The Scorpion's Reversible Evolution

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Blind scorpions that live in the stygian depths of caves are throwing light on a long-held assumption that specialized adaptations are irreversible evolutionary dead-ends.

According to a new phylogenetic analysis of the family Typhlochactidae, scorpions currently living closer to the surface (under stones and in leaf litter) evolved independently on more than one occasion from ancestors adapted to life further below the surface (in caves).

"Our research shows that the evolution of troglobites, or animals adapted for life in caves, is reversible," says Lorenzo Prendini.. "Three more generalized scorpion species living closer to the surface evolved from specialized ancestors living in caves deep below the surface."

The family Typhlochactidae includes nine species of scorpions endemic to the karstic regions of eastern Mexico. All species in the family have adapted to the dark with features such as loss of eyes and reduced pigmentation.

Three of the species (including T. mitchelli) live closer to the surface and are more generalized morphologically than the other six, making this family an excellent model with which to test and falsify Cope's Law of the unspecialized (novel evolutionary traits tend to originate from a generalized member of an ancestral taxon) and Dollo's Law of evolutionary irreversibility (specialized evolutionary traits are unlikely to reverse).

The results show that adaptation to life in caves has reversed among this group of scorpions: two of the less specialized, surface-living species, T. mitchelli and T. sylvestris, share a common ancestor with a much more cave-adapted species, and a similar pattern was found for the third less specialized, surface-living species, T. sissomi.

"Scorpions have been around for 450 million years, and their biology is obviously flexible," says Prendini. "This unique group of eyeless Mexican scorpions may have started re-colonizing niches closer to the surface from the deep caves of Mexico after their surface-living ancestors were wiped out by the nearby Chicxuluxb impact along with non-avian dinosaurs, ammonites, and other species." link

Ref.: Troglomorphism, trichobothriotaxy and typhlochactid phylogeny (Scorpiones, Chactoidea): more evidence that troglobitism is not an evolutionary dead-end L. Prendini, et al. Cladistics 26: 117-142.