亚速尔群岛圣豪岛

Guillaume Baviere, via Wikipedia, CC2.0

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一种zores 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

已经提出了许多可能的解释。一个是，亚速尔群岛太“年轻”（大部分土地面积少于100万年），因此缺乏新物种进化的时间。另一个是，这些岛屿在土地区域太小，或者在生态同质上太多，无法多样化。

一种more recent suggestion实际上，显然缺乏花卉多样性是由于缺乏分类学知识而引起的幻想。这种所谓的“ Linnean短缺”表明，更大的努力（例如使用现代遗传工具）可能表明，亚速尔群岛上的植物多样性要比目前想象的要高得多。

New research led by凯蒂·琼斯（Katy Jones）of Freie Universität Berlin andMark Carine伦敦自然历史博物馆published this week在BMC进化生物学，希望在植物的谱系中检验这两个假设 - Linnean缺口和生态同质性 -Pericallisendemic 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 inPericallis以及岛屿本身的地理变异，以确定亚速尔群岛是否根本缺乏足够多的栖息地来鼓励新物种的进化。

分类学或生态学没有短缺

研究人员在更广泛的样本中评估了形态和遗传多样性Pericallisspecimens 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 betweenPericallis形态学和微气候A中发现了一个物种；这意味着与金丝雀相比，生态学分化和物种在亚速尔群岛上有分歧和分裂的机会。

Distant neighbors

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.

一个物种的数量长geograp隔开hic 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.

金丝雀群岛就是这个经典岛自适应辐射的一个例子。地理上最远的岛屿在生态上也有所不同，鼓励遗传和形态差异以及加那片群岛不同岛屿独有的新物种的发展。

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, individualPericallisspecies 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.

一种n on-going enigma?

So have we solved the Azores Diversity Enigma? The researchers point out that the pattern they see inPericallismight 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.

但是，这项研究确实表明，亚速尔群岛和其他岛屿群岛之间存在真正的差异，而地理与生态学之间的异常关系降低了花卉多样性。可以追溯到达尔文的谜语似乎更接近解决。