The enigmatic islands
Islands have been termed ‘laboratories of evolution’, often being home to unique flora and fauna. Archipelagos (i.e. island chains) are particularly likely to host unique evolutionary adaptive radiations where one initial colonizing species evolves into multiple new species, each adapted to the differing ecological conditions of individual islands. Classic examples include Darwin’s finches on the Galapagos Islands and Lobelioid plants in the Hawaiian archipelago.
The Azores Islands present an exception to this rule. The Azores archipelago (located off the west coast of Portugal) forms part of the Macaronesian region, also comprising the Canary, Cape Verde and Madeira islands. While these neighboring archipelagos show the classic pattern of substantial floral diversity, with many species endemic to a single island, this is not the case in the Azores. Here floral diversity is much lower, with fewer species endemic to only a single island and few examples of new species evolving after arrival on the archipelago.
This unusual feature of the Azores has been known for some time; Charles Darwin himself commented on the puzzling pattern, more recently termed the Azores Diversity Enigma. What explains these two distinctive aspects of the Azores flora – limited evolutionary radiations and a lack of endemic species found on a single island?
Explaining an enigma
A number of possible explanations have been put forward. One is that the Azores are too ‘young’ (much of the land area being less than 1 million years old) and so lacking time for new species to have evolved. Another is that the islands are too small in land area, or too ecologically homogenous, to allow much diversification.
A more recent suggestion is that the apparent lack of floral diversity is in fact an illusion caused by lack of taxonomic knowledge. This so-called ‘Linnean shortfall’ suggests that greater effort (e.g. using modern genetic tools) might reveal that plant diversity on the Azores is much higher than currently thought.
New research led by Mark Carine of London’s Natural History Museum, published this week in BMC Evolutionary Biology, looks to test two of these hypotheses – Linnean shortfall and ecological homogeneity – in lineages of the plant Pericallis endemic to the Azores and Canary Islands.
The researchers assessed both morphological variation (i.e. variation in the different shapes plants take) and genetic variation in the Azorean and Canarian plants to test if current estimates of diversity are correct or if species diversity is underestimated. They also investigated the relationship between morphological and genetic variation in Pericallis and geographical variation of the islands themselves, to determine if the Azores simply lacks diverse enough habitats to encourage the evolution of new species.
No shortfall in taxonomy or ecology
The researchers evaluated morphological and genetic diversity in a much wider sample of Pericallis specimens than any previous study. Yet they still found no evidence of under-counting of species in the Azores or ‘over-splitting’ of species in the Canaries, with current species definitions largely supported. So it appears the lack of species in the Azores compared to the Canaries is no artefact, but a genuine phenomenon; the Azores Diversity Enigma is real.
The researchers also found no evidence that the Azores lack sufficient ecological diversity to allow differentiation and splitting of species. On both the Azores and Canary islands, there is a clear correlation between Pericallis morphology and the micro-climate a species is found in; this implies that there is no less ecological differentiation – and so opportunity for species to diverge and split – on the Azores than the Canaries.
So the researchers found no support for either of the tested hypotheses; can we then put forwards a new explanation for the Azores Diversity Enigma? The researchers suggest a potential explanation could be found in a complex relationship between geography and ecology.
Populations of a species separated by long geographic distances (e.g. on islands at opposite ends of an archipelago) will naturally begin to diverge genetically as the two separated populations can no longer exchange genes during reproduction. If the two distant islands also differ ecologically (in terms of climate, available habitats, etc.) then they will experience evolutionary pressure to diverge morphologically to better adapt to their differing climates. These two pressures – of geographic isolation and ecological differentiation – will eventually result in the evolution of unique species on different islands.
The Canary Islands are an example of this classic island adaptive radiation. Islands that are furthest apart geographically also differ the most ecologically, encouraging genetic and morphological divergence and the evolution of new species unique to the different islands of the Canary archipelago.
Conversely, in the Azores archipelago this correlation between geography and ecology is broken. It is often neighboring islands that differ the most ecologically, while distant islands have similar habitats. So while we do still see a correlation between geographic distance and genetic variation (i.e. species further away from each other geographically differ more genetically), this is not always reflected in morphology, due to the lack of correlated ecological differentiation.
Instead it seems there is a greater role for phenotypic plasticity; that is, individual Pericallis species in the Azores can adapt their morphology to differing environmental conditions without any underlying genetic changes. This seems to have broken the link between environmental variation and genetic variation; breeding still takes place between morphologically distinct populations on ecologically different (but neighboring) islands, preventing these populations separating genetically and resulting in less new species than we see in the Canaries.
An on-going enigma?
So have we solved the Azores Diversity Enigma? The researchers point out that the pattern they see in Pericallis might not reflect that in other Azorean plant lineages. Recent studies on other plants have suggested that, in these groups, more species might exist on the Azores than traditionally thought, suggesting we should not completely discount the ‘Linnean shortfall’ hypothesis.
However this study does strongly suggest there is a real difference between the Azores and other island archipelagos with floral diversity reduced by an unusual relationship between geography and ecology. A riddle dating back to Darwin appears that much closer to being resolved.