Earwigs and evolution

Usually found under our doormats and in our dahlias, the humble earwig or “forky tail” can teach us crucial lessons about evolution, according to a study published today (28 October 2004).

Dr Joseph Tomkins of the School of Biology, and his PhD student Gordon Brown, spent four years examining earwigs on numerous small islands off the British coast including the Farne Islands where population densities are so high that the earwigs can be collected by the handful.

The study, published in the current edition of Nature, illustrates the different “morphs” of earwig and the remarkable differences in morph frequencies which can exist between populations living just a few hundred metres apart.

There are two male “morphs” of the European (common) earwig. Each are thought to have different reproductive tactics – either guarding females or, alternatively, sneaking matings with females guarded by other males.

These different tactics are often accompanied by physical differences. In the larger of the two morphs, males are found to guard females with their 9mm forceps, while males of the smaller morph struggle to do so with forceps half that length.

The team found that island populations have a greater proportion of the bigger, long- forceped morph than mainland populations. They also found that amongst the islands of the Forth and the Farnes, there is extraordinary variation, ranging from populations with only one male morph to populations with two. Some of these islands support a phenomenal density of earwigs, the reasons of which remain obscure. The team also discovered that this high density has led to the evolution of the dramatic number of large-morph males in these populations.

Dr Tomkins said, “Darwin demonstrated how islands are fertile arenas for insights into evolution. We have shown how islands that were familiar to Darwin during his period of study at Edinburgh, before he set forth to the Galapagos, also have a dramatic evolutionary story to tell.

“We know of no other species where such extraordinary variation occurs. Our work highlights how evolution is a process that takes place continually and close to home in species that are commonplace enough to live in our gardens or under our doormats!”

In the 19th century, naturalists thought that the long forceped male morph was a different species but it is now known that a developmental switch transforms the males that encounter good conditions while they grow to become large and develop into the long forceped morph.

Dr Tomkins continued, “We have discovered that the size at which this switch activates has evolved across these island populations. We also show that, as the population density of earwigs on the island increases, so the threshold at which long forceped males are produced shifts to a smaller body size. This has the effect of increasing the proportion of the long forceped males in the populations where earwigs are most dense. As the density of earwigs increases, so the likelihood of males engaging in combat over females increases as they will encounter rivals at a higher frequency.”

The pair studied earwigs from two groups of islands. In Scotland, their studies took them to the Firth of Forth and to islands including Inch Garvie (near the Forth Rail Bridge), Bass Rock and the Isle of May. In England, they studied populations from the Farne Islands and Lindisfarne, off the Northumbrian coast.

The findings are of significance to researchers studying mating system evolution, the evolution of phenotypic diversity, phenotypic plasticity and threshold evolution, biogeography, phylogeography and developmental biology.

Dr Tomkins is funded by a David Phillips Postdoctoral Research Fellowship from the Biotechnology and Biological Sciences Research Council (BBSRC). Gordon Brown is funded by a studentship from the Natural Environment Research Council (NERC).

ENDS

NOTE TO EDITORS:

DR JOSEPH TOMKINS IS AVAILABLE FOR INTERVIEW IN ADVANCE ON TELEPHONE 01334 463598.

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