than other beetles on the surface.
Reduction of the larval instars
The Leptoridini beetles had a progressive reduction of the reduction of the number of larval instars from the ancestral 3 to 2 and ultimately a single instar, during which the larvae remains inactive, as it has in the egg of the egg sufficient nutrients to complete development without feeding.
The scientists have showed that this single larval instar appeared during the middle Miocene and that species with one single instar had a diversification rate higher than that of 3 or 2 instars lineages.
The genetic data allow concluding that all lineages analyzed have one single and common ancestor, a beetle that probably lived on the surface more than 30 millions of years ago and that had three larval instars.
“Our hypothesis is that reduction in the number of larval instars is an adaptation mechanism to the scarce food available”, points out Ignacio Ribera, CSIC scientists at the Instituto de Biología Evolutiva. “In caves there are few resources and larvae can hardly move, less than an adult”. If a larva requires covering long distances to get food, “it has little chances of surviving. On the contrary, a larva that doesn’t need feeding, which has all nutrients in the yolk of the egg, and that only requires water, it has more possibilities to survive”.
The study, which has been published recently in the Proceedings of the Royal Society magazine, concludes that a very specialized subterranean lineage can have a highly diverse and relatively widespread radiation with a very dynamic evolutionary history.
Life history specialisation was not an evolutionary dead end in Pyrenean cave beetles, Alexandra Cieslak, Javier Fresneda & Ignacio Ribera (IBE, CSIC-Universitat Pompeu Fabra), MNCN (CSIC), Museu de Ciències Naturals de Barcelona. Proc. R. Soc. B, http://dx.doi.org/10.1098/rspb.2013.2978