What is a Transitional Fossil?

(S. Joshua Swamidass) #1

Continuing the discussion from The Explanatory Power of Darwinism:

This is redefining the term “transitional” species. Are you familiar with Zeno’s paradox?

Not by a long shot. You are not justified in saying such a thing. Once again, do you know what Zeno’s paradox is?


Zeno’s Paradox in creationism: For every transitional form discovered, two additional gaps are created.

Emoticon added for communication context: :grin:

(Jon Garvey) #3

I’m not sure what work Zeno’s paradox does here. He proposed it in order to show the impossibility of change in the world, because of infinite gradation, so it has nothing to say on actual processes of change. He failed, under Aristotelian reasoning, because he did not realise that his infinite transitions were potential, not actual. And so a runner with actualized powers to run in 1 metre strides will divide his course into exactly 1 meter transitions. A runner with super powers following a laboratory accident can leap 100 meters in 1 transition. The number of transitions occurring depoends, in other words, on what actual processes are operating (Feser, Scholastic Metaphysics, p.31ff), and correspondingly the missing transitions acquire more than one explanation.

Applied to evolution, gradualism necessarily approaches the conditions of Zeno’s paradox - infinite small changes would never get anywhere at all. In practice, of course, the number would be finite, as mutations are “quantised,” and change occurs, but only via a huge number of forms - hence the 1-4bn species guesstimate.

But if the (much disputed) immediate precursor of Iguanodon possessed, or was given by God or external forces of nature, the ability to make a suite of changes in one generation - or even in some small number of generations - the actualized number of transitional forms would be the number of generations, no more and no less, down to zero in a true saltation.

The proportion of transitions to actual forms, then, provides evidence for the kinds of powers at work, ie the actual lengths of Zeno’s runner’s strides. If his footprints are only found every few metres, it could mean they’ve been erased, or it could mean he has huge strides, or anything in between, and cannot be evidence for one theory over the other. The one thing his footsteps do prove is that Zeno’s paradox is false, or he would never have left the starting blocks.

If one has reasons to suppose, however, that a vast majority of closely overlapping footprints have not been erased, then one is justified in supposing he takes reasonable strides, subject to the usual sampling losses, and that’s why Benton’s taphonomic, phylogenetic and historical analyses of the fossil record become important.

To paraphrase Argon’s comment. for every transitional form discovered that fits a species accumulation curve, several million species are rendered less likely.

What have I misunderstood about Zeno that you two philosophers know?

(S. Joshua Swamidass) #4

@jongarvey saw your post:

On another thread about evolutionary transitions, Joshua cited Zeno’s Paradox to me, in my view inapplicably. However, something like it seems to operate on any gradualist evolutionary scheme.

The issue here is Transition Fossils, which are defined as fossils we can find that are intermediate between forms we see today in the present. By this definition, the most important definition, there are transitional fossils everywhere.

There is a separate question all together about whether we see transitions between two forms only observable in the past. In this case a different challenge arises. Every continuum is infinitely divisible. So every transitions between two forms in the past will raise question about the transitions and the two end points. Produce another transition, and the questions can continue (now with two gaps to fill). Any continuum is infinitely divisible, this is not resolvable. Hence the reference to Zeno’s paradox.

For example, we indisputably find transitional fossils (first definition) between Sapiens and Apes (e.g. Neanderthals, Homo Erectus, and more). That doesn’t stop people from complaining about lack of transitional forms (second definition), because continuums are infinitely divisible, though taxonomical language is discrete (Zeno’s paradox).

Are you still so sure it is not relevant?

(Jon Garvey) #5

First of all, having been stung recently, I’d like to distinguish between transitional fossils and intermediate fossils. The number of true transitionals is small, and although this partly reflects the very nature of cladistics (nothing is ever assumed to descend from a known type, but shares a common ancestor, which is always a hypothetical node on the cladogram, not a species), it matters as soon as you move away from gradualism.

Zeno’s paradox does depend on the infinite division of a change, and so is most relevant to Darwinian gradualism - if the paradox were valid, then gradualism would be impossible in principle - but fortunately, the number of genes that can change is finite, and so the paradox doesn’t apply even there.

But the philosophical resolution of the paradox was achieved in ancient times by Aristotelian metaphysics, by saying that each of those infinite divisions was only a potency, and that in reality only a finite number of steps would be actualised - in the original paradox, the runner’s paces would actually be 1 metre, or whatever, long. In evolution, if evolution occurs by discrete changes of form, in contradiction of Darwin’s recycled scholastic dictum Natura non facit saltus, then nature too moves in “paces”, and not through an infinite (and therefore non-traversible, if Zeno were correct) continuum.

