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In this De-Natured segment of Nature League, Brit breaks down a recent scientific journal article about the rates of speciation of marine fish around the world.

Article citation:
Rabosky, D.L., Chang, J., Title, P.O., Cowman, P.F., Sallan, L., Friedman, M., Kaschner, K., Garilao, C., Near, T.J., Coll, M. and Alfaro, M.E.

An inverse latitudinal gradient in speciation rate for marine fishes

Nature, 2018

Article link:
https://www.nature.com/articles/s41586-018-0273-1#Sec17


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On Nature League, we spend the third week of each month exploring a current trending article from the peer-reviewed literature.

Scientific information isn’t /just/ for scientists- it’s for everyone! It just requires a bit of a break down. [CHEERY INTRO MUSIC].

For this month’s De-Natured segment, we’re going to look at an article released in July 2018 in the journal Nature. In this month’s Lesson Plan, we talked about the mechanisms of evolution. We also discussed the concept of speciation, and the different ways that new species arise on Earth.

One of the most general patterns of speciation we see on our planet has to do with /where/ we see it happening the fastest. In this paper entitled, “An inverse latitudinal gradient in speciation rate for marine fishes”, the researchers investigated the relationship between species richness, speciation rates, and latitude in marine fishes. So here’s what’s already known.

Evidence from hundreds of years of biological research suggests that species richness, or the total number of different species in a region, is highest at the equator and lowest at the Earth’s poles. I mean, when you think of dazzling displays of biodiversity, Antarctica doesn’t quite come to mind as fast as the Amazon Rainforest... This pattern is called the “latitudinal diversity gradient”, or LDG, and many studies suggest that this gradient exists because rates of /speciation/ are higher at the tropics.

This study focused on marine fishes, a group of organisms that also exhibits this pattern of high species richness at low latitudes. Just think of the incredible array of marine diversity in tropical, shallow waters- there’s a reason these are some of the most beloved places on Earth for divers, snorkelers, and ocean enthusiasts, myself included. So, is this diversity gradient a result of higher speciation rates?

Or something else? To address the relationships between latitude, species richness, and speciation rates, the researchers first assembled something called a time-calibrated phylogeny for more than 31,000 ray-finned fishes. In general, a phylogeny is a history of the lineages of a group of organisms.

Phylogenies can be visually represented as phylogenetic trees. Phylogenetic trees show relative times of when species diverge. In this study, the authors used a combination of genetic data and fossil species to create and calibrate their marine fish phylogenetic tree.

This calibration step changes the relative times of divergence to actual time. So, instead of saying, “species B diverged from species A before species C did it”, the tree can be read as, “species B diverged from species A 2 million years ago, whereas species C diverged 1 million years ago”. Sweet, so we have a tree...and a really detailed one at that.

In fact, the tree constructed by this team is the largest fish tree to date. Once they created this massive phylogeny of ray-finned fishes, the researchers used three mechanisms to estimate the speciation rates of these fishes: a framework called BAMM, a summary statistic called DR, and the interval-rates method. The team computed speciation rates using each of the three methods for 106 families of marine fishes.

The final piece was figuring out /where/ these species live, and the team gathered geographic ranges for the majority of known marine species by using an AquaMap algorithm and information from over 100 other organizations. The algorithm provided by AquaMaps estimates geographic ranges for marine fishes using a combination of factors like species occurrences, expert knowledge, and environmental preferences of the fish species themselves. With all of this info combined, the team was able to estimate /where/ speciation rates are fastest for marine fishes.

So what did they find? The first result the team notes is a strong latitudinal diversity gradient in marine fish species. This result is consistent with previous studies, and the highest species richness was found in the Coral Triangle of the tropical Indo-Pacific Ocean.

This goes along with the traditional biological wisdom...so far, so...unsurprising. But here’s the crazy part: they found the exact /opposite/ pattern in the speciation rate results. All three estimates of speciation rates were /highest/ at the poles of the Earth, and /lowest/ near the equator.

In fact, cold-temperate and polar lineages of marine fish had speciation rates almost /twice as fast/ as the average low-latitude lineage. In the last piece of their article, the authors offer that there are still some unknowns regarding their results. These unknowns represent future, exciting directions for their research- namely, investigating whether the rapid speciation rates they found in the cold regions of Earth’s oceans reflect a relatively new and ongoing expansion of marine biodiversity.

This article was published in the journal Nature, which is no easy feat. This journal typically only publishes studies with groundbreaking results that are widely applicable and really transform the face of science. Here are some reasons why I think this study wound up in this journal.

Here’s the thing. Scientists working in evolutionary biology, biodiversity, or related fields have time and time again found higher species richness at low latitudes. And, they’ve generally thought that this was a result of higher /speciation/ rates in those regions.

The results of this study provide evidence directly /against/ that claim. Not only that, but the results sort of fly in the face of several established relationships between thermal energy and speciation. I mean, just look at the biodiversity of fishes in the tropics.

It’s totally wild to think that speciation rates are significantly lower there than in the Arctic. But here’s where the study is really potentially transformative. If these results hold up for other groups of organisms, like species that live on the land, for instance, this could mean that the latitudinal diversity gradient we see for life on Earth could be due to something completely different than speciation rates.

This opens the door for a ton of exciting new research to come, which is certainly one of the reasons this article made it into such a top tier publication. As with any piece of exciting new research that goes against the grain of well-known patterns, there’s a potential for getting caught up in the excitement and overlooking confounding variables. The authors do a great job of pointing out these potential confounding variables and explaining how they addressed them.

One of these potential trouble areas has to do with whether a separate variable is associated with high rates of speciation. That could mean that the results they saw have nothing to do with latitude. The authors provide one possible variable like this.

They mention that instead of latitude, the high speciation rates could have to do environmental factors in these cold and dark regions. However, the team tested this by comparing deep sea fishes at low latitudes to deep sea fish at high latitude. They found that high latitude deep sea fishes have much faster speciation rates than low latitude fish- so hey, looks like latitude is still the biggest factor.

Confounding variables aside, my biggest issue with this study isn’t about the study itself- it’s about the way it’s being portrayed. Science is the process of ruling things out, and in fields like evolutionary biology, our understanding of other species on Earth is /constantly/...well...evolving! Unfortunately, these results have been portrayed in some write-ups as completely upending 200 years of biodiversity understanding.

I’ve even seen phrases like, “biologists have been wrong the /whole time/” and “everything we thought we knew is a lie”. You know, nothing like hyperbole and science…. Here’s the thing.

It’s not that their results mean that scientists have been wrong the whole time about speciation and latitude- they’ve simply contributed one more piece of the puzzle. And yes, that piece might look a little bit different than the others...but that’s science, and /good/ science at that. Scientists and the public alike must always keep an open mind when it comes to how much we simply /don’t know/ about the world around us.

And as long as we keep asking questions about these amazing puzzles, we just might keep finding some incredible pieces. Thanks for watching this episode of De-Natured here on Nature League. Nature League is a Complexly production: check out our History of Science videos on our sister channel Crash Course to learn more about the awesome intricacies of the scientific process.