Evolutionary Biology Blogι**=7/3ψ
by Lars Witting

The just-so stories of biological evolution

The eye of our cat Luni Sputnik. Evolution is full of contingencies, like those that caused the evolution of different eyes in different animals. But contingency is taken too far in evolutionary biology, where pseudo contingencies are used as ad hoc explanations at the cost of a more firm theory of evolution by natural selection. Photo L. Witting

It is sad to say, but the majority of our natural selection causality from evolutionary theory is really just just-so stories, where a pseudo form of evolutionary contingency is likely to produce ad hoc fallacies by the pitfall of evolutionary interpretation.

There is, of course, plenty of real contingency in biological evolution. This is of the unexplainable type; let it be pure random effects, or effects that are unpredictable because they are critically dependent upon too many initial conditions.

Pseudo contingencies, on the other hand, relate to just-so stories, where they are the evolved traits that are used to explain the natural selection of other traits. They include the trade-off between survival and reproduction in Lack's clutch size (Lack, 1947), and the resource transportation networks in the allometric model of West, Brown and Enquist (1997, 1999). Pseudo contingencies were introduced by Darwin (1859), and they are included in basically all the traditional life history models in Stearns (1992), Charlesworth (1994) and Roff (2002) [ see Witting (1997, 2008) for details ].

While the selection from pseudo contingencies may represent the essential selection in natural populations; the underlying causality cannot straightforwardly be verified due to the pitfall of evolutionary interpretation. Pseudo contingencies are therefore likely to produce ad hoc fallacies, with a flawed underlying causality.


  • Darwin, C. 1859. The origin of species. John Murray, London.
  • Lack, D. 1947. The significance of clutch size. Ibis 89:302--352.
  • Roff, D.A. 2002. Life history evolution. Sinauer Associates, Inc., Massachusetts.
  • Stearns, S.C. 1992. The evolution of life histories. Oxford University Press, Oxford.
  • West, G.B., J.H. Brown and B.J. Enquist 1997. A general model for the origin of allometric scaling laws in biology. Science 276:122--126.
  • West, G.B., J.H. Brown and B.J. Enquist 1999. A general model for the structure and allometry of plant vascular systems. Nature 400:664--667.
  • Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL http://mrLife.org.
  • 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.