Friday, December 14, 2012

Cat(fish) among the pigeons! Now how would an 'evolutionary biologist' explain it??

Here's a story -- catfish have been observed actually sneakily stalking, seizing, and eating pigeons!  Yes, real feathery pigeons hanging out on the riverbank, not some sort of pigeon-fish.  Here is the story that appeared in the journal PLoS One,  that has understandably been all over the news this week.

Now this is very interesting, not least because it's not clear how the fish do it.  Their vision is not generally thought to be good enough to be up to the task of spotting, then grabbing pigeons.  Instead, the 'whiskers' on the front of their face, technically called 'barbels' and from which the fish get their feline name, are sensory structures.  One explanation for their ability to hunt as predatory carnivores, and indeed prey on land-based, flying-capable, much more biologically advanced(?) birds, is that the barbels can pick up some aspect of the motion of the pigeons, perhaps the vibration that the birds make when they are drinking.

This may be so, but the video doesn't suggest that the pigeons, who appear to be searching for food at the river-bank's edge, are moving around or doing anything that might be detectable in that way.  Be that as it may, the fish are definitely succeeding as predators.

An evolutionary explanation?
Now, what would your normal Darwinian, selection-obsessed biologist (including reporters for leading science journals like the BBC and NYTimes) make of this?   Well, of course, they'd have to adopt some story about how the barbels evolved motion-sensing capability in times when water-based prey or other foods were unavailable, or for some other reason that led those with more sensible barbels lived longer, grew faster, had more spawn, and so on.  Over eons of time, the sensitive barbels evolved, and their duller compatriots went the way of all fish.

Such cock-sure explanations are the stock and trade of 'evolutionary biologists', a self-assigned title that seems to entitle a person to spin off any Just-So story that s/he can dream up. Darwinian adaptation via natural selection does seem to be real, and important, even if its actual day-to-day mechanism is much more elusive than is routinely assumed, and even if we can't really be sure of the 'why' of  adaptation even if we can be sure of the 'what' as it appears today.  That is, we can see what a structure can do, or does do, today, and nothing stops us from also 'assuming' that that same function is what the structure was evolved by natural selection for.  Indeed, we are making a leap of faith to assume that the function is what the structure is for today.

That would be like assuming my thumb is 'for' hitting the space bar, and evolved for that purpose back in Neandertal days.  Clearly my thumb is useful for such tool-related things, but just as surely it didn't evolve specifically for this type of use--or, perhaps, any type of tool use (though tool-use is certainly a plausible explanation).

Well, maybe not Just-So after all!
Unfortunately for our evolutionary biologist (or journalist echoing his/her story), these fish were observed pigeon-hunting in France, but are known historically to have been introduced from eastern Europe only just over a century ago.  They are, as far as we can tell from the paper, or another paper on the introduction of these fish into places like France (reference 11 in the PLoS One paper we link to above), not known to prey on pigeons or shore-wandering birds in the Old Country.

No surprise, the authors delve right away into showing, with a certain amount of technical flash, that the fish seem to be dividing up their ecology so only some prefer roast pigeon, others dine on more traditional fare.  And, right off the bat, they offer speculation about fitness and natural selection and adaptive strategies (with no direct evidence--as the authors acknowledge).  Even in something so quick to arise and so clearly behavioral, there seems to be a need to show that it simply must have 'evolutionary' importance.  It can't just be choice and intelligence and adaptability.

It may turn out that there are similar scenarios of birds on the river banks in the fish's homelands, and that the French fish's relatives do hunt them.  This could tend to confirm the story, and indeed it might add to its plausibility.  But the birds might be recent shore-dwellers there, thanks to human occupation or alteration of the local ecologies, too recent for selective stories to have much credence.  Or, the fish there could just be trolling without any genetic basis involved.

The absolutely vital research agenda
Now, premised on the belief that everything just has to be due to this kind of a Darwinian process, what can we expect from these new behavioral findings?

