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What's a "Species"?

5/29/2015

 
The naming of a new species of hominid -- Australopithecus  deyiremeda -- made a lot of news this week.  The purpose of this post is not to worry over the details of the fossils that were used to construct this new taxon, but to ask for some clarification about what is actually meant by the term "species" as paleoanthropologists use it.  I'm going to tell you what I think it means, then I'm going to complain about it a little bit, then I'm going to ask you to tell me what you think it means. (Full disclosure: I fall at the lumper end of the lumper-splitter spectrum, and I think there are too many named "species" in our family tree.) 

In their paper, titled "New Species from Ethiopia Further Expands Middle Pliocene Hominin Diversity" (Nature 521:483-488), Yohannes Halie-Selassie and colleagues use the word "species" 17 times but provide no explicit definition of the term.  What is a "species"?  What are the implications of defining a "new species" of hominid?

Like so many other things, it depends. There exists a smorgasbord of different species concepts to choose from. A "typological species," for example, is a classification based on the co-occurrence of shared features, while an "evolutionary species" is defined based on the integrity of an ancestral lineage (without branching, there is no new species).  A "biological species" is generally defined as a group of organisms that can breed with one another but not with other groups of organisms.  In other words, a biological species is reproductively isolated from all other biological species.  The biological species concept is perhaps the one most frequently applied in biology, especially to living populations of plants and animals. 

I think a "biological species" is also what most people, paleoanthropologists included, mean when they talk about "species" of hominids. 

The distinctions among the various species concepts are not just academic when they're applied to fossil hominids.  They have implications for our notions about what variability means in the fossil record and how we interpret that variability in terms of the patterns and processes of human evolution.  Because reproductive isolation is the entire basis of the biological species concept, individuals in a biological species (by definition) could not and did not interbreed with any of their contemporaries outside their own species.  There cannot be multiple, co-existing species of human ancestors: a "new species" is either a human ancestor or somewhere off on a side-branch of our evolutionary family tree.  The discovery of a human ancestor that pushes someone else off onto a side branch is much more exciting that the discovery of another non-contender. You can see that in the enthusiasm of headlines about Australopithecus deyiremeda such as "Doubt cast on Lucy's place in human evolution" and "New hominid discovery older than Lucy raises more questions on human ancestry."  They might as well read "Don't let the door hit you in the ass on way out, Lucy."

I'm not an expert on paleoanthropology, and I've never directly analyzed or attempted to describe or classify the remains of a fossil hominid.  But I am someone who regularly attempts to describe variability (mostly in lithic artifacts) and make sound interpretations about what that variability means.  In any case, when you're looking at continuous variability, you can split all you want. There is easily detectable variability in just about everything not produced by a machine, so ultimately it's no great feat to break continuous variability down into as small of groups as you want (groups of one, if that makes you happy).  But what do those groups mean?  That's a hard question to answer without having a sample of a decent size that let's you investigate how the variability you're looking at is structured. That's why I'm a fan of trying to understand how variability is structured before trying to create groupings that have some analytical value. 

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The pitfall of aggressive splitting of the fossil record is that the groupings you produce (the biological species) are tied to important assumptions about the processes that produced those groupings.  While I think the biological species concept is one that is clear and makes ecological and evolutionary sense, I also think its uncritical application to the fossil record is less than useful.  First, I think it's impossible to operationalize consistently and objectively (how can you determine if the populations represented by fossil individuals were capable of inter-breeding?).  So I don't trust that "species" that are equivalent in taxonomic terms reflect populations that are equivalent in evolutionary terms.  Second, I think naming lots of "species" short-circuits our study of what variability in the fossil record means by erring on the side of attributing it to species-level differences.  Under the biological species concept, species are non-overlapping, nominal categories, and speciation is a one-way street. By calling a fossil hominid a new biological species, you are making a statement about the nature of the relationship between that fossil and all other fossil hominids.  Given the sparse nature of the fossil record, I don't think that we can really make those kinds of statements with much confidence.  Using the scalpel of the biological species concept ties us to those assumptions, however.

So I'm very skeptical of the reality of the number of named species that currently inhabit the hominid family tree. How many are there now?  Twenty?  Thirty?  More?  I wonder what would happen if we started fresh and re-analyzed all the Pliocene and Pleistocene hominid fossils discovered over the last 120 years.  What would the structure of variability look like, and how would we interpret that variability if we erased all the existing species names and the historical legacies of discovery that accompanied them and based our groupings on patterns of variability across time and space? Who knows. I also wonder how many "species" of domestic dogs paleoanthropologists would define given a sampling of their bones.

