Long-term evolution on Earth has shown a directional trend. Although there are many exceptions it is evident that simple self-replicators have evolved into large multi-cellular organisms with high metabolic rates and complex behavioural interactions. But after Darwin's (1859) introduction of natural selection as a process of chance and history the consensus has arisen that there is no overall direction to the evolutionary process (e.g., Mayr, 1988; Gould, 1989; Williams, 1992). As concluded by Maynard Smith and Szathmary (1995) : ''On the theoretical side, there is no reason why evolution by natural selection should lead to an increase in complexity".

But this conclusion no longer holds. This is because the theory of Malthusian Relativity gives a strong theoretical background for the hypothesis that long-term evolution is directional. The direction arises from selection for a steady increase in the energetic state of the organism (energetic state is defined here as the net-assimilation of resource). This type of selection may be seen as an axiomatic rule of long-term evolution in stable resource rich environments. This is because the extra energy that is gained from the increase in energetic state can be invested in fitness enhancing traits like reproduction, survival, and interactive quality.

A continuous increase in the energetic state and the ability to exploit the resource has traditionally been part of the paradox of evolution toward extinction. Increased resource assimilation leads to increased reproduction and increased population growth, and thus in the long run it may lead to the over-exploitation of resources. And with a continuous increase in the exploitation rate, over-exploitation may become so server that both the population and the resource becomes extinct. But this is true only if we disregard the effects of competitive interactions. In Malthusian Relativity, with selection by density dependent competitive interactions, the increased population growth generates increased interactive competition and increased selection for interactive traits that trade-off against further population growth. The result is an abundance that stabilises at an intermediate level while the continuous increase in energetic state generates an exponential increase in traits like body mass. This type of increase has been observed, e.g., in fossil horses during the last 57 million years (MacFadden, 1986).

It has traditionally been argued that the relatively steady increase in body size that has occurred among the largest organisms on Earth is more likely to be due to evolutionary diffusion than to directional natural selection (e.g., Stanley, 1973). It is argued that this hypothesis is supported by the fact that many, if not most, organisms have been left aside from the increase and by the fact that many organisms even seem to dwarf in size. But the directional evolution of Malthusian Relativity is not in conflict with these facts. Instead the theory predicts that these patterns are what we might expect. This is because the predicted increase is based on unconstrained evolution in a stable resource-rich environment. The environment may instead impose a limit to the amount of resource that the organisms can consume, and as such limits will differ for different niches we can expect a variety of energetic levels and body masses. And if the available resource is extremely sparse the upper limit to resource consumption may be so low that the organism cannot evolve away from the low-energy self-replicator. Such low-energy self-replicators may also be unable to invade environments with more abundant resources if these are dominated by high-energy organisms that exclude the low-energy organisms by direct inter-specific interactive competition. And according to Malthusian Relativity a species may dwarf if its individuals have progressively less resource available, suggesting that dwarfing may be widespread during periods of environmental crisis.

References

• Darwin, C. (1859). The origin of species. London: John Murray.
• Gould, S. J. (1989). Wonderful life. New York: Norton.
• MacFadden, B. J. (1986). Fossil horses from ''eohippus" (hyracotherium) to equus: scaling, cope's law, and the evolution of body size. Paleobiology 12, 355--369.
• Maynard Smith, J. & Szathmary, E. (1995). The major transitions in evolution. Oxford: W.H. Freeman Spektrum.
• Mayr, E. (1988). Toward a new philosophy of biology. Observations of an evolutionist. Cambridge: Harvard University Press.
• Stanley, S. M. (1973). An explanation for cope's rule. Evolution 27, 1--26.
• Williams, G. C. (1992). Natural selection. Domains, levels, and challenges. New York: Oxford University Press.