Birds of a feather

The word species has got to be the mushiest term in all of science.

It's one of those situations where you think you know what something means until you start pushing on it.  When humans started to put a serious effort into categorizing other life forms -- Aristotle is usually credited with being the first to do this in a systematic way -- it seemed obvious enough.  Members of a species are similar morphologically.  Put more simply, you can tell a cat from a dog because they look different.

The problem is, this starts to cause problems just about immediately.  What about organisms that look very different, but we still consider to be the same species?  Dogs, in fact, are a good example.  Imagine you're an alien scientist arriving on Earth, and you're looking at a St. Bernard and a chihuahua.  If a human said, "These are the same species," my guess is you'd do whatever passes for laughter on your home world, then get back in your spaceship and fly away after writing "No intelligent life" on the map of the Solar System.

Dogs, of course, aren't the only ones; there are lots of examples in nature of different-looking organisms that are considered conspecific.  So in the 1800s, the definition was revised to, "a group of organisms that are capable of mating and producing offspring."  This worked until people started to think about mules, which are the offspring of a horse and a donkey (usually considered separate species).  Then, it was pointed out that although alive and well, (most) mules are infertile, so a word was added to take care of that problem: "a group of organisms that are capable of mating and producing fertile offspring."

It only got worse from here.  An awkward difficulty with the above definition is, what about asexual species?  They kind of don't fit in no matter how you look at it.  Oh, well, maybe they get their own version of the definition.  But what about ring species?  This is a group of populations, often arranged in a ring around a geographical barrier (thus the name) where all of them can interbreed except for the ones at the "ends" of the ring  It's been observed multiple times, including a group of salamanders in California, the Greenish Warbler of central Asia, and a ring of gull species -- the latter of which goes all the way around the world.

So do these represent one species, or many?  Within the ring, some of them are interfertile, and others aren't.  And splitting the ring doesn't help; then you're separating populations that are interfertile.  In fact, like asexual species, ring species seem to be unclassifiable with the canonical definition.

It all comes, my evolutionary biology professor said to us, from the desperation humans have to pigeonhole everything.  "The only reason we came up with the concept of a species in the first place," he said, "is because humans have no near relatives."

Of course, none of this sits well with the creationists, because a central tenet of their beliefs is that each kind of life form was created by God as-is and nothing's changed since.  Which is all well and good until you ask, "What do you mean by 'kind of life form'?"  They respond that God created "discrete forms with genetic boundaries to interbreeding," which they call baramins (a neologism coined from the Hebrew words for "created" and "kind").  So the ring species of gulls isn't a problem because gulls are a "kind."  In fact, you can define "kind" in this context as "a classification of life forms that conveniently makes all of the internal contradictions go away.  Now stop asking questions."  

In any case, there really is no good, consistent definition of species that covers all the exceptions.  Even now that we have genetic analysis -- which is currently the touchstone for classification -- it only further reinforces the fact that evolution generates a continuum of forms, and you're asking for trouble if you try to subdivide them.  Only in cases like ourselves, where there are no living near relatives, does it seem clear-cut.

Take the study out of the University of Colorado that appeared in Nature Communications this week.  It's about a trio of species of birds, so being a rather fanatical birder, it immediately caught my eye.  The species involved (and I use that term guardedly, for reasons that will become obvious) are the Common Redpoll, (Acanthis flammea) the Hoary Redpoll (Acanthis hornemanni), and the Lesser Redpoll (Acanthis cabaret), all types of finch with a characteristic red splotch on the forehead.  

Common Redpoll (Acanthis flammea) [Image licensed under the Creative Commons Cephas, Carduelis flammea CT6, CC BY-SA 3.0]

The Lesser Redpoll is only found in Europe, but the other two occur in North America.  They have pretty obvious color differences; the Lesser Redpoll is brownish, the Hoary Redpoll is almost white, and the Common Redpoll is somewhere in the middle, with reddish flanks.  The size differs, as well, with the Lesser at the small end and the Hoary at the large end.

Lesser Redpoll (Acanthis cabaret) [Image licensed under the Creative Commons Carduelis_cabaret.jpg: Lawrie Phipps derivative work: MPF (talk), Carduelis cabaret1, CC BY 2.0]

However, the differences aren't huge.  We get Common Redpolls at our bird feeders in winter fairly regularly, but Hoary Redpolls are a rare sighting in our area.  Every winter I scan the flocks of redpolls looking for whiter individuals, but I still have never seen one.  However, I may be able to cross that one off the list of "species I haven't seen" -- because the current study has shown that despite the differences in appearance, all three are a single species.