Anyway, splitters be splittin,' and there's not much I can do about it.  When I taught my 200-level Human Origins class last year, I made the decision to focus not on the minutiae of "species" in the fossil record, but on what various lines of evidence could tell us about the timing, processes, causes, and effects of changes in human anatomy and behavior over evolutionary time.  We talked about species concepts and why they matter, and I gave my class my opinion that we're too quick to name new species and perhaps too reluctant to first look at what variability might mean outside the constraints of a species-level classification.  Is Homo antecessor a legitimate "biological species"?  Is the Homo erectus/ergaster division useful? If we know that Neanderthals and Homo sapiens exchanged DNA, why are we still calling them separate species?

Maybe I've wrongly identified the dominant species concept that is active in the background of paleoanthropological thought.  Maybe a new name isn't meant to assume reproductive isolation and all that that implies. If I've gotten it wrong, please correct me.  Maybe I missed something somewhere.  In my own published work, I've been asked to provide clarifying definitions for such controversial terms such as "household," "process," "model," and "projectile point."  President Clinton famously debated the meaning of the word "is." Is it too much to ask for a clarifying definition of "species" when we define a new one? 


Update (6/5/2015): This post was discussed in Barbara King's blog post for NPR titled "Declaring The Discovery Of A New Species Can Get Tricky."

ResearchBlogging.org
Haile-Selassie Y, Gibert L, Melillo SM, Ryan TM, Alene M, Deino A, Levin NE, Scott G, & Saylor BZ (2015). New species from Ethiopia further expands Middle Pliocene hominin diversity. Nature, 521 (7553), 483-8 PMID: 26017448
Bob Jase
5/29/2015 04:27:17 am

I thought a biological species was a baramin that had been excommunicated.

For the little its worth, I agree that there is a lot of splitting going on. No one cheers for the discoverer of the fifth or sixth specimen of a named type but the first specimen, now that's special! You get to name it, even after yourself, and there are interviews and the crowd applauds.

I think this is evidence that scientists are actual members of H. sapiens, a species that often craves attention.

Kristin Wilson
5/29/2015 09:31:01 am

Yes. Exactly. This post will be required reading for my intro to biological anthropology in the fall.

Wes Copas
5/29/2015 10:21:22 am

In my sophomore-level physical anthropology class, I differentiate between "biospecies" (biological species as discussed in the article above) and "paleospecies," or arbitrary groupings of extinct organisms based solely on morphological characteristics - which may or may not reflect the biospecies that existed in the past. One of the great pitfalls in paleo-anything is confusing these two very different ways of classifying organisms. The second great pitfall is trying to reconstruct phylogenetic history of an extant biospecies based on minuscule sample sizes of fragmentary fossils. DNA evidence adds a wonderful new source of information, but this new area of study is limited in the same way: how can we say that Neandertals are a separate species from us (Homo sapiens), based on so few sequenced genomes? The end result of this, as I teach the subject, is that other than in general terms, we can't reconstruct phylogenetic relationships - or extinct hominin biospecies. Attempting to do so is a fascinating exercise, but without a vastly larger database, that's all it is - an exercise. If this is understood, the students are able to concentrate on evolutionary trends in biology and behavior, which is the most important thing to grasp at the introductory level. That's how I do it...

J.Mallet link
5/29/2015 01:55:46 pm

I'm also pro-lumping species and anti-splitting. However, a too-strict interpretation of the horribly named "biological species concept" is in my view not a good idea.

Darwin realized that species definitions are arbitrary, given they evolved gradually from other species. He was right. WE need to decide what is we mean by a species; biology does not make the distinction.

Darwin's solution was to use the same divisions that previous taxonomists had made between species (without understanding evolution) since Linnaeus, and before. He developed a theory that would explain the origins of the clusters of biological phenotypes people had called species, even though, at that time, these taxonomists mostly believed they species had been created by a benevolent God.

The problem with the later "biological species concept" (BSC) of the mid-20th Century is that it didn't seem to allow hybridization. Darwin argued against a reproductive isolation or "biological species concept" definition of species in his chapter "Hybridism." But later authorities misunderstood this chapter.

More recently, Jerry Coyne & H. Allen Orr argued for the BSC:
"In our view distinct species are characterized by substantial but not necessarily complete reproductive isolation. We thus depart from the 'hard line' BSC by recognizing species that have limited gene exchange with sympatric relatives."

