The tropics are, like many cities, hot, busy and crowded. It was previously thought1 that these conditions in the tropics generate a hotbed for the formation of new species (speciation). Species diversity is remarkably high in the tropics and declines toward the poles. However, newly developed tools to measure speciation rates, coupled with ever-growing global data sets, have enabled the surprising finding that terrestrial speciation rates for the past few million years are similar across different latitudes2 or increase outside the tropics3. Writing in Nature, Rabosky et al.4 document a speciation rate for marine fishes at high latitudes that is twice the speciation rate in tropical seas. This high speciation rate in cold, species-poor waters poses an interesting conundrum for evolutionary biologists and ecologists.
There are two potential drivers of high speciation rates in the tropics. First, the elevated temperatures in the region both speed up metabolism, increasing the number of mutations, and decrease generation times. This is a potentially powerful combination, producing more of the variation necessary for evolution and the possibility of faster evolution. A second possible driver is ecological opportunity. The energy-rich tropics offer abundant resources that can support many different niches. And the tropics are so rich in species that the interactions of members of a single species with its competitors, predators and parasites might differ from place to place, leading to different adaptations and eventual divergence into new niches1. Although this narrative makes for a compelling theory, Rabosky and colleagues’ discovery suggests a different story, at least for marine fishes.
The authors gathered genetic data for 11,638 species of marine and freshwater fish, along with information on inferred evolutionary relationships based on taxonomic groupings for 19,888 additional fish species for which genetic data were not available. Using these data, and information from 139 dated fossil fishes, the authors generated a large set of plausible phylogenetic trees detailing the evolutionary relationships between all living marine fishes, and, crucially, estimates of when different lineages diverged from one another. These dated trees enable speciation rates to be inferred on the basis of the branching patterns of the tree. Species connected by short branches, and with many close relatives, have high speciation rates, whereas species that are separated by long branches and that have few close relatives have low speciation rates.
Most taxonomic groups are made up of lineages with both low and high speciation rates. The marine fishes in the authors’ large phylogenetic trees were no exception, with speciation rates varying by more than 50-fold between lineages. The authors combined these values with global maps of where these species live, revealing a clear geographical structure to the speciation rates. Because small biases integrated over large amounts of data could produce misleading inferences in these sorts of studies5, the authors considered several ways to estimate speciation rates and to map species’ location. But whether the authors considered the patterns looking species by species, place by place or ecoregion by ecoregion, there was always a pattern of the average speciation rate increasing from the tropics towards the poles.
Rabosky and colleagues consider only a few potential mechanisms that might affect the rate at which marine fishes produce new species at high latitudes. At higher latitudes, marine fishes tend to have longer generation times and slower metabolisms6 than have fishes in the tropics, suggesting that such extended generation times and lower mutational input do not limit speciation rate in these cold-adapted lineages. The authors also tested and discounted the interesting possibility that high-latitude species are the descendants of tropical lineages that exhibited adaptations for cold-water living and also happened to have high speciation rates. This negative result suggests that a high-latitude marine environment, rather than the species that colonizes it, drives high speciation rates.
The correlation of high speciation rates with low diversity is consistent with the idea that there are unfilled ecological opportunities near the poles. However, ecological opportunity is something that is inferred rather than witnessed1. The shape of a phylogenetic tree can indicate slowing speciation as species numbers rise — a pattern that is consistent with diminishing ecological opportunity7. If closely related species in such clades occupy different niches, as might be the case for the high-latitude Antarctic icefishes8 (Fig. 1), this would be consistent with ecological opportunity having a key role in driving their diversification. Such analyses are needed to determine whether high-latitude groups have reached the ecological limits of their ecosystems or whether high-latitude fish diversity might be expected to continue increasing.
Continued diversification at higher latitudes might seem reasonable, given that Earth’s cooling over the past 30 million years or so9 has given rise to the present, relatively young temperate and polar realms. However, the rate of speciation at high latitudes reported by Rabosky (roughly 0.2 new species per species per million years) is high. If this rate had been sustained over the whole of the past 30 million years, high latitudes would have tropical levels of species diversity by now. Given that the accumulation of diversity depends on both speciation and extinction rates, one explanation that reconciles a high speciation rate and low current diversity is if both speciation and extinction are elevated outside the tropics10. This could result in a pattern in which an increase in the number of species is limited by high extinction rates, and poleward realms would have few, but relatively young, species. Measuring extinction rates is almost as difficult as trying to assess ecological opportunity, but new approaches that combine information on extinct species represented in the fossil record with information from their living relatives11 might offer a way to investigate whether extinction rates are greater at higher latitudes.
The view of high-latitude oceans as ‘sleepy’ backwaters remote from the exciting evolutionary bustle of the tropics will need to change if it turns out that both speciation and extinction of marine fishes occur at a faster pace beyond the tropics. Such a pattern would imply that living cheek by jowl, or rather gill by jaw, in the tropics is a condition that is more constraining than productive, such that the real biodiversity action is taking place where there is less, rather than more, biodiversity. Far from the madding crowd, as it were.
Nature 559, 341-342 (2018)