Remarks on the evolution of species: gradual or sudden?

This is not a raging debate. If it were, we would have heard about it, and that’s definitely not the case. But it was raging 150 years ago, when was published for the first time Darwin’s book On the Origin of Species. Do species evolve into different varieties and then into different species slowly, little by little over millennia? Or do they burst into life in great varieties together at the same time? Is the evolution of species gradual or sudden?

Admittedly, the debate wasn’t raging in pubs and street corners. It was between scholars with an interest in this question: naturalists, palaeontologists, archeologists. Today, the views of people in these fields on this and related questions are not so polarised: we have gathered a lot of data and acquired a greater understanding of the process of evolution as a whole—geological, biological and paleontological—over the last century and a half. But here is the thing that struck me last week when I got to Darwin’s discussion of this point, well into the second half of his book.

Several years ago, I read Stephen Jay Gould’s book Wonderful Life. I was amazed. This was surely in part because it was the first time I was reading a book of this kind, but it was also surely in part because it was simply fascinating to me to learn all these things I had never heard about anywhere else. Gould tells the story of the Burgess Shale in the Rocky Mountains of British Columbia discovered by the naturalist Walcott in 1909 and excavated by him and his wife over many years, summer after summer, slowly and painstakingly, during the following few decades. The distinct impression with which I was left was that Gould was arguing in support of a theory of evolution where widely different species appear in a short period of time (on evolutionary timescales), and then disappear or evolve more or less quickly depending on their particular abilities to adapt the the conditions under which they compete with others in their struggle for life.

This position and line of argument necessarily implied that there was a debate about the question of how species have evolved, and in addition, judging from the tone and general style of his argumentation, that he was himself campaigning to convince people of this, being somewhat isolated in this view. As a consequence, I was left with the impression that this was a modern debate, ongoing between Gould and others in the field from which (I only later found out) he was somewhat estranged. From Gould’s book, I was left thinking this view of a sudden evolution of species and varieties versus a slow and gradual process of accumulating genetic variations favourable for survival under changing conditions through the process of natural selection had been formulated recently. What a surprise it was to see Darwin himself addressing this question, countering well-formulated arguments and critiques of several eminent scholars he mentioned who favoured the hypothesis of a sudden rather than gradual evolution more than 150 years ago already!

There is no contradiction, though. The reason for this misinterpretation on my part is due to my own ignorance of the subject. Had I read a little more I surely would have understood that the debate was an old one. That Gould’s position, his line of argument, and the debate within which he was engaged during his life (1941-2002) as a scholar had a longer history. That the debate had been evolving slowly over the last two centuries, and that it has not suddenly appeared into the arena of hot topics in palaeontology, and that it must have surely have been most vigorous within Darwin’s own lifetime after the appearance of On the Origin of Species.

What the Burgess Shale revealed was out of this world.  But this was not understood for a long time: not until the latter part of the twentieth century. Walcott and his wife spent decades excavating, collecting, cleaning and classifying the specimens they would find doing this work on their own, summer after summer (this is how they spent their summer holidays). There were also generally very few people interested and  motivated enough to do this. In the end, it was their discovery, after all, and it was natural for them to work on it and for others to let them do it. The passing of a few decades, world war, major societal changes, and the Burgess Shale was largely forgotten. And besides, everyone wants to discover, to be the discoverer; nobody wants to rummage around someone else’s leftovers looking for something interesting they might have missed by sifting through decades of rubble: finding a gem in piles of rubbish is never very likely. But the gem was not to be found in the rubble; it was to be found in the re-examination of the fossils!

If you want to do your own discovery of the interesting history and intriguing story of the Burgess Shale and its weird animals, you should read Gould’s Wonderful Life. What I want to share with you here is that the fossils in these 530 million year old sedimentary layers were 1) exceptionally well preserved, 2) remarkably concentrated and large in numbers, 3) astonishingly wide-ranging in their variety of radically different body plans, major and minor structures, and physical appearance, and 4) all lived together, simultaneously and side by side, in the warm shallows of the primordial seas of the pre-cambrian. The ensemble of organisms in the Burgess Shale is one of the best examples of the pre-cambrian explosion: a very short period in our Earth’s history that saw, in several different areas of the world, an explosion of strange and wonderful creatures, a small number of which survived to become the ancestors to all currently living animals, including ourselves, of course.

In particular, it was found in the Burgess Shale one little creature, seemingly insignificant in comparison to the so many other larger, stronger and fiercer looking creatures, that is likely the first common ancestor to all vertebrates. That’s pretty huge if you consider that you will likely be hard pressed to think of more than a few animals that are not vertebrates. What this little creature looks like is nothing more than a tiny swimming spine covered in skin, with a little head and mouth at one end and bum hole at the other, with a thin digestive tube going along the spine from mouth to bum hole.

It is today plainly obvious that Darwin was correct in practically every conclusion is drew and explained in his writings, following a lifetime of observations, experiments, readings and lengthy considerations on the nature of evolution and on life itself. It is also clearly obvious that he was exceptionally gifted in his observational, scientific and intellectual skills, as well as remarkable forward-thinking and greatly ahead of his times. At the same time, it is obvious today that what the Burgess Shales tells us in a manner that makes it undeniable is that in the course of the history of life on this planet, explosions of species with widely different body architectures, sense organs and life systems have occurred. These species have coexisted for a time but then most have disappeared and left only a few of them as the progenitors for future generations of species and varieties within species.

We could say that Nature sometime plays a game of chance by producing as many different configurations and structures of living systems just so to maximise the probability that at least one of them will survive, evolve and procreate. Then again, we could also say that when the conditions are good, whatever is possible happens, more or less frequently depending on the requirements for this thing to happen, and therefore, without much restrictions on the conditions of life, a huge variety of organisms will appear in a relatively short span of time, that will look to us from the perspective we have today on the length of geological timescales like a wild and uncontrolled explosion of life.

Independently of interpretations or explanations, it is not debatable that new species sometimes appear very suddenly, and that sometimes they evolve from an ancestor very slowly over millennia. It is not debatable because both are seen in the biological and geological record, providing more than enough evidence to come to this conclusion. So what is there to debate? Why is there a debate? We can understand why there would have been one 150 years ago, but today? Maybe because the importance of each of these two different ways of evolution is difficult to asses. Maybe because some scholars grant to one a much greater importance while others disagree. Whatever the case is, it looks like I’ll have to keep reading and mulling over this matter to eventually come up with an explanation. And you, do you have any thoughts you’d like to share about this?