The Definition of Evolution

Today we understand evolution mainly as a fact about nature, but its historical root is that of an explanation of a much older fact: that the living world consists of different types of organisms. Where did theses species, as they were called, come from? In pre-Darwinian times, God was seen as having created them to be eternal, immutable entities arranged in a linear hierarchy. The Swiss naturalist Charles Bonnet wrote in 1779

Nul changement ; nulle altération ; identité parfaite. Victorieuses des élémens , des temps & du sépulchre, les especes se conservent…

No change; no alteration; perfect identity. Victorious over the elements, time and death, the species are conserved… (p. 231f., my translation)

With Darwin, species became fluid. They changed continually and transformed into each other in a process called evolution. Both evolution and the mechanism by which it was proposed to occur, natural selection, were debatable concepts. Both were, in fact, debated, the latter much longer than the former. But the existence of species as distinct phenotypic categories would not seem to be up for debate, for it was the very thing these two ideas were meant to explain. To change it would be to change the subject. And yet, later history revised the definition of evolution in such a way as to remove the target of explanation after the explanations themselves were already in place.

While Europe was swerving towards the abyss of World War II, the architects of the modern synthesis came to change the definition of evolution in a manner which reflects badly on scientists’ ability to apply even basic conceptual reasoning. Under the influence of Mendelian genetics, Wright, Fisher and others began describing evolution as a change in allele frequencies, because they believed that it is genetic changes, and only genetic changes, that constitute evolutionary change. The allele-frequency definition has gone on to become the canonical textbook definition of evolution and is still widespread, though not undisputed, today. Dawkins, for example, writes:

[When] there is a systematic increase or decrease in the frequency with which we see a particular gene in a gene pool, that is precisely and exactly what is meant by evolution (2009, p. 33).

But this seems entirely confused. Think about it: if evolution were change in gene frequencies in a population, Darwin would never have been able to know about it. More generally, the whole history of biology would have had to rest in suspension about the reality of what was called “evolution” and await the arrival of genetics to confirm that they were indeed talking about the real thing.

What’s going on, I think, is that the champions of the modern synthesis came to believe that changes in gene frequencies were both a necessary and sufficient condition for evolution by natural selection to occur. Equating the two might have seemed like a concise and parsimonious redescription of what evolution is.

But this is a simple conceptual mistake. Even if genetic change were the only way to get evolution from A to B, it would not follow that evolution is genetic change. The phenomenon to be explained still is the transformation of phenotypically defined classes of organisms. Imagine that species A had transformed into phenotypically distinct (incl. non-interbreeding) species B without any genetic change. Now imagine that species A had transformed into phenotypically indistinguishable (incl. interbreeding) species B under lots of genetic change. In which case is it appropriate to speak of two different species connected by evolution? It is clear to me that the answer is the first scenario. Genetics is important for evolution only insofar as it is (one of) its main mediating mechanisms, not because it would constitute the phenomenon of interest, which lies on the phenotypic level.

Evolutionary biology has developed a vocabulary and a way of thinking that tends to blur this distinction: populations consist of genotypes, not organisms; fitnesses are assigned to genotypes or genes, not organisms; genes and alleles play the role of traits; reproduction is genetic recombination, and organisms die instantly at the border of generations. Fisher said:

It is often convenient to consider a natural population not so much as an aggregate of living individuals but as an aggregate of gene ratios (1922, p. 340)

 

Convenient, maybe, but also dangerous, if you even lack the conceptual rigor to distinguish a phenomenon from its implementation.

Compare the following reductio: Evolution is nothing but change in frequency and constellation of organisms’ atoms over time. This requirement is as much a necessary and sufficient condition for evolution as are gene frequency changes. Evolution is only possible through changes in atoms (genes), and atomic (genetic) changes suffice for evolution. In reality we know that not all genetic change necessarily leads to evolution, so there’s no strict sufficiency, but that is paralleled by an analogous restriction for atoms. We also know that some forms of evolution (e.g. of social behaviors) might be possible in the absence of genetic changes, and again the same holds for atomic changes. The cases, I submit, are equivalent, but no one seems to think the atomic definition of evolution is a serious proposal. What is wrong is a simple conflation of a phenomenon and its proposed causal mechanism.

This is not a biggie if it happens to your grandma. But that not even scientists are  necessarily good at basic conceptual reasoning is important to know. If you don’t give them advance credit but just judge their pronouncements on their own merits, you can avoid some of the nonsense that even this extraordinary league of gentlemen* generates.

 

Bonnet C (1779) Oeuvres d’histoire naturelle et de philosophie, Volume 5‎. Neuchâtel: Samuel Fauche.

Dawkins R (2009) The greatest show on earth. London: Transworld publishers.

Fisher RA (1922) On the dominance ratio. Proceedings of the Royal Society of Edinburgh 42:321-341

 

*Women (unfortunately) included

 

Advertisements
1 comment
  1. Ratioxxx said:

    ‘Imagine that species A had transformed into phenotypically distinct (incl. non-interbreeding) species B without any genetic change.”

    Maybe I lack the necessary biological knowledge, but could you please elaborate on how this could be possible?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: