Asleep at the switch: Paleontological life lessons, stasis, and the genius of Yogi Berra
There are certain things most of us learned (or should have learned) as children, like, don’t run with scissors. (Although, we can rely on brilliant cultural rebels to abnegate such lessons,
with the help of Photoshop, the repeated jocular themes of abject stupidity in Weird Al’s music only serve to further drive the point home.) Well, if we consider the narrower disciplinary purview of paleontology, there are also certain equivalent life lessons, actually, “history of life” lessons, most of us paleontologists learned. One of the key examples we’ll focus on in this blog post involves the nature of evolutionary change within species. What pertinent history of life lesson’s have paleontologists revealed? The big one is that if you search for evidence of persistent, directional evolutionary change within species lineages in the fossil record, you don’t find any. You literally—and figuratively—slam into a stone wall. Indeed, regarding the literal part, this happens because you are most likely sampling up and down a cliff face at an outcrop somewhere—and you slam into that kind of stone wall. So, hey, don’t run with scissors and don’t run around outcrops.
Okay, so once you cut it out with the running, and start poking around the outcrop, using the trusty hammer you hopefully brought along to liberate a few fossils from the various rock layers, what do you see? First, we feel compelled to quote one of our favorite philosophers here, because this quote conveys such an important life and history of life lesson:
You can’t “see” anything without looking. Further, if you’re a scientist, what you should do is look around for patterns, try to synthesize them, and use them to test and refine hypotheses. When you look at specimens of a species from the bottom to the top of your rock wall, you’ll find that the fossils of the lineage you are focused on don’t seem to change very much. They all look pretty much the same: they display stasis. The first and still classic example, yeah Niles, it’s classic, we can say it, indeed, we can even call it the paradigmatic example of this pattern and the associated phenomenon of punctuated equilibria involves Devonian trilobites of the genus Phacops (which is now due to a fit of taxonomic pique sometimes called Eldredgeops).
So, if you haven’t given up at this point, and you still want to follow Yogi’s maxim and keep on looking, you shrug your shoulders, get into your car, and drive to the next promising outcrop. Let’s say the rocks here are said to be a bit older than the rocks you just banged up against (and banged upon) in the first quarry. And if we look for our species and find it the fossils will look pretty much the same as the ones you saw in that first place—there might be a few very minor differences, equivalent to the types of differences between closely similar populations of modern species that varied slightly across geographic space. This process of tracking down your chosen fossil species lineage, and looking for change within it, could go on and on till you run out of patience, or gas. Or, until you come upon an outcrop that’s too far back in time, that is built up of layers of rock deposited before your species evolved, or you reach an outcrop that contains rocks that are too young, and contains layers of rock deposited after your species went extinct. Given that on average marine invertebrate fossil species, like the aforementioned trilobite, persist for 3-10 million years, that’s a lot of time without a lot of net within-lineage evolutionary change. What does this mean about the nature of the evolutionary process? What is the history of the concept of stasis? What causes it? Questions of key relevance to macroevolution, and ones that we’ll begin to explore in this blog post, and take on more thoroughly in subsequent posts.
Stasis as a pattern
Now, the paucity of net change in morphology, could actually fit several different patterns: 1) obdurate monomorphism, with mean or median species morphology not diverging statistically from its initial morphology at any point throughout its history; 2) waggling with concomitant wiggling in morphology, involving repeated cyclical changes in mean or median species morphology (see figure below, modified from Eldredge et al. 2005. Paleobiology 31:133-145. Copyright the Paleontological Society.);
and 3) meandering morphological changes in mean or median morphology that lead nowhere that could be characterized in a gestalt sense as a directionless random walk (see figure below).
All of these patterns are compatible with stasis as originally defined in the first 1972 paper on punctuated equilibria by one of us (NE) and Stephen Jay Gould (SJG henceforth), and as characterized in many subsequent papers and books authored by these alone, together (including in collaboration with BSL), or in collaborations with others. It seems like some folks are confused and don’t get the fact that as long as the mean or median morphology of a species from its first and last occurrence does not differ, then what is observed is compatible with stasis as originally defined. This holds true even if there is some sort of random walk in morphology that occurs during the history of the species. We can say this because we’ve been there and produced the original box score if you will. Further we think Yogi would agree that before other scientists try to say that some result proves or rejects stasis they really need to read the original. Check the official record, and figure out what the original authors actually said. You really can see a lot just by looking.
