Fitness Landscapes in Early Vascular Land Plants

Lecture © Michael G. Riley
BIOL 606 Session, University of Alberta, February 28, 2001

A model, generally speaking, is a simplified representation of a more complex system. It provides an explanation or hypothesis that should make predictions that are testable through experimentation or observation. The study of fossil plants (or any extinct organism, system, or process) further complicates the model due to the possibility that experimental manipulation and direct observation of the subject is unavailable (Niklas, 2000). Does this mean that modelling the past is not testable by the standard scientific method, and is therefore, non-verifiable and more just an interesting idea rather than a directly verifiable hypothesis? No! If the laws of physics, mathematics, and chemistry are inviolable to all extant and extinct biological organisms, then there appears to be a solid foundation for modelling the past - a set of working principles that all life must adhere to.

The biological tasks (elements of fitness) plants need to survive are well understood. However, Niklas (1997) notes that evolution operates at the phenotypic level, so "…morphological components are arguably the principle foci of selection pressure." Hence, he proposes four fundamental and independent tasks early plants must perform in order to survive on land; conservation of water, light capture, mechanical stability, and spore dispersal (Niklas, 1997). The ability of a given morphological shape to maximise 'fitness', given one or more of these tasks, can be quantitatively measured relative to other variants using various biophysical and biomechanical functions. A computer is then used to generated a set of 'all' possible plant forms for each task in a 3D universe - a morphospace. The morphological forms are modelled around branching cylinders because these patterns tend to show correlations with major plant clades (Rothwell, 1995) which are useful for understanding the phylogenetic relationships between early land plants (Niklas, 1997).

Since the earliest known morphological land plant form is a dichotomous branching axis ((as seen in the fossil genus Cooksonia) (Gensel & Andrews, 1985)), Niklas uses this branch form as the starting point for the computer to begin its search. In addition, Cooksonia is best represented by a morphological shape generated when only water conservation is optimised. Consequently, Niklas begins each search (or adaptive walk) by maximising water conservation, then allowing the other three tasks to be added in sequentially. This creates a total of 15 morphospaces. Each morphospace is associated with its most fit forms. These forms can then be compared to the fossil record for similar patterns. Most of these forms can be seen in the records of fossil and extant taxa, so the model appears to mimic some of the diverse shapes found in the plant kingdom. Yet, the paper makes no predictions as to the possible forms that are 'least fit' (and should never appear in the fossil record), or the order of appearance that you might expect to see the 'best fit' forms appear. This makes it very difficult to test or make predictions based on the model. It was pointed out that the inclusion of some moderate and poorly fit forms may have helped in searching the fossil record for similar morphologies; for instance, you might expect intermediate or least fit forms as species evolve and move from one adaptive peak to another. Niklas also acknowledges that the model might mimic the morphological forms by chance or there may be many possible mathematical variations and combinations of the functions that could arrive at similar landscapes. Even if this is the case, the model can still be a useful tool in determining or eliminating the forces and processes that shaped the early land plants.

The model also predicts that as the number of tasks a plant must perform increases, the number of morphologies that can perform the task equally well also increase, while the overall height of fitness peaks decreases (Niklas, 1997). This is verified by the model. The fossil land plant record also reveals a similar increase in the number of morphological forms over time (Stewart and Rothwell, 1993)

Although the model makes many simplifications and assumptions about the biological tasks, physical processes, and biophysical laws acting on early vascular land plants, it does appear to mimic early fossil plant morphology. This model should be tested against the fossil record as it may provide a "…useful heuristic tool with which to explore the otherwise intractably complex relationship between organic form and function [ in fossil plants] and the environmental conditions attending their evolution," (Niklas, 1997).

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Gensel, P.G. and H. N. Andrews. 1985. Plant life in the Devonian. Praeger Press, New York, NY.

Niklas, Karl J. 1997. Adaptive Walks Through Fitness Landscapes For Early Vascular Land Plants. American Journal of Botany, 84(1):16-25.

Niklas, Karl J. 2000. Modeling fossil plant form-function relationships: a critique. Paleobiology 26: 289-304.

Rothwell, G. 1995. The fossil History of branching: implications for the phylogeny of land plants. Experimental and molecular approaches to plant systematics, 71-86. Missouri Botanical Garden, St. Louis, MO.