So moving away from a “change of gene frequencies” model of evolution to some variety of saltational model (including special creation of any kind that isn’t simply gradual gene substitution - for example, the “nudging” of an entire package of complementary genetic changes) - then the number of transitions that ever existed would be strictly finite. To apply that to your example, Australopithecus - let’s assume for argument’s sake a human ancestor - did not gradually morph into, or through, a continuum of forms, but truly transitioned, let’s say into or through H erectus, H Neanderthalis, Denisovans and a few more distinct forms that haven’t been discovered or were never preserved.

In that case, finding a new transitional just adds +1 to the limited menagerie. We are no longer assuming a chronological version of the scholastic “principle of plenitude” in which every variant must have existed in principle, even if there is no evidence for it.

Hybridisation of those few forms, as in our own case, increases the possibilities, but at most by creating one new form from two, or more likely by being absorbed into the existing form.

Evolutionary intermediates, in this scenario - such as a medium size Samotherium which, it is agreed, is not conceivably ancestral to the giraffe, become even less significant than under gradualism, because we are not committed to saying the giraffe neck must have been growing through many divergent and continuous branches. And as I pointed out before, it is as true now as in Darwin’s time that most “transitions” in the fossil record are constructed from intermediates not even believed to be in the direct line of descent.

The question then is whether the evidence favours either model over the other. As far as the fossil record goes, the stasis-saltation pattern has been well publicised since Gould (whose own explanation depends on “accelerated gardualist” assumptions below the resolution of the fossil record. There are good reasons to suppose the fossil record is not nearly as incomplete as gradualism demands.

Even simple sampling methodology confirms that: on an infinite continuum, one would expect to find few fossils of many forms, and seldom (unless in juxtaposition) more than one from any species. In fact, we find many billions of fossils from (currently) only around 250,000 species - a mere fraction of living species… of which incidentally a large poercentage are found as fossils.

That would a truly extraordinary ratio given gradualism, turning Darwin’s “How come we don’t find more species in the fossils?” into the largely ignored “How come we find so many of each species?”

Population genetics applied to transitions tells us only that there appears to be common descent, not whether the large-scale changes occured at a more or less constant rate or not, unless one makes the assumption (contested by many including Gould and Eldredge) that all evolution is essentially scaled-up population genetics.

And so I do say that Zeno’s paradox is irrelevant
(a) because if true it renders gradualism, and all change, logically impossible. That is the very thing Zeno sought to prove by it.
(b) because under the Aristotelian scheme that resolves the paradox, actualised forms are discrete entities anyway, and the number of transitions is strictly finite. It predicts that one will inevitably find variations (if there’s a small, a medium and a large, one can arrange them in ascending order, and that may or may not correpsond to some real sequence of changes), but that one will only actually find a limited number of such variations. The gaps will be real, and will require some explanation beyond “Join the dots.”

(George) #6


Based on your own analysis… it can’t be true … because you say it would mean change is impossoble.

(Jon Garvey) #7

You’ve understood Zeno’s paradox. But if untrue it’s irrelevant as I said… The trick to being a philosopher is understanding why it’s not true. And when you know that, it weakens gradualism (though not entirely, as I stated in my post, because even gradualism does not posit infinite numbers of changes).

(George) #8


And so the paradox is untrue AND irrelevant.

(Jon Garvey) #9


(S. Joshua Swamidass) #10

In context that is what I saying too. The “lack” of transitional fossils is not a valid objection. We do have them under the most important definition.

The distinction between intermediate and transitional is good @jongarvey.

(S. Joshua Swamidass) #11

@jongarvey, haven’t we moved past Darwinism too? Why argue against a long dead scientist whose views do not reflect modern evolutionary science?

(Guy Coe) #12

He’s critiquing evolutionary gradualism, and you know that. So, we can drop the sense of the antiquated scientist argument.

(George) #13


If only I could figure out to whom you wrote this post… Joshua or Jon?..

(Jon Garvey) #14

In this case because his problem remains - we find relatively few species, and usually many examples of each. The difference is that we have some strong evidence that his explanation - a woefully sparse fossil record - is not valid.

(T J Runyon) #15

What evidence @jongarvey taphonomy is one of my strengths.

(Jon Garvey) #16

I normally cite the work of Michael Benton in this area, but on this question the fossils themselves speak too. Prothero’s estimate that there are around 250,000 described fossil species still seems valid, give or take new species and taxa that become lumped.

Prothero is keen to point out what a small proportion of living species that number is (he cites 1.5m named living species, which is the direct comparison, but uses the possible upper limit of undiscovered species to arrive at his 5%… an interesting methodological choice, on reflection; like for like it would be 16%, but no matter).