Stasis as a process
Now, the process that produced the stasis we’d see in a lineage with morphology that oscillates cyclically through time is not the same as the process that would produce an obdurately monomorphic lineage, nor is it the same as the process that causes morphology to drift with evanescent directions in a random walk. But we’re not going to talk processes of stasis this time around, we’ll leave that for a future post. However, we provide you with this video (copyright Okay Go, UMG [on behalf of Capitol Records], UMPI, BMG Rights Management, UBEM, Broma 16, CMRRA, and 16 Music Rights Societies) by the band Okay Go (think of the band as analogous to a species) to reiterate that a lot may happen to the populations (represented by the different band members) of that species, but the species as a whole doesn’t get anywhere and then just disappears.
A conceptual history of stasis: the early years
That life has evolved has of course been completely confirmed by empirical and theoretical results from genetics, systematics—and the fossil record. In fact, not illogically, it was to the fossil record that the early naturalists interested in the history of life turned. They thought that fossils would reveal at least hints of what goes on during the origins of new species, including especially the species we see around us in today’s world. This scientific work all started in the earliest years of the 19th century. Among the most significant pioneers searching for clues to the dynamics of species origins were French and Italian “John the Baptists”!
Jean Baptiste Lamarck:
and Giambattista Brocchi,
who first wrote about evolution and fossils in 1801 and 1814, respectively.
What did they conclude about evolution in the fossil record? First up, Lamarck. He thought that species keep constantly changing. He was studying the fossil mollusks of the Paris basin—now recognized to be Eocene in age and thus roughly 56-34 million years old (not knowing the actual ages of his Eocene fossils, Lamarck nonetheless realized they had lived well-back in geological time). Arguing with his colleague Georges Cuvier at the Jardin des Plantes in Paris, Lamarck thought that species did not undergo true extinction—but they rather slowly and gradually simply evolved themselves out of existence. Lamarck thought that the species in the modern sea were the modified descendants of those he saw in the Parisian fossil record. However, the in-between-states in the gradual transformation of species between the Eocene originals and today’s highly modified descendants had simply not been preserved, such that the transition between the ancient and modern species appeared sudden and sans gradual transition.
Brocchi thought otherwise. He thought of species as having histories with births (speciation events) and deaths (extinction events)—just like individual organisms. Further, he thought species were generally stable during their existence. Aspects of Brocchi’s pattern matched up very nicely with modern theory on the evolution of species as seen in the fossil record, i.e., punctuated equilibria. However, we do not postulate true identity between Brocchi’s views and punctuated equilibria because Brocchi speculated that the extinction of species is somehow programmed into them, and that they might show some signs of aging near the end of their lifetimes; this notion of species senescence of course does not jibe with current theory.
He did realize though that sometimes species were killed off before they got old (presumably in the prime of life?!?) because of some environmental catastrophe—the geological species-level equivalent of getting hit by a Mack truck.
Brocchi’s view of species-level evolution (aka macroevolution) was also sophisticated as he postulated that species were parts of lineages, which comes close to matching the current modern view of species developed by Ed Wiley:
and referred to as the evolutionary species concept. To Brocchi, younger species replaced their progenitor’s as geological time marched on, but he stopped short of speculating how this replacement of old species by new, descendant species, actually happened. (We’ll talk about how that happens another time.)
Stasis as data
So, back to our hypothetical musings of chasing fossils up through time in outcrops, then through larger chunks of time as we visit more quarries in temporally and geographically expanding circles. We are talking here of the rich abundance of fossils left behind by marine invertebrate animals. Sample sizes are often large—meaning that heritable variation can be documented in a single locale (if not always single bedding planes)—and compared with the morphology and within-population variation in different places and slices of geological time. Both of us (BL and NE) have gone through this process using real-world examples—chasing certain lineages of trilobites (phacopids and asteropyginids) and brachiopods (Mucrospirifer and Athyris) over considerable spans of time (5+ million years, centering around ca. 380 million years ago) and stretches of geography (from New York west to Iowa, and Canada south to Virginia).
