In its rawest form, evolution is change over time. Many things evolve: chemical reactions, landscapes, behaviors, and of course species. The scale of time is rarely defined though. Naturally, it is assumed that the longer the amount of time, the greater the degree of change. But even under relatively shorter periods - for instance, the length it takes a human baby to become a grandparent - much change can occur in the world surrounding that person. Landscapes shift and the distribution and connectivity of species changes. Thus ecology, even those minute interactions that occur in only one or two generations, can be highly relevant to a species history.
Evolution is a harmonic composition that is balanced by the environmental forces that act upon ecosystems and the biological forces that mold populations and species. Like the epic concerti of the classical greats, evolution is composed of several movements. Elegant sonatas of populations steadily drifting apart are punctuated by candenzas of rapid adaptation of species as a result of newly arisen barriers. Some species are soloists amid an orchestra of diversity, but that orchestra inevitably evolves together da Capo al fine.
The first movement I want to discuss in the concerto of life is a slow movement. Much like an Adagio, macroevolution makes wide generalizations using elegant, simple chords "with a tense melodic line and taut harmonies" (NPR Music). The "tense melodic line" being the unbroken line of descent with modification from a common ancestor. Here, the scale of change occurs at and above the species level and deals with the origin novel traits. The appearance of feathers on birds or differences in leaf shape among oak trees, for instance. What we miss in this movement are the details: how did novel traits arise? The answer delves into each species' unique physiology, genetics and ecology.
Bordering on "just so" stories, macroevolution piques the interest and cues the emotions of the listener. Unlike one of the great concerti, we only get to hear the end of the Adagio movement. What we only now observe is a climax, the strings are taut and the harmonies reverberate through the concert hall, but the song is still far from done. It appears to us that everything we have listened to over the last several hundred years of natural history research has led up to this finale, but we would be fooling ourselves as this concert has been playing steadily for billions of years. At this point, we move from merely an audience to participants in the sonic storyline.
The slow, intentional pace of the Evolutionary Adagio belies the intensity of each note. A deliberate stroke on the string of adaptation echoes through to generations beyond leaving a modified draft of the song in each generation's wake. Yet every important note, every crescendo, and each major lift is hidden from our ears in this Adagio. It's as if we are digging through the world's greatest sonic masterpiece and only finding parts of a score here and there in the fossil record. We have enough pieces of the score to get idea of how the song should go and to understand the major features of the orchestra, but we do not know how each transition went.
Contrary to musical composition, in evolutionary science this isn't so much a problem. We know the general melodies of the theories and it is still very harmonious and a pleasure to listen to. The pieces fit nicely and the composition of the Evolutionary Adagio makes sense, if but a little staccato at times. This seemingly slow, deliberate change of macroevolution is exceeded by the magnitude of novelties that have arisen to fill new niches left over from a landscape in constant flux. We can measure differentiation and understand very well the pattern of speciation understanding very well the unique signature that every individual note leaves on the song as a whole. Adding up all the notes we get a plausible history, but more importantly we have a testable hypothesis with transcendent explanatory power.
This movement's strength comes from a deceptive simplicity: out of nowhere an eye arises, scales turn into feathers, or spines appear. But these novel traits are really proxies for entire sets of circumstances and reactions that affected ancestral populations. Hidden inside this drawn-out movement are other movements operating at more allegro tempos. The sum of which equate to an Evolutionary Masterpiece that underpins the entire symphony of biology.