The problem Coyne and Orr knew about was that the entities we'd like to call species, in birds, butterflies, plants, Anopheles mosquitoes, maybe even hominins, do hybridize (occasionally). Introgression occurs, meaning genes flow between species, and over time, the effects of gene flow on genomes accumulates, sometimes in a major way, without totally obliterating the strongly selected differences between species. Today, the more we sequence genomes, the worse this "problem" for the BSC becomes.

For instance, horses and donkeys that split 5 My ago have hybrids, mules, that are occasionally fertile (though they are normally sterile). Recent genomic analyses give evidence of gene flow between these ancestral species as well as among more recent species in the donkey-zebra lineage. See Jónsson et al 2014. http://www.pnas.org/content/111/52/18655.abstract

So how much gene flow between species should we allow? Coyne and Orr do not specify this.

A simpler idea of what we mean by species might be to go back to Darwin's idea that species are clusters. Today, we would argue for clusters of multilocus genotypes, or "genotypic clusters" as the taxa that we can recognize as species. Species are defined by gaps in variation that, under total panmixia, should be more continuous (the differences between species perhaps determined at relatively few genes scattered across the genome; other genes may flow more freely). If they can retain their clusterhood, in spite of gene exchange at large fractions of their genome, (maybe even more than half their genome, provided some important "species determining" traits still remain mainly distinct), then we can call them separate species.

I'm sympathetic to lumping when we only have a single specimen or very few. Your diagram with the morphing colours was very apt. Genotypic clusters cannot be detected without a reasonably large samples (say 5) of each taxon to ensure that there is strong association between multiple phenotypic characters or genetic marker traits, leading to clustering of genotypes.

Denisovans were identified based on a single bone, and also now extended to two teeth. Were they separate species from their closest relative, the Neanderthal? Were Neanderthals themselves separate species from humans? My Harvard genomics colleagues in this field don't really care, because the "species" label adds zero explanatory power to their work (I've asked them about this very question). In my view Neanderthals, believed to have a long history of coexistence with modern humans, probably were separate species (in the Darwinian sense above), because we have evidence for extensive coexistence of the two phenotypes before the eventual demise of Neanderthals.

But the Denisovans vs. Neanderthals question is hard to answer since it was the genetic data from so few specimens that led to the original identification of this new archaic hominin.

Right now, maybe we'd better not try to decide which of these is a species. Which is the main, and I think, most important point you make about doubting the new fossil evidence for a new Australopithecus.

@eratosignis

Zach Throckmorton
5/29/2015 05:45:56 pm

Great post, Andy. Thank you for producing and posting it. And nice comment, J.

Ultimately, species names are somewhat arbitrary labels we put on nature to suit our own purposes. Since those purposes vary, the way we label varies. I have my students read de Queiroz's 2007 paper (http://statgen.dps.unipi.it/courses_file/GdP/Papers/DeQueiroz_SpeciesConceptDelimitation_Syst.Biol.2007.pdf), mostly for its Table 1. I also like Mayden's 1997 chapter (http://philpapers.org/rec/MAYAHO-6), but it's a bit technical for undergrads and young grad students.

Randall Hayes link
6/2/2015 12:23:44 am

I really like the picture. This is a great way to illustrate something like the body size differences between us and Neanderthals or the height differences between us and the Flores 'hobbits.' Placing the actual data on a graph makes things clearer for students.

Jack Hunt
6/25/2015 11:20:41 am

Marvelous to find this. I've been making many of these same arguments for years.

I'll try to avoid repeating arguments already made

My first point is that we need to emphasize cladistics in taxonmy. Humans are monkeys Sharks aren't fish. Paraphyletic clades are invalid, and give people the wrong idea about evolutionary relationships.

You can't have Homo Sapiens arising from H Sapiens and H Neandertalis. You can't have H Sapiens deriving from H Erectus and then in- breeding ( a suggestion from Chris Stringer 's latest book).

We do need to revisit the whole idea of species. It is really a creationist concept and not an evolutionary concept. Species go extinct on a regular basis and yet leave descendents. 5 million years ago, there is some hominid who is your direct ancestor. In fact, you have an unbroken string of ancestors going back billions of years. You wouldn't call most of them human. We chop out little bits of that time line and call the individuals within a species, and yet, under that idea, my H Erectus great grandmother gives birth to a H Sapiens child. Ridiculous of course.

A final point is the politics of species Unfortunately, in the US at least, environmental protections are built around protecting species, and so, if you are trying to protect a habitat, it's far better to be pritecting a species than a sub species

jmv2009 link
10/27/2019 08:11:29 am

Just don't use "species", as it has too many unintended connotations. Use clades w/ admixtures.


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