In that grand endeavor we call science, one of the most beautiful phenomena is where nature repeats itself. In all the ogles of complexity, the great richness of natural forms, the immense diversity of physical entities in our universe, we are delighted when we find this simple, pleasing occurrence: consistency. Consistency, to us humans, seems to be the stuff of productivity. It leaves us feeling accomplished in our tasks, facilitates growth in ourselves, and reminds us of important ideas.
The recurrence of important ideas is a grand part of the search conducted by scientists. The Fibonacci Sequence, the Golden Ratio, the (another thing like the previous two…I need a series of 3 for aesthetic purpose). Whenever we observe any repeated phenomenon in the universe, it brings us closer to unification. The more a single phenomenon repeats, the more chances we have to understand just what ties these things together. Why? Because in the vastness of this universe, it remains extremely unlikely that any one thing will happen more than once for random reasons. It could be said of coincidence that the universe is rarely so lazy.
No, two occurrences of the same phenomenon are likely connected by causation. The chances are far greater that similar principles are behind these two occurrences rather than random chance. So in nature, consistency sings of big ideas, a song which allures the scientist to the opportunity for a unifying theory. It is upon unifying theories that so many scientists have based their careers. Newton determined laws of motion to explain the many consistent movements of matter. Darwin introduced natural selection to explain the many consistent histories of life. Einstein deduced relativity to explain the consistent workings of the universe at a large scale. Theoreticians delight in consistency almost as much as they delight in serendipitous randomness.
It is exactly this sort of consistency that makes convergent evolution so tantalizing.
Occasionally, when exploring the many floras and faunas of this planet, we come across species that look similar to species from back home. This similarity — or to use my language, consistency of appearance — can be due to two things: 1) The similar species are related, sharing a recent common ancestor, or 2) The similar species have evolved to look similar because the fill similar ecological niches, even though they are unrelated.
The first situation is the substance of taxonomy. If two species look similar, it has been assumed for a long time that they are related, and as such, names and classifications have been based on outward similarity. More on this in a moment.
The latter case, where unrelated species look similar, is substance of convergent evolution; when two species, filling a similar role in the ecosystem, evolve to look similar, they are said to converge upon the same morphology. Their evolution leads them to converge, in other words.
Before we had DNA, methods for discerning between convergent evolution and actual ‘relatedness’ were limited, and as a result, mistakes were made. Taxonomy, up until the genetic revolution in biology that occurred in the 20th century, was rife with false classifications. Birds are an especially good example of this. Just look around the world for species called robin, called chat, called warbler, called sparrow, called finch, and you will see that those who conducted the naming of things in the past had no idea what was actually related.
Let’s look at just one of these examples: robin. In Europe, the robin is Erithacus rubecula, a well-known and loved woodland species of songbird with a characteristic orange belly. European explorers came to North America and named a similar orange-bellied species American Robin (Turdus migratorius). Well, it turns out that these two “robins” aren’t even in the same family, let alone being in the same genus. Ideally, the second part of a common name — like robin — should denote genus, but in this case, robin serves only to confuse. Fooled by orange belly and lovely song, early namers mistook these two relatively distant songbird cousins practically as siblings. Oops. Thanks to genetics, we know better now.
So why are these two species similar if not for reasons of shared ancestry? Why has nature repeated itself, showing such consistency in morphology? Now we’re asking the right questions: undoubtedly, the orange-bellied, gray-brown upperside coloration has advantages in both of these species, let alone in the myriad of other bird species it has popped up in the world over. If we were to study the specifics of their colors in the context of their life histories, we would come closer to understanding this natural consistency.
But you know what? Nobody has studied this. The door’s open, fellow explorers.
You know what other convergence hasn’t been studied? That between Eastern Meadowlark and Yellow-throated Longclaw. Both live in open grassland habitats, characterized by a camouflaged back, yellow throat and belly, and black collar. Again, these similarities aren’t due to a shared ancestor; both of these species evolved this appearance independently, long after their common ancestor some 50 million years ago.
And here we reach the point of this essay: to document a previously undescribed form of convergence. Let’s imagine ourselves in a different place to set the mood:
Returning from our excursion into a dry, relatively high altitude aridland, realize this: that exact scene, focusing specifically on the bird, could occur on all continents (except, of course, Antarctica). Go to this kind of habitat in North America, South America, Africa, Asia, or Australia, and you are likely to find lanky brown birds with long tails and thin bills. The best part? These birds are totally unrelated. Let’s take a tour, shall we?
Along the jagged spine of the Andes, high altitude aridlands harbor a diversity of unique flora and fauna, becoming especially scrubby in places like Chile and Argentina. Here, sharing the slopes with guanacos and vicuñas are South America’s lanky brown iteration: the Earthcreepers. Specifically, the Earthcreepers that best match our mold are those in the genus Upucerthia.
In the Southwestern drylands nestled between the Rockies, the American birdwatcher knows the thrashers well. The Western counterpart to Mockingbirds of the East, the thrashers mimic other birds’ songs will inventing phrases all their own, with some songs going on for what seems like hours of inventive and imitational vocal play. The genus that matches the lanky brown morphology best, in this case, is Toxostoma.
In the desperately dry hills and valleys of northern Africa, there are few living things that can withstand such hot and dry conditions. Many birds do still prosper here, with sandgrouse and hornbills being two unique examples. With these and small set of other hardy life forms across this dry expanse lives a couple of species called Hoopoe-larks. This genus — Alaemon — matches the lanky brown morphology almost perfectly.
In the higher elevational bands of some of Asia’s great ranges, one genus has occupied aridlands and evolved into our morph of interest. Sharing the dry expanses with characteristically hooved Old World mammals (like wild species of goats and antelopes), a few babblers of the genus Turdoides now share the long and slender, dully brown look with counterparts around the world.
While we could continue our cosmopolitan tour further into Asia and into Australia, the trend has indeed been set. Something about arid habitats makes this slender build advantageous. In pondering an organism’s biology, one of the most powerful questions we’re equipped with is, “What is the adaptive significance of….?” With this question, we can start to delve into this array of shared traits, to productively ponder why such convergence has occurred. We can explain the long tail of Toxostoma, the slender bills of Upucerthia, the slender build of Alaemon, the dull brown of Turdoides. There are answers to these questions, answers which hint at the order evolved by the natural world. With its laws and mechanisms, the natural world crafts its inhabitants, with natural selection guiding them toward what is most advantageous.
This grand journey, afforded to all of life, is one of many paths. But paths cross; lineages converge. And in the case of these species, we can be sure that this convergence is indeed guided by the environment. All these species are adapted to survive. This morphology helps them do it in dry environments, and is apparently so helpful that unrelated species would all converge upon it.
Like with robin coloration, or with the coloration shared by Yellow-throated Longclaw and meadowlarks, we don’t yet know why this convergence has occurred. We don’t know what makes this morphology advantageous. You see why I keep returning to the open door metaphor?
Questioning like this allows us to remember that one scientific aphorism, a truth that we thinkers revere more than perhaps any other guiding principle:
We should not be afraid of the unknown, because ‘unknown’ is just another word for opportunity.
Will you take up the torch and learn what has never been learned before?