How the pace and pattern of species description informs our evolutionary understanding
Can we predict the unknown and the undescribed? Are new species just ‘more of the same’ or something truly different? Blackburn et al., 2019 investigated the tempo and predictability of new species descriptions and find that there’s a delightful, non-random pattern to how new species are described and added to the ever-budding tree of life. Here’s why that’s encouraging…
What difference does it make when (or where) species are described?
Macroevolutionary biologists concern themselves with the patterns and processes that give rise to new species, the maintenance and generation of biodiversity. Since these processes occur over vast, seemingly impenetrable time scales, much of what we do relies on estimates of ancient changes leading to the species we observe now. It’s a probability exercise. Something that is a pervasive problem in evolutionary biology is accounting for what we can’t possibly see – the sum total of mutations in our genetic code or all the lineages that have gone extinct, with or without being observed. Wait, what does ‘with or without being observed’ mean? Well, yes – we might observe animals or plants in the fossil record, but there are biases to what fossilizes – so all modern phylogenetic methods make assumptions about rates of extinction and speciation. ‘Known unknowns’ if you will, biases that we must account for using simulation. Phylogeneticists are well-acquainted with the uncertainty of the unseen and how that might affect our interpretations. But what about unseen things that still exist? How are undescribed species biasing our assumptions about the pace and rhythm of evolution? That’s what Blackburn and colleagues investigate – is the pace and pattern of taxonomic discoveries biased? And how might this bias change our perception of biodiversity?
Wait, there are still new species waiting to be discovered?!
Of course! Blackburn et al. mention that there have been some 150 amphibian species described every year for the past decade! In 2018, the Natural History Museum in London alone described 272 new species, from swamp eels to dinosaurs, nightshades to copepods. Most every time I’ve known an expedition to visit a remote part of South America for example, we usually find several undescribed species of fish.
Are undescribed species truly unknown? Not to that species (assuredly) and usually not to any of the indigenous peoples living in or around those habitats. But for evolutionary biologists hunkered down behind a computer screen (e.g. writing this blog post), or a museum curator changing specimen labels, pending a revised affiliation for some species or another, those species in the bush may very well be unknown. Or understanding of what unifies life on earth depends on placing as many organisms into our shared framework for biological understanding as possible. In fact, according to many of the authors cited by Blackburn et al. – perhaps the most pervasive hurdle to our understanding of the Tree of Life is poor taxon sampling – not having enough species to ‘know’ the true phylogeny.
What patterns are there to how species have been described?
As time has passed, Blackburn and colleagues found that newly-described species are typically more closely related to some other known species, rather than being a member of some drastically divergent branch of life. This makes intuitive sense – the chances of finding something truly bizarre and novel diminish with time (and seemingly therefore, with sustained scientific effort). As such, Blackburn et al. describe a decrease in branch lengths – which represent the length of time (and by proxy, differences) between sister species. And this pattern holds true for the flowering plants, most reptiles, and certain insects that the authors surveyed. But not everyone… lesser known groups like Onychophorans (velvet worms) show a more spastic pattern of how we have described their diversity – a sign that Blackburn et al. attribute to our taxonomic knowledge of that clade not being quite ‘mature’ yet. What’s the immediate relevance of this finding? Well, for organisms like velvet worms, it means that there are probably still really weird, divergent lineages waiting to be discovered! There’s also a pattern to what regions continue to produce undescribed, diverse taxa (at least squamate reptiles) – perhaps unsurprisingly, those continents with tropical climates – Africa, Australia, and South America. Biodiversity hotspots continue to surprise us with new and strange critters.
What’s a gamma? (No, not agama).
Gamma is a common measure that phylogeneticists use to consider the ‘shape’ of a phylogeny. Phylogenies look like trees (they’re dendrograms after all) and gamma describes how much that tree has either an acacia shape or looks more like a series of bamboo shoots. Specifically, gamma looks at where nodes (splits turning one lineage into two) are distributed within a phylogenetic tree – towards the root (base) or towards the tips (species). What I mean by that is, did all the diversification among lineages happen early on or more recently, in a clade’s history? Does the tree look bushy at the top or has many long, long branches. Blackburn et al. looked at how gamma changed through time for certain clades, asking: as time progressed and species are described, how has the pattern of diversification changed for a group of organisms? Were a lot of species described early and then not a lot later on? Or, as time progressed, have younger lineages been the only newcomers to our attention… while older lineages have been well established for some time?
What can reptiles (well, squamates) tell us about the history of taxonomy?
Blackburn et al. describe that some lineages do differ drastically in their history of descriptions… a by-product of not the animals, but the researchers that love them. Take, for example, pleurodontan lizards (iguanas, collared lizards, casquehead lizards) and snakes (Serpentes)… which show a certain kind of what the authors call ‘taxonomic maturation’ whereby our knowledge of radically different lineages within pleurodonta and snakes have not changed appreciably in well over a hundred years. This doesn’t mean that we haven’t discovered new iguanas or snakes, rather that the species we have described are very similar to species that were named decades ago. Geckos (Gekkota) and frogs (Ranoidea) on the other hand, exhibit a drastically different pattern from iguanas and snakes – truly radical lineages continue to be found in recent history, almost at random. What this suggests (besides that there are seemingly many geckos in many places) is that researchers may not yet have a holistic grasp on what the sum-total gecko diversity is yet.
Blackburn et al. close with an interesting question: ‘Is it worth focusing on species discovery and description for a given clade? Or, instead, is the diversity sufficiently known where investigating evolutionary patterns will be worthwhile because most major lineages are probably now known?’ The authors argue that for most groups of organisms, we’re adding new species towards the tips of the tree – new species that are closely related to more well-known (or at least, less recently described) species. BUT this is not consistent across the Tree of Life, rather there are many clades for which we still know very little.
By Dr. Matt Kolmann (@KolmannMA)
Dr. Kolmann is an NSF-funded Post Doctoral Research Fellow at George Washington University studying the biomechanics of skulls, teeth, and muscles in piranhas, pacus, and their relatives. He is interested in how evolution has shaped predators according to the properties of what they eat, using any and all fishes as study systems. He gained a grudging appreciation for hummingbirds, now grown to full-blown admiration, working in the lab of his avian collaborator and committee member… on stingray feeding. https://mattkolmann.jimdo.com/