It is likely that we'll see proposals saying that studies are 'needed' to identify the genetic nature of this adaptation.  Some large number of poor unlucky catfish will suffer gruesome 'experimental' alterations or removal of  their barbels to show what it is about them that 'causes' this specific behavior.  These procedures will be dressed in language ('barbelotomies, sensory barbelneurectomies, barbelhemisections, translateral barbel reimplantations,....) so research review boards can approve them as not inhumane and worth doing for human good on various rationales.  Tissue will need to be imaged by CT scans and the fish wired for fMRIs as they hunt.  Costly  high-speed videography will need to be done to document the behavior down to the microsecond.  Then thousands of fish will have to be sequenced to find genotypes in those that hunt along the shore versus at the bottom in the usual catfishy way.  Big GWAS will find candidate genes, and then thousands more catfish will be studied with genetic variants introduced that makes them no longer able to tell a pigeon from a rock.  The news media and journals will hype periodic Big Stories about this, of course.

This will not be done by small fishery research labs, but by the major medical schools, who will snap up money in big gulps from NIH, faster than, say, a catfish grabbing pigeons.  Society will be the pigeons in this case, and the reach into our pockets will be justified by the argument (excuse?) that this will tell us about sensory behavior and eventually lead to cure all sorts of human behavioral diseases.  The promise will be that at birth people will then be 'diagnosed' if they have variants in the evolutionary same gene, so we can apply preventive measures.

Are these just our own kind of fish stories, or is this how it might actually go?

A cat among the pigeons?
In any case, the automatic assumption of a plausibility story as the true story is unjustified no matter how routinely such things are offered up.  There is an aspect of evolution, called genetic assimilation, that could be involved if there is any fitness (reproductive) advantage, even if there is no genetic basis for the behavior today.  In genetic assimilation, when or if genetic mutations arise that tend to push a fish to do this successful pigeon-shoot, and hence to have higher reproductive success, then Darwinian natural selection could favor those individuals and over a long time period the behavior really would become 'genetic'.  The species of catfish would become mandatory birdivores.  But this is many steps beyond the current data and there is no reason to think that such selection would occur.

Indeed, it might be--we'd say might far more likely be--that being smart enough to find different types of food is what's good for fitness, and getting hard-wired for skeet could make you vulnerable.  After all, if there's a cat among the pigeons, the pigeons that don't go to the shore for their vacations will live to fly another day and either for genetic reasons, or just because they're smart enough to stay away from danger, there would come a day, perhaps sooner than the catfish could evolve their birding behavior, when there just weren't any birds to hunt!  Then, their 'adaptive' species would simply starve to death--unless they were smart enough, and could override their genetic birding mandate,  to figure out what their distant ancestors did, and go scruffing around the river bottom for the less glamorous sources of food. 

A genetically-based Darwinian explanation is simply not required. Animals figure things out without having to be genetically programmed to do the specific thing they're figuring out. The most plausible Darwinian explanation is that fish evolved flexibility and sufficient brainpower to do what best presents itself to them.  Work by Victoria Braithwaite and a friend of ours named Paula Droege, here at Penn State, are in fact showing remarkably complex behavioral abilities in fish.

We are by no means the first to argue that reflex selectionism is a mistake, not science.  Even Darwin occasionally tempered his rhetoric about adaptation.  But his intellectual descendants (us) rarely have his level of IQ points, and are all too susceptible to giving standard sermons (that may be the right word for them) about how Nature is.  Sometimes they may be true.  Sometimes they may be true but not demonstrable by sufficient evidence--and in the absence of videos from thousands or millions of years ago, that makes them suppositions that are no more than  plausible from the point of view of science.

One should be more cautious if one wants to claim to be doing 'science'.


Patrick Clarkin said...

Ken, thanks. These are important lessons. I think it's good for us to remember that flexibility itself is an adaptation, behavioral flexibility in particular. A gene that led to a protein/phenotype capable of working in a narrow ecological range would be even more adaptive if it could work in a wider range, or even respond to novel challenges. Another way of looking at it might be that a gene capable of doing many good somethings is better than a gene capable of doing just one good something. Catfish capable of exploiting a new food source seem like a good example of this.

Ken Weiss said...

I agree entirely. I think it is easier to envision 'fitness' advantages of alleles 'for' adaptability, esp. cognitive adaptability, than reflexes.

There also doesn't need to be any negative aspect of greater flexibility. So it needn't be a trade-off.

If it were actually advantageous to be rigidly programmed, even if it starts out as a behavioral decision, then Waddington's idea of assimilation could hard-wire it.

But this catfish behavior shows clearly the lack of need for that (assuming this hasn't been going on for millennia in eastern Europe where they come from)