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A hotspot on your doorstep

For a continent whose flora has been studied for longer and by more botanists than any other, Europe can still spring surprises. As researchers from the Royal Botanic Gardens of Madrid and Kew have discovered, one familiar group of plants – the carnations and pinks – diversified into today’s huge array of species at record-breaking speed. The rate of speciation in the genus Dianthus outstrips anything found in regions regarded as hotspots of rapid evolution.

As part of a broader study of biodiversity in the Mediterranean region, Spanish botanists Luis Valente and Pablo Vargas decided to investigate one very diverse genus and picked Dianthus. There are some 300 Dianthus species worldwide, but the largest group, with more than a hundred species, is found around the Mediterranean. To learn more about the origins of this group, Valente and Vargas teamed up with Vincent Savolainen in Kew’s Jodrell Laboratory, where they could work out the relationships between species using the latest techniques of molecular phylogeny. These studies revealed little genetic variation between species, suggesting that Europe’s carnations had experienced a sudden and rapid burst of speciation.

Such an explosion in the number of species is generally associated with more exotic locations, such as equatorial forests, oceanic islands, the Andes and the Cape region of South Africa. Intrigued by the possibility that Europe might also be an evolutionary hotspot, the team searched for an explanation.

Using well-established molecular techniques to calibrate the Dianthus family tree, they found the ancestral Dianthus gave rise to new species at an unprecedented rate of almost eight species per million years – faster than any other group of plants. This rate was not constant however. To begin with, new species appeared slowly, with at most 0.3 species per million years. Then, at some point between 2 and 1.3 million years ago, the rate of diversification rocketed to seven or eight times as fast (Proceedings of the Royal Society B, vol 277, p1,489). ‘We thought that carnations were a young group, but never suspected they had diversified so rapidly,’ says Valente.

Above and below: Dianthus evolved into many different forms to attract pollinators

What triggered this blooming of carnation species? The likeliest answer is the sudden drying out of the Mediterranean Basin 2 million years ago, when the hot, humid climate gave way to a seasonal one, with mild, wet winters and hot, parched summers. Today’s flora is dominated by plants that bloom in spring, when insects are plentiful, but most Dianthus bloom in summer, when insects are scarcer. Valente suspects that pressure to attract rare pollinators drove the evolution of different floral features and then geography did the rest. The Mediterranean region has a remarkably varied topography, with hundreds of islands, peninsulas and many mountains – a landscape that helps to isolate plant populations and encourages divergence into new species. ‘In theory, this is the ideal setting for fast speciation, and for some reason that we haven’t got to the bottom of yet, carnations seem to have been particularly prone to it.’

Carnations and pinks diversified into today’s huge array of species at record-breaking speed

Other European genera with large numbers of species, such as Silene, Astragalus and Centaurea, are likely to have behaved in the same way as Dianthus, suggests Valente. ‘Hopefully this study will persuade young botanists to take more interest in Europe. To think that the field next to where you live may be a hotspot for the origin of new species is an exciting thought.’

12 l KEW Summer 2010