Hoary Redpoll (Acanthis hornemanni) [Image licensed under the Creative Commons Ron Knight from Seaford, East Sussex, United Kingdom, Arctic Redpoll (Acanthis hornemanni) (13667519855), CC BY 2.0]

The color and size differences, the researchers found, are due to a "supergene complex" -- a single cluster of genes that work together to produce a specific phenotype.  What's striking is that despite the differences in that gene complex between the three different groups of redpolls, they are otherwise about as genetically identical as it's possible to get.  And... they're all potentially interfertile.

"Often times we assume that a lot of traits can act independently, meaning that different traits can be inherited separately from one another, but this particular result shows that sometimes these traits are actually tightly linked together," said Erik Funk, lead author on the paper, in an interview in Science Daily.  "At least for these birds, they're inheriting a whole group of traits together as one."

Birders tend to hate it when confronted with "lumpers," as they call researchers who merge species together, therefore reducing the number of potential birds to chase after.  They much prefer "splitters," who take previously single species and subdivide them, like another "winter finch," the Red Crossbill (Loxia curvirostra), which according to some taxonomists isn't a single species but several -- possibly as many as seven.  In any case, my point here is that this kind of thing happens all the time.  Like I said at the beginning, we think we have a clear idea of what's meant by a species until we start examining it.

But to me, this only increases my fascination with the natural world.  It's a beautiful, subtle, and complex interlocking web of organisms, and maybe the most surprising thing of all is that we do think it should be simple and easily classifiable.  As usual, our scheme for understanding the world turns out to be woefully inadequate -- and once again, science has come to the rescue by turning a lens on a small and unassuming bird as a way of pointing out how much more we have to learn.

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As I've mentioned before, I love a good mystery, which is why I'm drawn to periods of history where the records are skimpy and our certainty about what actually happened is tentative at best.  Of course, the most obvious example of this is our prehistory; prior to the spread of written language, something like five thousand years ago, most of what we have to go by is fossils and the remnants of human settlements.

Still, we can make some fascinating inferences about our distant ancestors.  In Lost Civilizations of the Stone Age, by Richard Rudgely, we find out about some of the more controversial ones -- that there are still traces in modern languages of the original language spoken by the earliest humans (Rudgely calls it "proto-Nostratic"), that the advent of farming and domestication of livestock actually had the effect of shortening our average healthy life span, and that the Stone Age civilizations were far more advanced than our image of "Cave Men" suggests, and had a sophisticated ability to make art, understand science, and treat illness.

None of this relies on any wild imaginings of the sort that are the specialty of Erich von Däniken, Zecharia Sitchin, and Giorgio Tsoukalos; and Rudgely is up front with what is speculative at this point, and what is still flat-out unknown.  His writing is based in archaeological hard evidence, and his conclusions about Paleolithic society are downright fascinating.

If you're curious about what it was like in our distant past, check out Lost Civilizations of the Stone Age!

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Fishy business

My evolutionary biology professor told our class, many years ago, "The only reason we came up with the word species is because humans have no near relatives."

It's a comment that has stuck with me.  We perceive species as being these little cubbyholes with impenetrable sides, and once you've filed something there, it stays put.  Of course a polar bear and a grizzly bear are different species.  How could they be otherwise?

But when you start pushing at the definition a little, you find that it gives way almost immediately.  Ask some non-scientist how they know polar bears and grizzly bears are different species, and you'll likely get an answer like, "Because they look completely different."  And, to be fair, that's more or less how the father of taxonomy, Carl Linnaeus, did it.

Problems creep in almost immediately, though.  The "of course different species" polar and grizzly bears look far more alike than do, say, a chihuahua and a St. Bernard.  (Imagine trying to convince an alien biologist that those two are members of the same species.)  So very quickly, scientists were forced into refining the definition so as to capture the separateness of two different species in such a way that the term could be applied consistently.

What they ultimately landed on was the canonical definition used in just about every biology textbook in the world: "Members of the same species are capable of potentially interbreeding and producing viable and fertile offspring."  (The "fertile" part had to be added because of the famous example of a horse and a donkey being able to produce a viable hybrid -- but that hybrid, the mule, is almost always infertile.)

