Thursday, 17 March 2016

Even Fish Walk All Over Creationists

Comparable disparity in the appendicular skeleton across the fish–tetrapod transition, and the morphological gap between fish and tetrapod postcrania

How creationists love those gaps!

Real or imaginary, you'll find creationism's magic creator stuffed into as many gaps as they can find to stuff it into. Until, that is, science actually looks at the gap. At that point there is always one of two outcomes:
  1. The magic creator isn't there and the gap is full of perfectly natural, scientifically understandable stuff.
  2. The gap wasn't there in the first place. It was either made up, the product of creationist wishful thinking, or, rarely, due to a scientific mistake.

Our work investigated how quickly the first legged vertebrates blossomed out to explore new skeletal constructions, with surprising results. We might expect that early tetrapods evolved limbs that were more complex and diverse than the fins of their aquatic predecessors. However, although radically different from limbs, the fins of the distant fish-like forerunners of tetrapods display a remarkable array of subtly varying traits. This variation may point to a previously unsuspected range of biomechanical functions in their fins, despite the fact that those ancestors lived exclusively in water.

Dr Marcello Ruta. Co-author.
Quoted in ScienceDaily
One of creationism's favourite gaps is the supposed gap in the fossil record showing how a fish could walk on land before it had a limb evolved for walking. It was a version of that old favourite, 'What's the use of half a wing?' question, which they like to imagine stumps every evolutionary biologist.

In fact, this was never a problem for evolutionary biologists because we have examples today of fish that walk on land and fish that walk on the sea bed. However, there was a gap of sorts which was something of a puzzle, one which is probably too subtle for creationists, but a little niggle for those really interested in the precise mechanism of the transition between fish and terrestrial tetrapods and in how evolution proceeds generally.

Now it looks very much like this gap was a rare example of a scientific mistake of sorts - there was an assumed 'gap' which probably wasn't there in the first place.

This particular 'gap' came from the understanding that the major structural changes in the tetrapod limb would have evolved shortly after the first salamander-like fish began to crawl out onto land, so a range of different solutions would be expected to evolve from the general fish fin; yet that radiation isn't there in the fossil record. As this transition onto land occurred the transitional fish/amphibians limb would have needed the additional function of bearing the weight of their owners as well as locomotion. In water, weight-bearing is not a major problem.

According to classical understanding of evolution, what should have happened next was a rapid burst of evolution with lots of different solutions to the new problem and a radiation into new clades each based on a different solution, similar to what we see when dinosaurs transitioned into birds.

However this is not what we see in the terrestrial tetrapods. All tetrapod limbs have the same basic plan, notwithstanding that there have since been modifications to the feet and the angle at which the hip joins the pelvis. All the later descendants of these early tetrapods - amphibians, reptiles, mammals and birds - belong to the same super-clade. There are not a lot of different solutions to the basic problem of walking whilst weight-bearing.

So why didn't this radiation take place? Is there a gap in the fossil record? Are there lots of extinct evolutionary 'experiments' which failed which we haven't found yet?

According to a paper published by Dr Marcello Ruta from the School of Life Sciences at the University of Lincoln and Professor Matthew Wills from the Milner Centre for Evolution at the University of Bath in the UK, it looks like that gap may not be there. The tetrapod limb might have evolved out of fish fins that had already started to evolve solutions to locomotion in water.

Abstract
Appendicular skeletal traits are used to quantify changes in morphological disparity and morphospace occupation across the fish–tetrapod transition and to explore the informativeness of different data partitions in phylogeny reconstruction. Anterior appendicular data yield trees that differ little from those built from the full character set, whilst posterior appendicular data result in considerable loss of phylogenetic resolution and tree branch rearrangements. Overall, there is a significant incongruence in the signals associated with pectoral and pelvic data. The appendicular skeletons of fish and tetrapods attain similar levels of morphological disparity (at least when data are rarefied at the maximum sample size for fish in our study) and occupy similarly sized regions of morphospace. However, fish appear more dispersed in morphospace than tetrapods do. All taxa show a heterogeneous distribution in morphospace, and there is a clear separation between fish and tetrapods despite the presence of several evolutionarily intermediate taxa.

Ruta, Marcello and Wills, Matthew A. (2016)
Comparable disparity in the appendicular skeleton across the fish-tetrapod transition, and the morphological gap between fish and tetrapod postcrania.
Palaeontology, 59 (2). pp. 249-267. ISSN 0031-0239 DOI: 10.1111/pala.12227

Copyright © 2016, John Wiley and Sons. Reprinted with kind permission under licence #3830930662453.

It has usually been assumed that when organisms evolve novel attributes that enable them to colonise fundamentally new environments -- as in the move from water to land -- this should trigger rapid evolutionary diversification and be accompanied by an increase in structural variety. Our work challenges this received wisdom, and shows that, at least in the case of the evolution of early tetrapods, key innovations did not quickly lead to greater anatomical variety.

For the first time, legs had evolved to fulfill new functions. Not only must they be able to support the weight of the body on land, but they also needed to enable the animal to walk. Perhaps these dual requirements limited the number of ways in which these first legs could function and evolve, thereby constraining their range of variability.

Professor Matthew Wills. Co-author.
Quoted in ScienceDaily
This challenges the long-held assumption that when a 'key innovation' evolves, there will normally be a rapid burst of evolution, as species evolve into the new niches this innovation makes accessible. However, fishes may already have begun this radiation before leaving the water. The authors think that the range of anatomical possibilities may have been constrained by the need to walk and weight-bear simultaneously using a fin already modified, so the opportunities for an evolutionary radiation were limited by the time the transition to terrestrial life began.

In order to even begin to transition into terrestrial tetrapods the fish would have needed an already modified fin. That modification could have had some other benefit for aquatic vertebrates and only incidentally made it possible to walk and weight-bear on land. There is, of course, no reason to assume purpose or intent in evolution and evolution is nothing if not opportunistic.

The interesting thing here is that a new finding is challenging some basic assumptions, in complete contrast to how creationism deals with inconvenient facts - like this assumed gap not being there, for example. New information is never allowed to challenge creationism's basic assumptions. With science, however, new information which challenges basic assumptions is not only looked for but welcomed. There are few real scientists who don't enjoy the challenge of having to rethink basic assumptions.

How much easier it would be, but how utterly pointless, to always have to arrive at the 'Goddidit!' conclusion for everything, because either your livelihood depends on it or you are too afraid not to in case a magic invisible man in the sky gets them.

Incidentally, if any creationist managed to get this far, did you notice how the authors of this paper wrote as though the Theory of Evolution is a theory in crisis as more and more evidence against it is found, like creationist pseudoscientists tell you? Me neither! The entire thing was based on complete confidence that evolution is the only scientific way to explain the appearance of living things. They didn't once need to invoke magic!

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1 comment :

  1. Rosa, you wrote: Incidentally, if any creationist managed to get this far [in my blog post]".

    No, I don't think they managed to do that. You seem to be too optimistic about the reading & understanding capacities of creationists.

    ReplyDelete

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