New research on horseshoe bats published in the journal Nature provides a fascinating example of how a central barrier to speciation – the evolution of reproductive isolation – can be overcome, without any need for geographical separation.  Although the process of speciation - how one species evolves into another - has occurred millions of times, it remains controversial. 

 Traditionally, a key problem for evolution - and evolutionary biologists - is how diverging new species might avoid mating with each other, thereby escaping the mixing of genes that results from sexual reproduction, and allowing new adaptations to accrue.  This has led to the suggestion that in most cases, reproductive isolation and speciation must result from populations becoming physically separated, such as through the formation of a mountain range, or a period of glaciation.

Since 2000, Drs Tigga Kingston (Boston University) & Stephen Rossiter (Queen Mary, University of London and University of Bristol) have spent their summers studying bats in the remote rainforests of Sulawesi, southeast Indonesia.  This region, named ‘Wallacea’, after the great nineteenth century explorer and evolutionist, Alfred Russell Wallace, harbours a staggering diversity of bat species.  Kingston and Rossiter noticed that one species, the large-eared horseshoe bat (Rhinolophus philippinensis), occurred as three size forms: small, medium and large.  When they recorded their echolocation calls, they found that the three size forms called at different harmonics of the same frequency, 13.5 kHz. The large form called at 27 kHz, the medium one at 40.5 kHz and the small at 54 kHz. The bats had switched harmonics, and the resulting frequency differences are likely to impact profoundly on the diet of the different bats.  Whereas the low frequencies used by the large bats are particularly good for detecting large insects, the higher frequencies used by the smaller bats prove ideal for detecting smaller insects, which are acoustically invisible to the large bats. When a resource base (insects) can be partitioned in this fashion, it becomes much easier for incipient species to diverge.

An evolutionary hop across harmonics will also influence communication.  Horseshoe bats’ ears and brains are hard-wired to the frequency of their own echolocation calls, but are insensitive to hearing below and above this frequency.  Consequently, the large and small forms are almost certainly deaf to each other’s calls, and any ability to hear the medium bats’ calls will not be mutual.  Therefore, if acoustic communication is important for identifying mates, as is often the case in bats, harmonic-hopping could instantly hinder mating between the three sizes.  Kingston and Rossiter’s analysis of the bats’ DNA strongly supports this – the three forms do not reproduce together.

It appears that Indonesia is not the only location in which this species has diverged via harmonic hops.  In Australia, the big and intermediate size bats occur in close proximity to each other, calling at the same frequencies as they do in Sulawesi.  However, when Kingston and Rossiter compared DNA sequences of the Indonesian bats with their Australian counterparts, they found that the different size forms were more closely related to each other within each country than between countries.  This suggests divergence via harmonic switching has occurred independently in two places.

There are approximately 70 species of horseshoe bat worldwide; in South East Asia over 30 species have originated in the last five million years alone.  The mechanism of divergence revealed by this piece of research might go some way to explaining this diversity and the rapid radiation of this group. This research was funded by a UK eco-tourism organisation called ‘Operation Wallacea’, which runs expeditions for university students.   Kingston and Rossiter will return to Sulawesi this year, hoping to find evidence that harmonic hopping has occurred in other island populations of the large-eared horseshoe bat.