Hi, I'm Tim Tyler, and this is a video about evolution, devolution and provolution.

Adaptive evolution can usually be thought of as entities climbing adaptive peaks in a multi-dimensional fitness landscape. This is a postive, constructive process. However evolution does not always work like that. As is well known, there is another type of evolution - caused by random mutations and genetic drift - which opposes selection and unravels adaptations.

The deleterious effects of random mutations are normally kept down to managable levels by error detection and correction mechanisms.

However, high levels of random mutations can arise because of radiation, toxic chemicals and stress. Also, the effects of genetic drift can be amplified by phenomena such as inbreeding and small population sizes.

High levels of mutation and drift can become problematical if they rise to levels higher than selective processes can compensate for. If this happens it can cause a range of problems - from adaptations stating to malfunction to complete extinction of the population.

These problems have been given a variety of names - error catastrophe, extinction vortex and mutational meltdown are some of the terms involved.

However, I think the terminology in this area of biology has got into a bit of a mess.

  • "Mutational meltdown" - due to a rather unfortunate historical accident - has been defined to refer to the accumulation of deleterious mutations due to small population sizes. Yet there is no hint of population size in the term itself. There are plenty of other cases of mutations and drift causing problems besides this one.

  • "Error catastrophe" - by an even more unfortunate historical accident - has been defined to refer to accumulating errors during the process of protein synthesis. I think this terminology is very misleading. The term "error catastrophe" should refer to any runaway autocatalytic error amplification process, and should not be specific to protein synthesis.

Rather than trying to reform the existing terms, I think an attractive way forwards here is to introduce some fresh terminology. In particular, I think we can usefully divide evolution into provolution and devolution. Provolution is snappier synonym for adaptive evolution, while devolution refers to the systematic loss of adaptations that occurs where there is an excessive mutational load.

  • Provolution - refers to climbing on the fitness landscape;

  • Devolution - refers to descending down into its valleys;

  • Evolution - is usually composed of a combination of the two.

Taking a more historical perspective:

  • Provolution - has led gradually from primitive simplicity to the powerful adapted complexity of modern living things;

  • Devolution - represents the opposite force - it leads gradually from adapted complexity back to primitive simplicity again.

In devolution, instead of climbing peaks in fitness landscapes, organisms are progressively displaced further and further away from them - usually due to mutational load or genetic drift.

Devolution is slightly different from the concept of an "extinction vortex". Devolution does not necessarily lead to extinction. For example, sometimes, populations can use recombination to make use of existing variation to defend themselves against against the stressor causing the increased mutation rate - thereby rescuing themselves from extinction. In those cases, use of the term "extinction vortex" seems inappropriate. An extinction vortex is a type of devolution that leads towards extinction.

A few corner cases should help to clarify the concept:

  • What about a cave fish losing its sight? Is that devolution? If it is losing its eyes because doing so saves energy and is adaptive - which is quite likely - that would not be devolution in the sense described here.

  • Is it possible to have localised devolution - only affecting one mutational hotspot? Yes, that concept seems as though it makes reasonable sense to me.

  • What about adaptive hill-climbing on a mountain that is sinking into the sea? Is that provolution? Or devolution? Provolution, I think - though this is a tricky case.

I don't think I am attempting to hijack the "devolution" term. Rather, I think my usage of the term "devolution" lies within the spirit of the conventional meaning of the term - which is to do with backward or degenerate evolution - whilst providing scientific rigor to the concept.




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