Did Madagascar's famous biodiversity evolve on the island itself or did it arrive from the African continent? Malacologist Frank Kohler reports.

The island of Madagascar harbours a spectacular diversity of endemic plants and animals which has been a source of wonder and fascination for biologists interested in understanding the factors behind the evolution of this remarkable biodiversity.

Studies of Madagascar’s vertebrate animals – its mammals, birds, geckos, snakes, frogs and freshwater fishes – have shown that much of the diversity has resulted from so-called adaptive radiations into ‘species flocks’.

It has been assumed that this diversity was the result of the island’s extended period of isolation from other landmasses spanning more than 130 million years.

But more recent molecular studies suggest that most of these radiations are no older than 50 million years, indicating that ancestors of many Madagascan groups probably arrived on the island relatively recently and randomly by trans-oceanic dispersal, often from the African continent.

The studies also suggest that evolutionary radiations happened relatively quickly, driven by the availability of previously unexploited ecological niches on the island.

However, vertebrate groups account for only a comparatively small proportion of Madagascar’s biodiversity.

Researchers from the Australian Museum, Sydney, and the Natural History Museum, Berlin, have asked whether corresponding patterns are found among the island’s diverse groups of invertebrates, such as insects and molluscs.

Studying a small group of freshwater snails, the researchers found six separate species, three of them new, in a group that was previously thought to comprise only one or two species.

With their limited abilities of dispersal and confined to fast-flowing freshwater streams, it is unlikely that these snails overcame the dispersal barrier of the ocean to arrive on the island from elsewhere.

The researchers concluded that these species therefore represent an endemic radiation on Madagascar much like the known radiations of vertebrate groups.