October 18, 2018 - Elizabeth C. Millerfish diversity Biogeography Evolution

High species richness in the Central Indo-Pacific explained by time and many colonization events

The biodiversity of most marine organisms peaks in the Central Indo-Pacific region and declines with distance from this hotspot. Although this striking pattern has fascinated researchers for over 60 years, its origins have remained unclear. In a study published in Proceedings of the Royal Society B in October 2018, Miller et al. explored how the timing and rates of fundamental evolutionary processes—speciation, extinction, and colonization events—have jointly shaped differences in regional species richness in percomorph fishes.

To investigate this question, the researchers first delineated eight major marine regions based on prior biogeographic studies encompassing the world’s warm and cold oceans. The geographic distributions of over 12,000 marine species, including 72% of all known marine fishes, were determined using occurrence data compiled from OBIS, supplemented with and validated against data from other repositories (e.g. FishBase). These distributional data were combined with an extensive time-calibrated phylogeny to reconstruct the history of colonization events and to quantify diversification (speciation minus extinction) rates in these regions. The researchers then tested whether present-day species richness patterns were associated with (i) the number of colonizations, (ii) the age of colonizations, or (iii) diversification rates.

The researchers found that a synergy between the timing and frequency at which fish lineages colonized marine regions was the key to explaining the discrepancies in present-day species richness. Specifically, regions with higher “time-for-speciation” were found to have higher species richness. That is, there has been more time for lineages to diversify in situ and build up richness as a result of relatively early colonization. This process occurred repeatedly among independent lineages in the Central Indo-Pacific, which experienced many colonizations from 34 to 5.3 million years ago enabling more species richness to build up over time in comparison to other warm oceans. Diversification rates, however, were highest in high-latitude cold oceans and comparable among warm oceans, and were thus uncoupled from present-day species richness patterns.

This study not only provides an explanation for the high diversity of the Central Indo-Pacific region compared to other marine regions, but also adds to the growing body of work illustrating that both the timing and rates of macroevolutionary processes must be considered in order to explain large-scale biodiversity patterns. Importantly, the results of this study reflect the geologic history of the global oceans, and therefore may be broadly applicable to other groups of organisms as well. This study is also an example of how the wealth of biodiversity data made available through databases such as OBIS, coupled with modern analytical techniques, can help provide new insights into long-standing questions in ecology and evolutionary biology.

Reference: Miller, EC, Hayashi, KT, Song, D, Wiens, JJ. 2018. Explaining the ocean’s richest biodiversity hotspot and global patterns of fish diversity. Proceedings of the Royal Society B: Biological Sciences 285, 20181314. doi:10.1098/rspb.2018.1314