When we did this we ran smack (metaphorically, because we never run around outcrops, although one time when NE’s dog got lost in a quarry we did walk briskly in search of him … thankfully we found him, muddied but otherwise happy and unscathed) into the stone wall of implacable net evolutionary stasis. Sure, there is always some detectable variation within single samples (if you look hard enough at least!). And yes, even more easily detectable variation when you compare samples from the oldest, middle and youngest rocks containing elements of what is referred to as the “Hamilton Group fauna”. But, when all is said and done, our fossils looked pretty much the same from their earliest recovered samples right on up through to the very youngest specimens.
Think of it this way: say you go to the beach and collect an assortment of mollusk shells cast up by the tide along the strand line. It’s easy to sort them into discrete piles of the “same” shells.
And if you go to Cape Cod one day, and Long Island on another, you can make pretty much the same assortment of discrete piles of the “same” species. These separate species have pretty much a 1:1 relationship to the populations of still-living species under the waves offshore.
What we’re saying here is you can do this same sorting game in big-time three dimensions in the fossil record: through prodigious chunks of geological time by collecting vertically, and through space by collecting horizontally over equally prodigious hundreds, sometimes even thousands, of miles-wide chunks of geography. The local beach scene writ large, scaled up strikingly. That’s what stasis is: finding those same piles of shells on nearby beaches through non-trivial amounts of time and space.
Back in the 1960s, paleontologists were still being trained to expect to find a pattern, a detectable trace, of slow steady gradual change; at least if their samples were large, and spaced closely together. In actuality, sensu Yogi, they thus couldn’t see a lot, because they weren’t really looking for what they were finding. The expectation of slow, steady gradual change as the hallmark signature of evolution in the fossil record was, of course, Darwin’s principle legacy to paleontology—and the wider circle of evolutionary biologists in general. Even our good sampling (which is better for modern paleontologists looking in younger rocks, as Lamarck and especially Brocchi had the good fortune, or was it sagacity?, to work on), failed to reveal the hoped-for slow and steady change that paleontologists were supposed to find.
Ah, the stony evolutionary silence of fossil species that didn’t change much throughout their history. The way out was the realization that there are changes, marked changes that lead to new species, themselves going on to become stable hallmarks of descendant species, showing up laterally (geographically). That’s the story of “punctuated equilibria” (hereinafter “punk eek”) to be told another day on this blog.
The story here is of this key component of net stability displayed by most species from the start to the end of their evolutionary careers. NE and SJG dubbed this stability “stasis” (SJG was very good at coining names for new terms!). Stasis is one of the two main empirical underpinnings of punk eek.
And stasis, instead of the mark of failure by greenhorn paleontologists to find the gradualism that their elders had told them they must find if they did their jobs right, instantly became to us and a few other likeminded paleontologists in the 1970s a phenomenon that cried out for explanation in modern evolutionary biological terms. It took awhile for many paleontologists, and even longer for most other evolutionary biologists unfamiliar with fossils, to stop insisting that gradualism must be right, and stasis itself an impossibility. You really can see a lot just by looking, but only if you’re actually looking. As SJG put it, turns out that “stasis is data.” By now, nearly 50 years later, stasis has been pretty well recognized, though not always as thoroughly incorporated into evolutionary theory as we think it should be (that topic will also be forthcoming in a subsequent blog post).
Concluding remarks on stasis (for this post)
Well, if stasis is indeed data, how do we explain it? How in the world can a species remain evolutionarily stable, despite some oscillatory wobbling in its features as it plods its way through its time on earth. (Not really “sleeping” as our title suggests, but sort of “dynamically dormant” for as many as five million years or more). For it is indeed true, as evolutionary biologist John Thompson points out in his book Relentless Evolution,
that the genome of any given species is in constant turmoil, from place to place, from year to year. And so species or their parts are always changing or always moving, yet, overall, species don’t seem to “get anywhere” (just like the band in the video above) in terms of morphological change (which is, after all, the initial and still very much the original problem for evolutionary biologists to explain). We’ll leave you with that for now. Please tune in for subsequent posts with more discussion of the processes of stasis and more integration between punk eek and macroevolution.
This blog was written by Niles Eldredge and Bruce S. Lieberman. The current manager of the website is named below.