Friday, 1 August 2014

The Incredible Shrinking Dinosaurs

Dinosaurs shrank for 50 million years to become birds - life - 31 July 2014 - New Scientist

I was surprised and more than a little disappointed to to see the angle taken in the above article. It looks almost designed to supply a handy quote for quote-mining creationist pseudo-scientists to mislead their credulous victims with.

"No other dinosaur group has undergone such a long and extended period of miniaturisation," says Mike Lee of the South Australian Museum in Adelaide. "Statistically this trend was far stronger than by chance, analogous to flipping a coin a dozen times and getting all heads."

[...]

The analysis reveals that the ancestors of birds shrank without interruption. "What was impressive was the consistency of the size change along the dinosaur-to-bird transition, with every descendant smaller than its ancestor," says Lee.

It's not till we get to the final sentence in that last paragraph that we discover there is a simple and obvious answer and one moreover which illustrates how Natural Selection makes evolution non-random.

Getting smaller must have offered advantages at every turn.

So why not say so? There is no highly unlikely probability to explain here; Evolution by Natural Selection provides a complete explanation.

The gradual evolution of smaller and smaller body size would have allowed the bird predecessors to explore novel niches and body plans off limits to their larger relatives. It would have permitted them to chase insects, climb trees, leap and glide, and eventually develop powered flight.

Michael S. Y. Lee, Author
In fact, the statement in the article that these evolving proto-avian theropod dinosaurs "shrank without interruption" lacks finesse. It is true to say that the specimens examined showed a consistent shrinkage but, since no fossil sequence can ever be complete for every generation we actually don't know what size the missing specimens were so we can't say for certain that it was "without interruption", though the overall trend was undoubtedly and markedly towards smaller size.

But perhaps I'm being a little over-sensitive, having had a couple of run-ins today with creationists, one of whom was an obvious fraud selling quack 'pheromone' potions to credulous simpletons and vulnerable, inadequate people in the best traditions of the snake-oil trade and who would have no hesitation in quote-mining this and presenting it as an example of science disproving evolution by showing it is statistically impossible. It does no such thing, of course and shows precisely the opposite but creationist frauds can usually rely on their victims not following up a source and checking it for themselves.

As we can see, the authors make no such claim in the paper and never raise the question of probability:

Abstract
Recent discoveries have highlighted the dramatic evolutionary transformation of massive, ground-dwelling theropod dinosaurs into light, volant birds. Here, we apply Bayesian approaches (originally developed for inferring geographic spread and rates of molecular evolution in viruses) in a different context: to infer size changes and rates of anatomical innovation (across up to 1549 skeletal characters) in fossils. These approaches identify two drivers underlying the dinosaur-bird transition. The theropod lineage directly ancestral to birds undergoes sustained miniaturization across 50 million years and at least 12 consecutive branches (internodes) and evolves skeletal adaptations four times faster than other dinosaurs. The distinct, prolonged phase of miniaturization along the bird stem would have facilitated the evolution of many novelties associated with small body size, such as reorientation of body mass, increased aerial ability, and paedomorphic skulls with reduced snouts but enlarged eyes and brains.


Size reduction, whatever processes drove it, certainly seems to have allowed the bird lineage to fill niches that small-bodied animals can, and to undergo a fairly extensive radiation into these.

Bhart-Anjan Bhullar, Yale University
What this study shows is that at each stage in the evolution of birds from large theropods there was a distinct advantage in being small. The fact that the rate of change was four time faster than the change in other theropods which did not evolve into birds shows that there was strong selection pressure in this evolving branch favouring smaller size. The probability of getting smaller at each 'stage' (what stages are there in a continuous gradation?) is irrelevant since Natural Selection loaded the dice.

The reason theropod dinosaurs in that clade were able to exploit each new niche that a further reduction in size enabled them to move into, was that there was no real competition. The smaller dinosaurs were more successful than their larger siblings and so tended to leave more descendants. The 'decision' to get smaller was never taken on the flip of a coin or the roll of a dice because there was no decision involved. Probability is only involved in determining the range of variation on which Natural Selection acts. Smaller dinosaurs in that clade were more successful and that's all it takes. Had larger ones been more successful, the same range of variation would have resulted in larger dinosaurs.

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1 comment :

  1. I find it very fascinating that birds are in fact living dinosaurs in the same sense that humans are a type of mammal.

    The other day i read an article that most, maybe nearly all, dinosaurs were feathered. Maybe these feathers were evolved for warming up the animal. And/Or to impress on other dinosaurs in the mating process. Then later on the same feather producing genes proved useful to a totally new trait: namely gliding and flying. Cf gliding/flying snakes; see: http://www.flyingsnake.org/ .

    Back now to the dinosaurs. The miniaturisation process those animals went through has a pre-history as well. See this article: http://theconversation.com/take-a-t-rex-and-a-chicken-and-youll-see-how-dinosaurs-shrank-survived-and-evolved-into-birds-29996 . Here are some quotes from that article:

    1) The Lee study comes on the heels of another fascinating paper on dinosaur size evolution published in PLOS ONE in May by Roger Benson at the University of Oxford.

    2) Benson and his colleagues [...] found that the earliest dinosaurs, living more than 200m years ago, underwent a spectacularly rapid rate of body size increase as they evolved from housecat-sized ancestors. This happened as the Earth was recovering from the worst mass extinction in its history .

    3) Over time these high rates of growth decreased in most dinosaur groups, as they usually do when ecosystems become saturated with specialised species. But there was one exception: the lineage leading towards birds. This peculiar group of dinosaurs experienced a reversal in size growth and experienced elevated rates of size decrease for more than 100m years.

    4) The Lee and Benson studies tell us some neat things about the evolution of birds. It seems like small size was crucially important to birds and their closest dinosaur relatives. That is the only way that such fast rates of size decrease could be maintained for so long. And perhaps it is that very ability to evolve so fast for so long that set birds apart from other dinosaurs, drove them to diversify into so many species, and even enabled them to survive the extinction that wiped out Tyrannosaurus, Triceratops, and all of the “conventional” dinosaurs.

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