Geology 107: Paleobiology
Bauplane can and do change. For example, the amphibian Bauplan must at some time have changed into the reptilian one, and so on. Again, as in speciation, one may postulate rather special circumstances in which stasis breaks down. Is the Bauplan modified only in some special process, or in some ordinary extrapolation of normal microevolutionary processes? Note that if the Bauplan is not ordinarily operated on by selection, changes in it cannot be accomplished by ordinary adaptation, and we must appeal to some unusual process.
I don't like any of this. I see stasis in species as maintained continually by stabilizing selection, and I see the Bauplan as similarly, continually, subject to selection. Clades persist because their constituent populations are well enough adapted to reproduce themselves, and the Bauplane of the constituent individuals are a part of that adaptive complex. We see dramatic selection against radical departures from normal Bauplane in many mutations in humans, domestic animals, and crops.
But we do have to face the question whether every character we see in organisms past and present is a good adaptation, molded and perfected by natural selection. Are there characters that are not adaptive?
Note that the two constraints were built into Rudwick's arguments, in which he specified that the paradigm was the best structure that the organism in question could have built. But the power of Seilacher's analysis is that it explicitly separates out the constraints that prevent a structure from being optimal from the functional component would otherwise drive it toward optimal morphology.
Seilacher's approach has come to be known as Konstruktionsmorphologie, or Constructional Morphology. Later modifications and discussions have not altered its basic structure. Thus Raup added the components of chance and phenotypic response to the list of agents that can prevent a structure from being optimal in morphology, and Hickman separated out further subcategories of constructional and historical constraints. Cowen suggested that constraints might not be as important as we might think. Structures might be closer to optimal than they look because they are built to be cost-effective rather than perfect. For example, people might like to own a Mercedes but often buy a Volkswagen instead. If you like, you can think of cost-benefit considerations as a separate, and very important, constraint on morphology.
Altogether, then, it's clear that we cannot expect perfection in structures that have evolved in real organisms under natural selection. The question then becomes, how can we tell which aspects of a structure are adaptive, and which are molded more by the various constraints that act on them? The techniques involved are included in the exercise of functional analysis of fossils, and they have two major aspects.
1. Comparison with Living Organisms. We can make observations and experiments on living animals which allow us to identify adaptations. For example, the peppered moth of England suffers predation that depends on its capacity to find a background that will give it camopuflage against the birds that hunt it by sight. Dark and light morphs are differentially selected, depending whether the trees they use as perches are dark or light. In this case, color is an important component of adaptation.
In assessing the morphology of fossils, then, an immediate task is to find homologous or analogous morphology that has been carefully studied among living organisms. Direct comparison can then be made against the detailed morphology of the fossil.
2. Engineering or Thought Analogues. Sometimes there is no available homologue or analogue, and so we make comparisons with structures that are man-made. Thus the flight of pterosaurs is being studied by building full-size replicas, and the plates on the back of Stegosaurus were studied by building models. In these cases the analogues can be made as exactly applicable to the fossil as ingenuity permits, whereas the availability of homologous or analogous living organisms is not predictable, and the comparisons may not be directly relevant. But the non-biological comparisons are often difficult because the constructional materials do not match exactly, and there are no comparable historical constraints on man-made objects.
Any functional analysis of a fossil can be tested only by the goodness of fit to the predictions of a hypothesis, and goodness of fit is not a numerical, objective parameter. It is certainly best if one can design a series of tests, all of which must be successfully met. Thus the lens of a trilobite eye must be interpreted in a way that makes optical sense, the lenses of the eye must then work together to make up an operational organ, and the eyes must operate in a biological reasonable way in the animal as a whole. Any reconstruction of trilobite vision therefore must survive tests at multiple levels, and we may feel confident about a hypothesis that survives such testing.
Such analyses, applied to individual species, or evolutionary sequences of species, may tell is a great deal about the way they operated in their environments, and therefore about the selective agents that brought about the adaptive features noted on the organisms. In the end, they add more information that we can use to reconstruct evolutionary mechanisms, so that we can deal with the paleobiology of the organism in its environment, instead of lists of characters that its fossils share. Return to 107 main menu