Mind the Gap

When I’m thinking about the evolution of animal forms, as one does, mostly I’m considering animals that exist, or that previously existed, and wondering what conditions and adaptations led to their rise and persistence. Which beak shape gave that bird an advantage where seeds were like stones? Which wing length made sense for a bird that needed to soar, not hop? What combination of traits meant fitness, and thus survival? And why the goofy feet, regardless?

But here’s a different question, one that takes one’s thinking in another direction: Are there forms that would seem to fit well into a particular niche but that never arose to fill it? If so, why the evolutionary no-shows? What stopped them from being?

I interviewed a biology graduate student recently named Stephanie Chia who has asked that question as part of her doctorate research. She focused on the largest group of songbirds, a superfamily called the Passeridae, which includes our little friends the sparrows and finches and warblers. She applied a tool to the problem called persistent homology, which is not just a great band name but a mathematical approach rooted in topographical data analysis. Along with some methods for piecing together ancestral shapes, it let Chia chart existing birds based on their individual combinations of morphological traits, estimate how their ancestors looked, and then find the gaps, or combinations of traits that reasonably should have, but didn’t, make up a bird in that group.

When something doesn’t arise, there’s a reason, of course. It might be an environmental factor (or more than one) that kept that form from taking shape, for example, or it’s possible the form did arise but quickly went extinct, or that some outsider snapped up the spot our bird required. In the case of these birds, the study showed that competition from a bird in another family was likely the limiting factor. Some sneaky passerine-adjacent bird, a Smith instead of a Jones, outfitted itself in the exact traits our bird would have had and leapt into the space our bird would have occupied, leaving our hypothetical feathered friend out in the evolutionary cold.

The victorious bird wasn’t even a quirky or colorful or weird bird; it would have looked and acted very similar to its kin, albeit with some tiny tweak that gave it its own identity.

I’m sharing this academic tidbit here because it struck me as fascinating, a way of thinking about lifeforms that might not come to mind organically but that demands exploration just the same. Like me, you might be interested to know why life on Earth looks like this and not that; here’s a way to peek at the not, to better understand what could have been but never was.

An elderly woman I used to know, when referring to something being done out of the typical order, would say it was “down the back stairs Bessie.” That’s how I think about this approach to evolutionary questions.

Meanwhile, I see it’s April Fools’ Day. But my missing bird, to which I’ve become rather attached, is no joke. There was a songbird-shaped gap that was pilfered by some no-name songbird-shaped bird. The missing songbird isn’t laughing, I’ll tell you that.

I’ll end with an apology. The above image was generated by AI because in these troubled times it’s becoming difficult to find images you can download for free. Plus, I needed the ghost bird in the middle. Which, as you now know, isn’t a ghost at all. Just an imposter and a thief.

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