Monday, May 6, 2013

The lungless salamander and the historical narrative

David Haskell's The Forest Unseen: A Year's Watch in Nature is a most beautiful book.  Haskell teaches biology at the University of the South, in Sewannee, Tennessee.  For the book he observes a single square meter of Tennessee forest for a year, his tools being a magnifying glass, binoculars and a breadth of knowledge and powers of observation that let him see much more in that square meter, which he calls a mandala, than most of us ever could.  Fortunately for us, he's also an expert story teller.

Plethodontid; Allen Blake Sheldon, Animals Animals in
the New York Times
Haskell writes about moss, hoof prints of a deer, a tiny snail, warblers, wildflowers, the smell of leaves in autumn, the cycling of the seasons.  His chapter about the Plethodon he observed crawling through his mandala one day caught our attention, a salamander of a family that lives in North, Central and South America, southern Europe and Sardinia.  The Plethodontidae comprise the largest family of salamanders known; some are aquatic, some semi-aquatic, some live entirely on land, some in trees.  Haskell's Plethodon was a land dweller, a small salamander just twice the size of his thumbnail, an amphibian, yet one that never sees the water even in its larval stage. And, oddly enough, it's got no lungs, instead respiring through its skin and the lining of its mouth.
No one knows how Plethodon salamanders arrived at their lungless condition. Their relatives all have lungs, although those that live in mountain streams have rather small lungs.  Cold streams have plentiful oxygen, so stream-dwelling salamanders can use their skin as a breathing organ.  Perhaps the terrestrial lungless salamanders evolved from these stream-dwelling kin?  This was biologist' favorite explanation until researchers looked more closely into the geological record. 
Indeed, a 1920 paper (Wilder, I. W. and E. R. Dunn. The correlation of lunglessness in salamanders with a mountain brook habitat. Copeia 1920:63-68) proposed that these salamanders were lungless as an adaptation to fast-flowing streams in Appalachia.  Lungs would make them too buoyant, and too likely to drift downstream, so the story went, so better to ditch the lungs. It wasn't until the 1980's and 90's that this was challenged by geologic, paleontological, and other evidence (e.g., here).  In addition, the suggestion has been made that they in fact evolved on land, not in water.
The rocks told an inconvenient story: the eastern mountains were small undulations when Plethodon salamanders evolved.  Sunch gentle includes could not have produced the cold, rapid streams inhabited by the small-lunged salamanders.  So, we are left without a historical narrative for the Plethodon's lungless condition.
Well, maybe not.  In fact, we've got a perfectly serviceable historical narrative, and that is genetic drift.  Unless there is specific evidence to the contrary, it could very well be that some mutation(s) led the ancestors' skins to absorb oxygen.  There need have been no particular advantage to this, and indeed if there was an advantage it may have been related to some other effect of the genetic change since, after all, the individuals had already been able to breathe using their lungs!

But once established, perhaps by chance in some small local group, or favored for some other reason, the individuals were able to breath without using lungs.  There need have been no disadvantage to lungs, but if it so happened that lungs weren't needed, then mutations disrupting their development would not have been selected against. It is of course possible that some disadvantage accrued, but it's as plausible that they simply didn't need lungs.

The problem with seeking plausible selective explanations for traits we see to day is that we feel compelled to do that.  Since Darwin's brilliant insight as to how life could have diverged from a single common ancestor, the default explanation has changed from "God willed it so" to "Selection made it so."  Selection is a real kind of factor in reproductive success, but it isn't the only one, and in fact the default explanation should be genetic drift -- things arose by chance -- and the burden of proof should be on those who want to suggest otherwise.

Selective explanations may be correct, and seem sometimes obvious, but our ability to understand or reconstruct selective scenarios for organisms that evolved in the long, deep past is very limited, and often there is little evidence to allow us to chose between fiction and such scenarios.  It is in this sense that excessive, or reflex, or assumed seletionism is a scientific flaw.

The subtle point, perhaps, is that some selective explanations may be correct, but we invoke them as if we actually knew what they were or even when they were called for.  And that is where science becomes dogma, and Darwin is treated like a causal God.

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