Different evolutionary routes to becoming diversity hotspots

Alpine forest landscape in the Himalayas.

How to tease apart the evolutionary mechanisms underlying global biodiversity patterns.

A major question in evolution and ecology is why biodiversity is so unevenly distributed on Earth. This geographic pattern of global diversity has been extensively analyzed in plants and vertebrates, and has been suggested to be attributed to climatic and topographic variables. However, environmental factors can not directly change the regional richness of species in the absence of evolutionary processes such as speciation, extinction and dispersal. From an evolutionary perspective, high regional diversity may be the result of high net diversification (speciation minus extinction) rate, multiple immigration events from adjacent regions, and/or a long time available for the accumulation of species. However, the relative importance of these different evolutionary processes in shaping regional diversity patterns is poorly known.

Image: Alpine forest landscape in the Himalayas. Photo: Tianlong Cai.

FROM THE COVER: read the article on which this post is based …
Cai T, Shao S, Kennedy JD, et al. (2020) The role of evolutionary time, diversification rates and dispersal in determining the global diversity of a large radiation of passerine birds. J Biogeogr. 47:1612–1625. https://doi.org/10.1111/jbi.13823.

To examine the different routes to the build-up of global diversity patterns, we focused on the biodiversity hotpots in the Sino-Himalayan Mountains (Image 1) and oceanic islands of the Indo-Pacific and Indian Ocean regions using the large avian babbler radiation as a model system. The babblers include more than 450 species belonging to five families, with highly diverse morphological and ecological adaptations (Image 2). The group reaches its highest local diversity in the Sino-Himalayan Mountains, with 87 morphologically diverse species in a grid with the resolution of 0.5 geographical degrees, and in the Indo-Pacific and Indian Ocean islands, with ~100 morphologically similar species in the genus Zosterops. We were intrigued by such a high diversity of babblers in two different settings, i.e. a montane region and oceanic islands, and the underlying mechanisms shaping this pattern. We were interested to unravel the relative roles of diversification rates, evolutionary time and dispersal for the build-up of the babbler diversity.

Image 2: Representatives of the seven babbler families. Family Sylviidae: (1) Blackcap Sylvia atricapilla. Family Paradoxornithidae: (2) Fire-tailed Myzornis Myzornis pyrrhoura, (3) Fulvous Parrotbill Suthora fulvifrons, (4) Golden-breasted Fulvetta Lioparus chrysotis. Family Zosteropidae: (5) White-collared Yuhina Parayuhina diademata, (6) Chestnut-flanked White-eye Zosterops erythropleurus. Family Timaliidae: (7) Chestnut-capped Babbler Timalia pileata, (8) Streak-breasted Scimitar-babbler Pomatorhinus ruficollis. Family Pellorneidae: (9) Rusty-capped Fulvetta Schoeniparus dubius. Family Alcippeidae: (10) David’s Fulvetta Alcippe davidi. Family Leiothrichidae: (11) Black-headed Sibia Heterophasia desgodinsi, (12) Red-billed Leiothrix Leiothrix lutea. All photos: Per Alström..

Based on a near-complete time-calibrated phylogeny and a reconstructed evolutionary history, we found that babblers originated in the Sino-Himalayan Mountains, suggesting a long time for diversification and species accumulation within this region. We concluded that regional diversity of babblers could be well explained by the timing of the first colonization events, while differences in rates of speciation or immigration have had far weaker effects. Nonetheless, the rapid speciation of Zosterops, which we suggested was facilitated by repeated sea level fluctuations during the Pleistocene, has accounted for the babbler diversity on oceanic islands. Our findings support two different evolutionary scenarios: long-time accumulation of species in mainland montane regions and high speciation rate on oceanic islands.

Breeding habitats
Image 3: Views of main breeding habitats of Variegated Laughingthrush (Trochalopteron variegatum) in the Himalayas. ­(A) Rapid changes in vegetation landscapes within a very short geographic distance along elevational gradients. The mist conserves warm and humid climates in the valleys below snow-clad mountains. Three eggs of Variegated Laughingthrush are shown in the left bottom. (B) Alpine forest in the mountains at ~3000 m. Photos: Tianlong Cai.

The question why there are so many babblers in the Sino-Himalayan Mountains was addressed already in the 1980s by the father of Chinese ornithology, Tso-hsin Cheng. After analyzing distributions and morphological variation of the genus Garrulax, he postulated that the Hengduan Mountains in the Sino-Himalayas was the main diversity hotspot for the babblers because this region was the center of origin for the group due to the historically stable climate and long time for accumulation of diversity – in agreement with our findings. The Sino-Himalayan Mountains provide suitable habitats with a stable climate for diversification of babblers, leading to high regional diversity. Many of the deep valleys along the margins of the large highlands of the Sino-Himalayan Mountains would be places of atmospheric inversions, where cold air would sink down into the valleys at night, creating distinct mist zones that would maintain high humidity and cloud forests (see Image 3). In contrast, the Indo-Pacific and Indian Ocean islands underwent repeated contact-isolation circles due to the sea-level fluctuations during the Quaternary glacial cycles, which likely accelerated the speciation rate of the Zosterops group by vicariance, leaving a large number of geographically isolated and morphologically homogeneous descendants. Our study highlights how assessing differences in macroevolutionary history can help explain why biodiversity varies so much worldwide. Further studies based on analyzing functional traits of related species would help us to understand how species can coexist in these hotspots.

Written by:
Tianlong Cai and Shimiao Shao – Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China. 

Per Alström – Animal Ecology, Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden

Additional information:
@TianlongCai; @AlstromPer;  https://katalog.uu.se/profile/?id=N99-636

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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