The problem was, even that wasn't enough to clarify things. Polar bears and grizzly bears, for example, can and do hybridize in the wild, and the offspring (the rather unfortunately-named "pizzly bear") are almost always fertile.  This isn't an aberration.  These kinds of situations are common in the wild.  In fact, in my part of the world, there are two birds that look dramatically different -- the blue-winged warbler and the golden-winged warbler -- but they will happily crossbreed.  When the hybrids were first observed by scientists, they were different enough from both parents that it was thought they were a third separate species, which was called Brewster's warbler.  It was only after long observation that biologists figured out what was going on -- especially given that "Brewster's warblers" are potentially interfertile with either parental species.

In fact, the more you press the definition, the more it falls apart, the more exceptions you find.  Today's taxonomists are usually wary about labeling something a "species" -- or when they do, they're aware that it's potentially an artificial distinction that has no particular technical relevance.  They are much more comfortable talking about genetic overlap and most recent common ancestry, which at least are measurable.

The reason all this comes up is because of a startling discovery brought to my attention by a friend and long-time loyal reader of Skeptophilia.  Researchers in Hungary have produced a hybrid between an American paddlefish and a Russian sturgeon -- two species no one could confuse with each other -- and they appear to be fertile, and normal in every other way.

The more you look at these "sturddlefish," the more shocking they get.  Sturgeon and paddlefish are not only separate species, they're in separate families -- two layers of classification above species.  "I’m still confused," said Prosanta Chakrabarty, ichthyologist at Louisiana State University.  "My jaw is still on the floor.  It’s like if they had a cow and a giraffe make a baby."

He quickly amended that statement -- giraffes and cows have a recent common ancestor only a few million years ago, whereas paddlefish and sturgeons have been separate lineages for 184 million years.  To get anything comparable, Chakrabarty said, you'd have to have something like a human coming out of a platypus egg.

The scientists believe that the reason this happened is because of the relatively slow rate of evolution of both lineages (especially the sturgeons).  Sturgeons now look pretty similar to sturgeons two hundred million years ago, while almost all of the mammalian biodiversity you see around you -- divergence between, say, a raccoon and a squirrel -- happened since the Cretaceous Extinction, 66 million years ago.  But even so, it's pretty remarkable.  To my eye, paddlefish and sturgeon look way more different than lots of pairs of species that can't interbreed, so once again, we're confronted with the fact that the concept of species isn't what we thought it was -- if it has any biological relevance at all.

Atlantic Sturgeon (Acipenser oxyrhynchus)

American Paddlefish (Polyodon spathula)  [Both this and the above image are in the Public Domain]

This brings us back to the unsettling (but exciting) fact that whenever we think we have everything figured out, nature reaches out and astonishes us.  It's why I'll never tire of biology -- to paraphrase Socrates, the more we know, the more we realize how little we know.

But one thing I know for sure is that the biologists really need to come up with better names than "sturddlefish" and "pizzly bear."

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This week's Skeptophilia book recommendation of the week is by the brilliant Dutch animal behaviorist Frans de Waal, whose work with capuchin monkeys and chimps has elucidated not only their behavior, but the origins of a lot of our own.  (For a taste of his work, watch the brilliant TED talk he did called "Moral Behavior in Animals.")

In his book Mama's Last Hug: Animal Emotions and What They Tell Us About Ourselves, de Waal looks at this topic in more detail, telling riveting stories about the emotions animals experience, and showing that their inner world is more like ours than we usually realize.  Our feelings of love, hate, jealousy, empathy, disgust, fear, and joy are not unique to humans, but have their roots in our distant ancestry -- and are shared by many, if not most, mammalian species.

If you're interested in animal behavior, Mama's Last Hug is a must-read.  In it, you'll find out that non-human animals have a rich emotional life, and one that resembles our own to a startling degree.  In looking at other animals, we are holding up a mirror to ourselves.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

All in the family

Over my nearly thirty years of teaching AP Biology, one of the topics that changed the most was taxonomy.

This might come as a surprise, given the changes in fields such as genetics, but honestly the two are closely related.  When I started my career, classification of species was done primarily by morphology (shape and structure), and the identification of which characteristics of an organism were plesiomorphies (structures inherited from, and therefore shared with, the ancestral species) and which were apomorphies (structures that were innovations unique to a single branch of the family tree).