What he does not mention is that a conservative estimate of the number of described fossils in museums around the world must be in the order of >200 million (Graeme Lloyd of the London Museum of Natural History estimates each museum has several million, and Wikipedia has a list of institutions). Granted there is misattribution, and that new species are no doubt hiding in boxes, it seems the right order of magnitude.

On that basis, a crude average representation of each named species is 800 fossils. Prothero’s assumption for the total of species that have lived is >>100 times the present, so conservatively lets make that 1 billion. On that basis (other things being equal), only 0.025%, or 1 in 4000, fossil species have been discovered.

And that means that our 200m fossil specimens ought to represent only 20% of the total of species that have lived, and it ought to be uncommon to find two of the same species.

Now, all things are not equal, and good fossil-forming conditions are uncommon. If many specimens are found in a single geological formation, that will bias the results significantly. But conversely, identical species are found widely scattered - just as one example, Iguanodon is represented, it seems, by just two species across three continents.

If Prothero’s estimates are right, then there would seem to be an extraordinary set of conditions operating to lose most of the world’s species entirely, but represent the preserved remainder abundantly; and not only that, but representatively enough for us to be able to construct a reasonably complete tree of life from what ought to be such a tiny sample as to be unrepresentative of anything.

The representative nature of the fossil record has been assumed for ecological studies, such as Bob Bakker’s comparison of herbivore and carnivore fossil representation to conclude that the latter were homoiothermic like mammals, and not cold-blooded like reptiles, a conclusion now taken for granted.

(T J Runyon) #17

I don’t really find this troubling. As far as multiple specimens of the same species go, like you said many come from the same locale. And from those that are more geographically dispersed, they are going to live in the same type of environment which means the same potential for fossilization. When you look at the fossil record I don’t know how you don’t come to the conclusion that the lack of fossils is due to preservation issues. The species with the most lacking record are those who live in environments where fossil preservation potentional is crazy low. That’s why we don’t have many tree-dwelling primates. Jungle floors suck at preservation. As far as intermediates and transitionals, a high number of which exist, a consequence of cladogenesis is that the populations where these transitions are taking place are small so they are even less likely to preserve. May be accused of explaining it away but this is just reality. My mosasaur is missing 5 verts. Obviously I’m not going to conclude this specimen had less Verts than other specimens. The fossil record is made up of specimens that lived in areas with more preservation potentional. I mean you have your rare freak specimen and your handful of Lagerstätten. Early homo is so messy because of the environment where this all took place isn’t good for preservation. So we just arent getting any appendicular elements. Which we really need to draw relationships and to determine which traits are derived.

(Mark M Moore) #18

You guys may have no trouble sorting the difference in your heads. I like to remember it with an example which dates me. An El Camino is intermediate between a station wagon and a pick-up, not transitional from a station wagon to a pick up. Or to go even further, its not an intermediate, its a composite. And I do think that is a lot of what we see. Creatures are found which are a surprising mix of traits found in other forms. This does not make them transitionals or intermediates between the forms whose traits they share. They are a composite, like the platypus. It can all be written off as “convergent evolution” of course, even when bats and dolphins use the same genes for echolocation, because evolution explains both why things are different, or why they are the same, or whatever one needs it to explain.

(George) #19

To all readers:

I am not opposed to the intentional use of these 2 terms to mean 2 different things. But we need to remember to include working definitions each time both terms are used.

Even today, you can be confident that (if the author does not offer a distinction) that a casual use of the term “Transitional” was/is INTENDED with same meaning as “Intermediate” !! For decades… these terms have been synonyms.

I once did a very in-depth analysis of these terms for @Marty… and I was surprised that what I THOUGHT was the meaning is not how sciences usually intended the meaning to be.

It essentially put me off Cladistics in general. There are less controversial evidences out there!

(Marty Pomeroy) #20

Thank you George for reading me in.

I don’t see mentioned here that there are fossil beds that “collected” specimens for millions of years somewhat continuously. Since some beds were active for several million years, and five million years would be 1% of the history of complex life, one would expect to see 1% of the changes in that locale recorded in some detail.

In these beds, one would expect to see gradualism played out somewhere. But there are always steps. Somewhere we should see a movie-like sequence of one species turning into two with a morph-like continuum. None.

Please feel free to correct me because I am not a paleontologist. If there are examples of this, please link them. If not, why not?

This, to me, is the best evidence that gradualism is not the means.

As regards the meaning of “transitional” in regard to fossils, the current definition is not the same as the definition used by Darwin. It has been modified to fit the data. But as George mentioned, this also distorts the common way the term is used outside of paleontology. I am of the opinion that this is partly strategic, so that when someone quotes Darwin on “transitional forms” to point out the problems still exist in the data, the supporter of atheistic evolution can respond “there are huge numbers of transitional fossils.” Then they lay out the modern definition so that the quoter of Darwin appears out of touch.