21 June 2006

Gavin de Beer and mosaic evolution

"In a broad sense, all organisms can be said to be mosaics, with some characteristics so ancient in origin that they have changed little and some so recent, geologically speaking, that they have changed more than a little. It rarely occurs to most of us that we share ancestral characters with such different organisms as, for example, a flowering shrub."

George Gaylord Simpson, 1983

Detail of a mosaic from Foça (ancient Phokaia) in western Turkey. Photographed in 1995.

On this date in 1972 the English evolutionary biologist Sir Gavin R. de Beer died (born 1899). One lasting contribution of de Beer to evolutionary theory was the concept of mosaic evolution that he developed in a 1954 paper1.

Although a mosaic-like pattern of evolution is becoming more and more apparent in many major evolutionary transitions, including the evolution of humans from ape-like ancestors, I have not been able to find a recent general review of what mosaic evolution is all about. Stebbins published a review in 1983, but he seems to have confused mosaic evolution with adaptive radiation2. Recently, Mayr3 and earlier, Simpson4 gave brief explanations of the concept with both authors properly distinguishing mosaic evolution from adaptive radiation. Curiously, Ridley’s textbook on evolution5 doesn’t mention mosaic evolution at all.

To my great satisfaction, however, I have found in de Beer’s succinctly written original paper a full explanation of his idea.

As the title of his paper implies, de Beer derived the concept of mosaic evolution from his study of the fossil Archaeopteryx and by comparing it with the bones of reptiles and birds. He found that Archaeopteryx had both reptilian and avian features:

"All these are characters which would not be in the least out of place if found in any reptile. On the other hand, there are a number of features in Archaeopteryx which are absolutely characteristic of birds"

This comparison led him to conclude that

"…it is clear that Archaeopteryx provides a magnificent example of an animal intermediate between two classes, the reptiles and the birds, with each of which it shares a number of well-marked characters."

And this led to the formulation of mosaic evolution (content in brackets mine):

"…the statement that an animal was intermediate might mean that it was a mixture and that the transition affected some parts of the animal and not others, with the result that some parts were similar to those of one type [ancestor], other parts similar to the other type [descendant], and few or no parts intermediate in structure. In such a case the animal might be regarded as a mosaic in which the pieces could be replaced independently one by one, so that the transitional stages were a jumble of characters some of them similar to those of the class from which the animal evolved, others similar to those of the class into which the animal was evolving."

He then applied these ideas to the fossils exemplifying the transitions from fish to amphibian and from amphibian to reptile and finally, from reptile to mammal. In each case, his observations derived from specific examples can be turned into general statements of mosaic evolution.

"But the fact that an animal can be at one and the same time show so many features which would make it an ideal transitional form, and also spoil this picture by possessing one or two characters which rule it out as a direct ancestor, is itself an argument in support of the principle of mosaic evolution, with the different pieces evolving separately, and some of them too fast. This phenomenon is found again and again in the study of transitions from one type of animal to another, and appears to be of general applicability. It would be more difficult to understand if the transitions took place by a gradual and simultaneous conversion of all the parts of the animal."

This was followed by the notion of the evolution of different organs at different rates:

"Just as in some cases…an animal may show characters which have evolved too fast relatively to the other characters, in other cases certain characters may have been left in a profoundly archaic condition."

De Beer even dealt preemptively with potential objections to his idea:

"Organisms are delicately balanced and adjusted mechanisms, and on the average, changes are more likely to upset than to strengthen them. Selection may therefore be expected to have acted with greater rigour against organisms vaying in more than one direction at a time, unless the directions were correlated…"

All of this terminated in a final conclusion:

"A necessary consequence of mosaic evolution and of the independence of characters evolving at different rates is the production of animals showing mixtures of primitive and specialised characters."

A technical paper discussing de Beer’s significant accomplishments in embryology is available here.

Some recent technical papers on mosaic evolution:

Andrew N. Iwaniuk, Karen M. Dean, John E. Nelson. 2004. A mosaic pattern characterizes the evolution of the avian brain. Proceedings: Biological Sciences 271, S148–151.

Robert A. Barton and Paul H. Harvey. 2000. Mosaic evolution of brain structure in mammals. Nature 405, 1055-1058.

Todd C. Rae. 1999. Mosaic Evolution in the Origin of the Hominoidea. Folia Primatol 70:125–135

1. De Beer, G.R. 1954. Archaeopteryx and evolution. Advancement of Science 11:160-170.
2. Stebbins, G.L. 1983. Mosaic evolution: an integrating principle for the modern synthesis.
Experientia 39:823-834.
3. Mayr, E. 2001. What evolution is. Basic Books.
4. Simpson, G.G. 1983. Fossils and the history of life. Scientific American Books.
5. Ridley, M. 1996. Evolution. 2nd ed. Blackwell.

Cross posted at Transitions.

1 comment:

Anonymous said...

thanks,I was wondering about the mosaic analogy