On The Nature Of Evolution

The first ever tree of life, as drawn by Charles Darwin. For Stephen
Jay Gould this would represent just one possible history of life

Evolution polarises those who believe in it and those who do not. Its believers can also be split depending upon their view of how the phenomenon operates. The American palaeontologist Stephen Jay Gould famously penned some of the most controversial theories in evolution, including the view that the story of life has played out randomly.

If we were to rewind to the Cambrian Explosion or to the dawn of life itself, the progression we would see in the fossil record would be radically different to the one we are familiar with.

Additionally Gould views evolutionary change as both irreversible and constraining further changes. Tetrapods randomly evolved to have four limbs because evolutionary pressures acted on fish ancestors with four fins. It is for this reason that there will never be any six-limbed tetrapods - evolutionary pasts constrain evolutionary futures. Gould's views of randomness, irreversibility and constraint are controversial, but a recent study has provided molecular evidence to support these.

A research team from the University of Pennsylvania examined the effects of purifying selection on a bacterial protein. Purifying selection favours mutations which have little to no effect in a fixed environment. This is in contrast to adaptation by natural selection, in which mutations are selected if they increase an organism's fitness in a new environment. Purifying selection is by far the more common type of selection.

'It's the simplest, most boring type of evolution you can imagine,' said Professor Joshua Plotkin from the University of Pennsylvania. 'Purifying selection is just asking an organism to do what it's doing and keep doing it well.'

The bacterial protein used was argT. It has a known three dimensional structure and is small enough to allow the effects of mutations to be accurately modelled. Computer simulations were made of the effects of introducing small mutations into the protein's structure over a 10 million year period.

'The very same mutations that were accepted by evolution when they were proposed, had they been proposed at a much earlier in time, they would have been deleterious and would have been rejected,' said Plotkin. What this shows is that the effects of certain mutations are contingent on what other mutations are present.

The structures of proteins are remarkably complex and their
functions are dependant on their structures remaining sound

As mutations gather, the degree of contingency in a protein increases and the mutations become entrenched. As time progressed, it became increasingly difficult to reverse mutations without de-stabilising the protein's structure due to the complex web of contingency involving those mutations. Entrenchment and contingency are known to exist in adaptation, but this is the first time they have been seen in purifying selection.

The entrenchment also serves to constrain what mutations could be selected for in the future, serving to narrow the possible evolutionary future of the simulated protein. While Gould's comment about replaying the tape of life was based on the large amount of randomness inherent in evolution's path, this study suggests a more nuanced reason that the playback would appear different.

It highlights the effects of entrenchment and contingency on a single protein. Yet imagine their effects on a complex multicellular organism with tens of thousands of proteins all contingent not only on their individual mutational histories, but one another. Proteins not only play roles in cells such as signalling or catalysing reactions, but also in guiding the development of multicellular creatures from embryo to organism - perhaps the most contingent of all biological processes. By scaling the results of the 10 million year protein study, it becomes easy to see how the large scale changes seen in the fossil record over 3.8 billion years could have occurred and how they represent just one possible constrained history of life.