One of many difficulties with this approach is that useful innovations can evolve more than once, and therefore aren't necessarily indicative of common ancestry.  This process, called convergent or parallel evolution, can generate some amazingly similar results, the most striking of which is the flying squirrel (a rodent) and the sugar glider (a marsupial), which look nearly identical at a quick glance (even a longer one, honestly).  To be fair, the fact that the two are not very closely related would be evident on any kind of moderately careful analysis, where giveaways like tooth structure and the presence of a pouch in the female sugar gliders would be enough to show they weren't on the same branch of the mammalian family tree.

Southern flying squirrel (top) [Image is in the Public Domain] and sugar glider (bottom) [Image licensed under the Creative Commons Joseph C Boone, Sugar Glider JCB, CC BY-SA 4.0]

But sometimes it's more difficult than that, and more than once taxonomists have created arrangements of the descent of groups of species only to find out that further study shows the original placement to be wrong.  As one of many examples, take the two groups of large-eyed nocturnal primates from southern and southeastern Asia, the lorises and tarsiers.  Based on habits and range, it's understandable that they were lumped together as "prosimians" on the same branch of the primate tree, but recent study has found the lorises are closely related to lemurs, and tarsiers are closer to monkeys and apes -- despite the superficial similarity.

Slow loris [Image licensed under the Creative Commons David Haring / Duke Lemur Center, Sublingua of a slow loris 001, CC BY-SA 3.0]

 

Tarsier [Image licensed under the Creative Commons yeowatzup, Tarsier Sanctuary, Corella, Bohol (2052878890), CC BY 2.0]

These revisions, and the sometimes surprising revelations they provide, have largely come from a change in how taxonomy is done.  Nearly all classification is now based upon genetics, not structure (although certainly structure plays a role in who we might initially hypothesize is related to whom).  But when it comes down to a fight between morphology and genetics, genetics always wins.  And this has forced us to change how we look at biological family trees -- especially when genetic evidence is obtained where it was previously absent.

This all comes up because of a discovery of intact DNA in a fossil of a primate much closer to us than the tarsiers and lorises -- a species from our own genus called Homo antecessor.  The species name suggests it was one of our direct ancestors, which is a little alarming because there's good evidence it was cannibalistic -- bones of the species found in Spain showed clear evidence of butchering for meat.

Now, however, the recovery of DNA from a tooth of an H. antecessor fossil -- at 800,000 years of age, the oldest DNA ever recovered from a hominid fossil -- has shown that it probably wasn't our ancestor after all, but a "sister clade," one that left no descendants.  (Bigfoot and the Yeti notwithstanding.)  The study was the subject of a paper in Nature last week, authored by a team led by Frido Welker of the University of Copenhagen, and required yet another reconfiguring of our own family tree.  The authors write:

The phylogenetic relationships between hominins of the Early Pleistocene epoch in Eurasia, such as Homo antecessor, and hominins that appear later in the fossil record during the Middle Pleistocene epoch, such as Homo sapiens, are highly debated.  For the oldest remains, the molecular study of these relationships is hindered by the degradation of ancient DNA.  However, recent research has demonstrated that the analysis of ancient proteins can address this challenge.  Here we present the dental enamel proteomes of H. antecessor from Atapuerca (Spain) and Homo erectus from Dmanisi (Georgia), two key fossil assemblages that have a central role in models of Pleistocene hominin morphology, dispersal and divergence.  We provide evidence that H. antecessor is a close sister lineage to subsequent Middle and Late Pleistocene hominins, including modern humans, Neanderthals and Denisovans.  This placement implies that the modern-like face of H. antecessor—that is, similar to that of modern humans—may have a considerably deep ancestry in the genus Homo, and that the cranial morphology of Neanderthals represents a derived form.

I find that last bit the most interesting, because it turns on its head our usual sense of being the Pinnacles of Evolution, clearly the most highly evolved (whatever the hell that actually means) species on the planet, definitely more advanced in all respects than those brute Neanderthals.  What this study suggests is that the flatter face of the Neanderthals is actually the apomorphy -- the more recently-evolved, "derived" characteristic -- and our narrower, more protruding faces are a plesiomorphy, inherited from our older ancestors.

This kind of stuff is why I'm endlessly interested in evolutionary biology -- as we find more data and develop new techniques, we refine our models, and in some cases have to overturn previously accepted conventional wisdom.  But that's what science is about, isn't it?  Basing your model on the best evidence you've got, and revising it if you get new and conflicting evidence.

 Just as well in this case.  One less cannibal in the family tree.  Not that there aren't probably others, but my genealogy already contains some sketchy enough characters.  No need to add more.

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This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago.  Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.

Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet.  It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike.  Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.