Bill Dembski was asking recently about probability estimates in evolutionary biology, ranging from effectively 1.0 to effectively 0.0. Given the importance of evidentially grounded probabilities to the overall argument of The Design Inference (which Bill and Winston Ewert are revising for a 25th anniversary second edition), this is an important area about which to have clarity.

In reply, I pointed out that evolutionary theory makes abundant use of extremely small probability singularities while reconstructing the history of life on Earth. Postulating such events is widely seen NOT as an inferential defect, but in fact a positive and biologically realistic aspect of the theory.

This article illustrates my point: “Key steps in evolution on Earth tell us how likely intelligent life is anywhere else.” Adam Frank writes:

A hard step is an evolutionary change that has only occurred once in the entire history of the planet.

Chordates and Company

Examples include the origin of life itself, the origin of eukaryotes, the origin of any metazoan phylum (e.g., Chordata, Arthropoda), or the origin of language. I don’t have time today to spell out the full structure of these inferences, but phylogenetic reasoning SEARCHES for singularities, as these enable one to tie together (into a monophyletic clade) what are otherwise very different species. How can you be certain, for instance, that you, horned lizards, and brook trout share a common chordate ancestor?

Chordates arose only once.

The Signal Weakens

But as the probability of an evolutionary transition moves away from effectively 0.0 (a singularity) towards 1.0 (bound to happen), the historical signal of monophyly correspondingly weakens. This goes some way towards explaining the paradox that evolutionary theorists such as Jacques Monod, or Richard Dawkins, are quite happy (eager) to say that the antecedent probability of the natural origin of life on Earth was indistinguishable from zero. As Monod famously put it:

The present structure of the biosphere far from excludes the possibility that the decisive event occurred only once. Which would mean that its a priori probability was virtually zero.

J. Monod, Chance and Necessity (1970), p. 144; emphasis in original

If abiogenesis represents a singularity, the Biogenetic Law (omne vivum ex vivo) gives one — for free, so to speak — universal common descent, or Darwin’s Tree of Life. I’d take that deal in a heartbeat, if what I really wanted was a single Tree of Life, without much, or any, further explanatory effort.