Evolution… at a snail’s pace?

The revival of the Snail Kite population in Florida – recently reported in the New York Times – is a happy story, on the surface. The Snail Kite is considered endangered in the US, and had been in decline for years, as its habitat eroded and disappeared. Adding insult to injury, a South American snail invaded Florida. The snail was Pomacea maculata, a species of Apple Snail. (like that? Pomacea? Pom? Apple? Ok, never mind) Probably maculata found its way into the Everglades as a result of an aquarium dump. This wasn’t Monsanto destroying small farmers one GMO seed at a time, or Exxon fouling entire ecosystems in one great gush. This was individual people. Though it must have happened over and over again, I imagine some guy who lost his house in the Great Hedge Fund Land Grab of 2008, and had to empty his aquarium – and all its gastropod contents – into the nearest body of water as he downsized.

If I close my eyes, trying to imagine myself doing this, I just can’t. Though I have spent plenty of time and money adding Gastropoda, Cnidaria, and Actiniaria to my own living room aquarium.  Though each of us is a walking gates-thrown-open menagerie from the bug-infested mud on our shoes, to the plants in our non-native garden, to the Demodex crawling all over our skin, to the viruses we sneeze all over our colleagues. Perhaps the guy just accidentally dumped a snail or two during a weekly water change. Anyway, invasive species are everyone’s fault. And they’re probably unavoidable.

Back to the snails. These chubby little guys are much larger than the endemic Pomacea padulosa, which went into decline upon the maculata’s arrival. This would be a classic invasive species outcome. Alien species lands in new environment. Outcompetes native species for resources: eats the native’s food, eats food the natives don’t, isn’t as susceptible to native predators, changes the environment to make it less favorable to natives. But this story isn’t about the status of Florida’s native snails. We learned from the snail darter that people care more about charismatic megafauna.

The Snail Kite isn’t really megafauna, but it is a big, beautiful bird. Much prettier than snails. But eats snails, eponymously. And when its diet of padulosa began to be swallowed by the invader, the Kite turned to maculata. And you might think, voilà! escargot = problem solved! Snail Kite Cleans Up Invasive Species! But it could have gone the other way, too. Since Pomacea maculata is so much bigger than Pomacea padulosa, the Kite couldn’t eat it as effectively, and it might have been that the bird would decline even more rapidly in the decade following maculata’s introduction.

Enter a group of scientists at the University of Florida. They noticed a sub-population of Snail Kites with much larger beaks than the norm; Christopher Cattau is first author on a couple of great papers on this topic by this team (1,2). The birds were better able to eat the new snails than their small-beaked brethren, opening a door for the species’ survival and potentially for mitigating against the maculata threat.

The Florida group noticed that, within about a decade, baby kites in maculata-invaded juveniles survival rates increased by 50%. Not only that, but they saw that the kites’ success happened as their populations were shifting to what was considered a poor habitat.

So the big-snail-eating kites were doing better than baseline in a sub-baseline environment!

How did they do it, you ask?

By growing bigger beaks!

No, really, they did it that way.

So. The bird grew what it needed to survive!?

Kind of. In case you’re not steeped in the Theory of Natural Selection, that statement is what those in the trade call “Lamarckian” after Jean-Baptiste Lamarck, who supported a version of evolution called “soft inheritance”. He envisioned a system where animals developed traits that they needed, and passed them down to offspring. Giraffe needs to eat leaves in high places? Grow a bigger neck. You’re welcome kids!

If the idea sounds familiar, don’t worry, it’s what most people have believed or intuited since Aristotle.

But it’s not the way we think of evolution any more, since Darwin. And part of the problem is that there’s no mechanism that explains Lamarckian adaptation. How do you grow a longer neck, just because you stretch it out every day? And even if you could, how do you pass on the stretched-out neck to your kids?

