To the naked eye, there’s not much to distinguish between them. Both are small, with black-and-white heads and gray-black wings, seed-cracking bills, and a gregarious manner. For a long time, they were even thought to be the same species. The only obvious difference, at least with the willow tit I saw, was a duskier olive underbelly coloration.These two nearly identical birds are a nice jumping off place for a discussion of diversity in evolution.
Posted By Brandon Keim on Jul 02, 2014
The easily confused willow tit and black-capped chickadee f.c.franklin via Flickr / Brandon Keim
ONE OF MY favorite pastimes while traveling is watching birds. Not rare birds, mind you, but common ones: local variations on universal themes of sparrow and chickadee, crow and mockingbird.
I enjoy them in the way that other people appreciate new food or architecture or customs, and it can be a strange habit to explain. You’re 3,000 miles from home, and less interested in a famous statue than the pigeon on its head?! Yet there’s something powerfully fascinating about how familiar essences take on slightly unfamiliar forms; an insight, even, into the miraculous essence of life, a force capable of resisting the universe’s otherwise inevitable tendency to come to rest.
Take, for example, a small songbird known as the willow tit, encountered on a recent trip to Finland and closely related—Poecile montanus to Poecile atricapillus—to the black-capped chickadee, the official bird of my home state of Maine. To the naked eye, there’s not much to distinguish between them. Both are small, with black-and-white heads and gray-black wings, seed-cracking bills, and a gregarious manner. For a long time, they were even thought to be the same species. The only obvious difference, at least with the willow tit I saw, was a duskier olive underbelly coloration.
Which raises a question, asked by Darwin and J. B. S. Haldane and generations of biologists since: Why? Why is a bird, similar in so many ways to another, different in this one? It’s a surprisingly tricky question.
Generally speaking, we tend to think of evolution in purposeful terms: There must be a reason for difference, an explanation grounded in the chances of passing on one’s supposedly selfish genes. Perhaps those olive feathers provide a better camouflage in amidst Finnish vegetation, or have come to signify virility in that part of the world. As evolutionary biologists Suzanne Gray and Jeffrey McKinnon describe in Trends in Ecology and Evolution review (pdf), differences in color are sometimes favored by natural selection—except, that is, when they’re not.
Often differences in color don’t have any function at all; they just happen to be. They emerge through what’s known as neutral evolution: mutations randomly spreading through populations. At times, this spread, this genetic drift, evenly distributes throughout the entire population, so the whole species changes together. Sometimes, though, the mutations confine themselves to different clusters within a species, like blobs of water cohering on a shower floor.
One can imagine life evolving again and again, crashing on the rocks of time and circumstance, until finally it hit upon just the right mutation rate—one that eons later would produce organisms and species and ecosystems.Given enough time and space, these processes can—at least theoretically, as experiments necessary for conclusive evidence would take millennia to run—generate new species. Such appears to be the case with greenish warblers living around the Tibetan plateau, who during the last 10,000 years have diverged into multiple, non-interbreeding populations, even though there are no geographic barriers separating them or evidence of local adaptations favored by natural selection. The raw material of life simply diversified. One became many, because that’s just what it does1.
Through this lens, evolution is an intrinsically generative force, with diversity proceeding ineluctably from the very existence of mutation. And here one can step back for a moment, go all meta, and ask: Where does mutation itself come from? How did evolution, and evolvability, evolve?
It’s a question on the bleeding edge of theoretical biology, and one studied by Joanna Masel at the University of Arizona. Her work suggests that, several billion years ago, when life consisted of self-replicating chemical arrangements, a certain amount of mutation was useful: After all, it made adaptation possible, if merely at the level of organized molecules persisting in gradients of heat and chemistry. There couldn’t be too much of it, though. If there were, the very mechanics of replication would break down.
Molecular biologist Irene Chen of the University of California, Santa Barbara, has further illuminated that delicate balance. Her work posits that, as an information storage system, DNA was less error-prone than RNA, its single-stranded molecular forerunner and the key material of the so-called RNA world thought to have preceded life as we now know it.
So, then, one can imagine, early in Earth’s history, life evolving again and again, crashing on the rocks of time and circumstance, until finally it hit upon just the right mutation rate—one that eons later would produce organisms and species and ecosystems that reproduce themselves and persist across time and chancellor.
That’s the remarkable thing about life: It continues. It keeps going and growing. Barring catastrophic asteroid strikes, or possibly the exponential population growth of a certain bipedal, big-brained hominid, life on Earth maintains complexity, actually increases it, even as the natural tendency of systems is to become simpler. Clocks unwind, suns run down, individual lives end, the Universe itself heads towards its own cold, motionless death; such is the Second Law of Thermodynamics, inviolable and inescapable.
Yet so long as Earth’s sun shines and genetic mutations arise, evolution may maintain its own thermodynamic law. Black-capped chickadees and willow tits diverge. Life pushes back.
1. To be sure, the concept of neutrally driven biodiversity isn’t universally accepted. There may be subtle, intrinsic advantages to diversification. An example comes from the models of James O’Dwyer, a theoretical ecologist at the University of Illinois: Simply by virtue of their novelty, new species may be intrinsically less vulnerable to pathogens that afflicted their evolutionary parents.
So, then, perhaps willow tits and black-capped chickadees evolved slightly different feather patterns because they provided some immediate, direct benefit; or maybe it happened just because, for no reason at all, really; or maybe there was a subtle benefit intrinsic to the process of variation itself; or maybe it was some combination of all three, varying by time and place.
So, it’s complicated. But whatever the complications, all these processes share something very fundamental: the emergence of variety over time as life’s essential property.
Brandon Keim (@9brandon) is a freelance journalist specializing in science, environment, and culture.