The Chilly Cradle of Life

Species richness is much higher in waters near the equator, but do we see that in a phylogenic tree?

Species richness is much higher in waters near the equator, but do we see that in a phylogenetic tree? (Image Credit: Rich Brooks, CC BY 2.0)

An inverse latitudinal gradient in speciation rate for marine fishes (2018) Rabosky et al., Nature  doi:10.1038/s41586-018-0273-1

The Crux

The tropical regions of the Earth are the most species-rich and diverse ecosystems on the planet, with this diversity and species-richness declining as you move further and further from the equator. One hypothesis explaining this is that speciation rates are simply higher in the tropics, meaning that more species are evolving in a given time in the tropics than anywhere else. To test for this, the authors used the largest phylogenetic tree available and analyzed speciation rates (how many new species evolve from older species) per million years.

Did You Know: Phylogenetic Trees

This paper is all about using phylogeny to answer questions about speciation rates, but what exactly is a phylogeny? Think of those images of the Tree of Life, with the animals and plants we are familiar with on the tips of the “branches”, the branches going back to join the “trunk” which eventually leads all the way back to the “root” of the tree. These tips represent extant species, and where the branches join together are where organisms on the tip of the branches had a common ancestor, and the root represents the common ancestor of all life on Earth.

Using phylogenies to answer questions can be very useful, especially when the tree has data on the length of the branches so that we can know how long ago organisms split from one another. These trees allow researchers to answer questions about evolutionary relationships and what may have happened in the past. For example, a famous court case from the 90’s in the United States used phylogenetic analyses to prove that a dentist was responsible for infecting several of his patients with HIV-1, because his strain of the virus was more closely related to the strains found in the victims than to random strains.

How it Works

The authors assembled a tree of 11,638 species of ray-finned fishes using genetic data available on GenBank (an online repository of genetic information), and added an additional 19,888 species, totaling 31,526 species. The authors only had genetic data for 1/3 of the species on the tree, which can be a problem when trying to analyze speciation rates, as you have to know how species are related to one another in order to fit them onto the tree. To get around this, the authors used a mathematical method to estimate the species’ position on the tree based off of the closest relative that they did have genetic data for. Basically, the authors know species B, C, and D are closely related to species A, but they only have genetic data for species A. So they put species B, C, and D on the tree near species A and estimate the time since those species split from a common ancestor.

In addition to the latitudinal locations of species, the authors also compiled data on the depth, temperature, salinity, and productivity of the environments that these fish species are coming from, to see if any of these environmental factors correlate with speciation rate.

What They Found Out

As expected, the species richness at the equator was much higher than it was towards the poles. In contrast, speciation rate was strongly and negatively correlated with both latitude and sea surface temperature, meaning that the speciation rate is higher in the colder water towards the poles and lower in the warmer tropical areas.

These findings mean that classic ideas explaining the higher diversity at the tropics, such as the higher productivity of those ecosystems promoting higher rates of evolution and speciation, need to be revisited and reconsidered.

Any Problems?

A few actually, and big ones at that. While the authors used a variety of analytical methods and considered many different causes for the results that they found, the huge issue with all of this is that 2/3 of the data used for all of those analyses was effectively made up. Now, the math that they used has a lot of support in the literature, but the fact remains that the majority of this tree was all estimated and not based off of real data. This specific mathematical method has really caught on in recent years, but only in the realm of phylogenetic studies. If a researcher tried to submit a paper where they collected data on average seed consumption by a specific bird species in one state, and then “estimated” the seed consumption rates for every other state and tried making statements with that data, the paper would be rejected.

In addition to the issue with estimating 2/3 of the data used, the authors also neglected to consider extinction rates when calculating speciation rates. You may be asking yourself: how can we say anything about the birth of new species if we are not also thinking about the death of older species? That’s a great question, and one that I think draws attention to some issues in the methods used in this paper.

Ernst Haeckel's Tree of Life, an early representation of phylogeny

Ernst Haeckel’s Tree of Life, an early representation of phylogeny (Image Credit: Ernst Haeckel)

So What?

Despite its possible methodological shortcomings, this study showed that a classic explanation for why the tropics have more species may not be valid. We may be tempted to think of scientific explanations as fact, but this paper draws attention to the fact that until you actually go out and do the work to see if a hypothesis holds true, you cannot truly explain a pattern in nature. Some may think of this as a bad thing, when you get the exact opposite result to what you were expecting. However, all that this means is that we need to get creative and think more about what could be leading to these patterns. Findings like those in this paper are the impetus leading to new ideas and research.

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