On the surface, this is what evolution looks like to humans: the need is there, and the trait follows. Everywhere we look we see organisms that are finely adapted to their specific environment. The Xanthopan hawkmoth grows a 30cm proboscis, perfectly suited to lapping up sugar from the Angraecum orchid’s 30cm nectar spur. This is a mutualism – the orchid rubs pollen onto the moth’s fuzzy head as it strives to push its proboscis to the base of the spur. Such interactions are common enough in biology, but the point is that it makes sense… the moth needs nectar, and if it grows a tongue of the right length, it can have its own private food supply.

Natural selection – unlike Lamarck’s model – provides a mechanism for change. Or anyway it does when combined with a little molecular biology. Stir in DNA and sex, add a sprinkle of random mutation for each little wriggling gamete, boil at the reproductive rate of the organism, then stand back and let the environmental conditions decide which offspring are the best at, say… eating bigger snails than grandpa ate. Yes, many offspring will die. It is a sad, short, and hungry juvenile lifespan for those normal beaked Snail Kite babies… but their long-billed brothers and sisters will make it, and thrive to pass on their big-beaked DNA to their offspring, carrying Snail Kites into the a happy future with their new, invasive food supply.

So is it still the Snail Kite, if it has a longer beak? “Alpha” taxonomists spend their time looking for physical traits that allow them to create the fine distinction that separates one species from another. They might even look specifically for different beak to rostrum height ratio as a means of distinguishing between bird populations.

Not so fast, Blakie. In this case it really is still the Snail Kite. Cattau and colleagues looked for signs of whether this was an actual genetic change, and found instead evidence of phenotypic plasticity. That’s a kind of built in Lamarck-ish tool that allows an organism to present a range of phenotypes, beak sizes in this case, that might in turn allow the organism to cope with a change in conditions. Stop and think about that.

You’re born with your genes, but those genes give you options: if your nest happens to sit in swamp S, you grow beak S. If instead your nest is in swamp L, you grow beak L. Cattau’s team uses an 80 year old tool called the breeder’s equation to assign the Snail Kite’s adaptation to phenotypic plasticity. To avoid putting you to sleep with the genetics details, let’s just say the breeder’s equation allows us to tease apart to what extent variation is due to environment or genetics. The authors argue that its environment is flipping the switch in this case, not selection. And it’s not simply that more food means bigger babies. Yes, they are bigger… but their beaks are bigger still.

It may be that, at this point, the Snail Kite has impressed you enough. It’s got a genetically stacked deck. It says, “Hey, I’m fine with my diet of tiny snails. But you change it up on me, give me a bigger snail… Whatevs, I got a bigger beak waiting right here.” Phenotypic plasticity offers the species options in a changing environment, can perhaps mitigate against the uncertainty that is the ecosystem. An especially powerful tool for threatened species to respond to the habitat and prey loss humankind has inflicted.

But this was not the story I was hoping for. I wanted to see selective pressure pushing so hard that, in a mere 10 generations, this bird had adapted an entirely new phenotype. That all the weak, small-beaked offspring had fallen away, and the more fit, strong-billed mutant had pushed its way to the front. I’m a bit crestfallen that the Snail Kite’s just pulled a trick, that it’s just got a genetic backup plan for hard times.*

Most people won’t care. “Yay! The Snail Kite survived!” And the statistics required to decide between the bird’s genetic sleight of hand and “true” evolution can cause the audience to stifle yawns and check watches, though some folks will be excited just having learned that phenotypic plasticity is a thing.

And the door is still open for the Snail Kite to truly evolve. Cattau, et al., think that the morphological changes induced by environmental perturbations set the stage for “real” selective adaptation to new conditions. My read? If those new beaks confer survival and reproductive advantages, then the Snail Kite will have truly evolved.

That’s where my interest in this story began. I wondered at the rapidity of the response reported in the Times, and my mind leapt immediately to the meaning of the New Snail Kite to its poor, small-beaked predecessor. To me, the distinction between what the bird was and what the bird is, is significant. Not because I think the only Snail Kite is the Pure Snail Kite of the pre-maculata days, but because I’m fascinated by change. By the fact that natural selection seems to demand change. Simply by presenting a challenge to the status quo (new habitat, new prey), we have a new organism – perhaps within the time it took my baby to get to fifth grade. Look, that invasive snail might also have been farted out as an undigested egg by an off-course Brazilian sea gull, the effect would be the same… just look at what a couple of confused Darwin’s finches did to the Galapagos. No environment is static, and so no species is static. And if no species is static, no ecology is static, since it’s a web not a chain. There’s a push-pull literally shaping the living world around us.

