Thursday, 28 November 2013

Selection vs drift - the conceptual mess at the heart of evolutionary theory

This post criticizes modern usage of the term "natural selection".

Natural selection is often defined as being non-random change. Genetic drift is defined as being random change. However, what constitutes randomness is a philosophical quagmire - and it is often difficult to determine whether a given change is genuinely random or not.

Further, some would say that genetic drift occurs when change is mostly random. It turns out empirically that the split between drift-like phenomena and directional selection is not a binary division into two distinct categories, but rather a sliding scale - going from pure randomness and drift at one end - to pure directional selection at the other.

Nature can choose randomly - and the term "natural selection" makes no mention of non-randomness. The terminology doesn't mean what the words say. It's simply confusing for people to learn.

So: why does the terminology of evolutionary biology enter into this philosophical quagmire? Why does it foist a false dichotomy on us? Why doesn't the terminology mean what it says?

I argue that this is a locked-in historical accident. I think that there's a much better classification scheme out there - involving natural production and natural elimination.

We could redefine the term natural selection to cover all changes in the frequency of births and deaths - and then write the equation:

Natural selection = natural production + natural elimination

...instead of today's:

Natural selection + genetic drift = natural production + natural elimination

Today's concept of genetic drift would become "random natural selection". Today's concept of "natural selection" would become "non-random natural selection". These terms are rather useless, and I doubt they would see much action - due to the difficulty of establishing randomness and the "false dichotomy business" described above.

I think this would be a big breakthrough in the way in which evolutionary theory is learned - and taught.

It would also make the term "natural selection" much more useful.

It a creature dies, that's natural selection. If a creature reproduces, that's natural selection. No more time-consuming and pointless enquiries into whether a given death or birth was "random" or not.

Non-randomness was not part of the definition of natural selection in the modern synthesis. In Evolution: The Modern Synthesis (1942), Julian Huxley wrote:

The term Natural Selection is thus seen to have two rather different meanings. In a broad sense it covers all cases of differential survival: but from the evolutionary point of view it covers only the differential transmission of inheritable variations.

Randomness doesn't get mentioned here.

However, the origin of the idea can probably be traced to Darwin's usage in the Origin. Darwin wrote:

if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.

Here, Darwin was rather vague about exactly what qualified as "natural selection" - but you can see where people are getting the idea from.

The modern usage probably ossified in the 1970s. In The Units Of Selection (1970), Lewontin defines "natural selection" in a way that excludes genetic drift. This was an influential paper at the time. The polarization associated with the controversy about the significance of the work of Motoo Kimura on genetic drift may also have contributed.

Of course, redefining well-established terms increases your crackpot index. However, in this case, there is a clear rationale:

Placing the concept of "randomness" at the heart of evolutionary theory is an unnecessary bad move. Trying to create a dichotomy out of a continuum is another bad move. Today's terminology incorporates a philosophical quagmire, represents poor-quality classification and it doesn't mean what the words literally say. This terminology isn't just bad, it is obviously bad. It's time for a change.

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