The potentially simplest mechanism for a causal theory of evolution is the intrinsic selection that emerges from heritable difference in the rate of self-replication. This route, however, is unlikely to be productive. It is already taken by classical selection theory, where it was necessary to include contingency in order to get the theory running. The result is either the uncertainty for flawed evolutionary interpretations, or a causal theory that predicts the continued evolutionary maintenance of nothing but self-replicating molecules.
There is, however, an essential intrinsic selection that points in the right direction. This is the selection for a sustained increase in the net energy that is available for self-replication. When other things are equal, and selection is unconstrained, it generates the direction of an exponential increase in the net energy of the organism.
But intrinsic selection will only allocate this energy into fast replication. This is why the theory of Malthusian Relativity is build around the feed-back selection of density dependent interactive competition. This selection is another essential consequence of self-replication, as self-replication generates population growth, and with an increased density of individuals, the individuals will eventually meet in interactive competition over limited resources.
These two mechanisms -- of intrinsic selection for increased net energy and interactive selection for competitive quality -- is all that it takes to get a causal selection theory running. It explains not only the evolution of high-energy organisms with large body masses and sexual reproduction between a female and male individual, but also the evolution of co-operative and eusocial reproduction, as well as the evolution of the body mass allometries that describe the life history as a function of mass (see Witting, 1995, 2002, 2008).
- Witting, L. 1995. The body mass allometries as evolutionarily determined by the foraging of mobile organisms. Journal of Theoretical Biology 177:129--137.
- Witting, L. 2002. From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61:171--195.
- Witting, L. 2008. Inevitable evolution: back to The Origin and beyond the 20th Century paradigm of contingent evolution by historical natural selection. Biological Reviews 83:259--294.