It’s lovely, really. And I don’t care if phenotypic plasticity and cryptic genetic variation are required to set the stage for the actual adaptation.

So, back to Lamarck. If we think of evolution as presenting – via plastic traits – a range of relevant morphological possibilities, then maybe the distinction between Lamarck and Darwin is a bit academic, or anyway overly mechanistic. To make Lamarck work, though, we’ll still have to abandon the idea that the giraffe acquired that long neck through repeated stretching.

No, it didn’t. It will acquire that trait, but only for its offspring in a kind of unconscious tough love for all its sperm, eggs, pollen, or spores waiting in its reproductive tract to be cast out over the world. That’s an overstatement, at least in the Snail Kite’s case: phenotypic plasticity set the stage for larger beaks. Natural selection may lock that phenotypic “choice” in place.

But to take the selection case to its extreme, it’s like Coop said: Murphy’s law doesn’t say only bad things happen. It says everything that can happen, happens. Bigger beaks can appear. That sucks for last year’s beak model. And it means that life can present as though the Snail Kite’s beak grows to match its prey. It’s a fine distinction, since the beak size is a plastic response to its environment. And if selection fixes that plastic trait, then what?

It makes me mourn that pre-maculata Snail Kite, the phenotype lost to the big snail invasion. Imagine or remember that the Tree of Life is actually a Tree of Death, with countless evolutionary failures hanging there like so many little Odins, teaching us about how adaptation is not a fix for the present, but a gift for the future, with new shoots constantly emerging from the scars of the old. And that even the current successful version is only a temporary fix.

Some will read this (haha, right) as an apology for humankind’s disruptive, destructive behavior. As though I mean that changes we wreak on our environment will lead to a new Eden of organisms perfectly adapted to the post-apocalyptic landscape. I don’t mean that. There are plenty of examples of buds on the Tree which have been permanently pinched off, their future-promise gone for good. Which truly sucks, especially for Pomacea padulosa. No, biodiversity is the answer, we know that. I’m merely reflecting on the fact that evolution at least has the capacity to swallow humanity’s bruisings, and regurgitate something new. That the Snail Kite may have a genetic hedge against environmental change is a fact that stretches our neck a little closer to those hard-to-reach branches of evolutionary understanding.

1) Ecological Applications (2016) doi:10.1890/15-1020.1 (link)
2) Nature Ecology & Evolution (2017) doi:10.1038/s41559-017-0378-1 (link)

* You want to trip harder on evolutionary mechanisms? After you study up on phenotypic plasticity, go read about cryptic genetic variation.


One response to “Evolution… at a snail’s pace?

  1. Nicely done! Have you been reading Quammen lately?

    A question likely to reveal my ignorance: Is the undercurrent here true, that ‘actual’ or ‘true’ adaptation is always only sufficient and not more than sufficient? In other words, do the big beaked kites continue to eat small snails, thus giving them a truly better beak more adapted to a wider niche or to changing circumstances (such as people mobilizing to eliminate the big snails at some point in the future to save some other endangered creature)? So, if the big snails disappear, will the big beaked kites still have an advantage, and will that adaptation perhaps disappear more slowly than it appeared because big beaks might be more useful than small beaks in ways that don’t have to do with big snails?

    An association: Lewis Hyde in “Trickster Makes this World” (a book worthy of the description “magisterial”) writes about mythic stories of creative omnivores like Coyote as being about the roots of creativity–it’s so helpful to have more than one way to fill a belly. I don’t think I’m suggesting that the phenotypic plasticity of kites is creative in quite that way, but it’s good for them that they have a little flexibility. Yet when I filter this through new materialist theory about “thing-power” and the self-organizing creativity of material assemblages, I wonder if we need to adjust our notion of creativity beyond that which humans most exemplify.

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