Darwin's Logical Argument for Natural Selection

One of the things I occasionally like to do is to re-read books that had an early influence on my thinking. It is an instructive exercise. Sometimes, when you read a book early in life you are easily impressed by its ideas and arguments. Oftentimes, this because so many of them are new to you. They have, as a result, an outsized influence on your worldview. When you re-read them, you often find them less compelling. You will have learned so much in the intervening years that the ideas and arguments start to seem obvious and stale.

There are some exceptions to this trend. One example of this, for me at any rate, is Daniel Dennett’s book Darwin’s Dangerous Idea. I first read it in my late teens. I loved it at the time. I was new to debates about Darwinism, its scientific basis, and its philosophical implications. I lapped up everything Dennett had to say. Re-reading it now, I still find it compelling. To be clear, a lot of it is not as impressive as I thought at the time. For example, I used to like Dennett’s somewhat imperious and bitchy style of writing -- so critical and dismissive of his peers -- but I don’t like that so much anymore. Nevertheless, I was pleased to find that the book is still full of interesting metaphors and thought experiments: universal acid, skyhooks and cranes, the Library of Mendel, the Two-Bitser machine and so on. All of these get you to think about the world in a new way and many of them still resonate to this day.

That’s a long introduction — a mini-book review of sorts — to what is going to be a very simple post that doesn’t really have anything to do with Dennett’s book.

One of the things I re-read in Dennett’s book was the summary passage from Darwin’s Origin of Species in which Darwin sets out the logical argument for evolution by natural selection. Typical of a lot writing — particularly 19th century writing — Darwin expresses the argument in a convoluted style. Here it is in all its original glory:

If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But 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. 

(Darwin, Origin of Species, 1st Edition, pg 127)

 

Regular readers of this blog will know that one of my hobbies is to extract logical arguments from long prosaic summaries. Indeed, it is an exercise I often set for students in my classes. Reading through this passage, it seemed obvious to me that there is a much more straightforward and logically compelling way of expressing Darwin’s argument. I thought it might be interesting to show how to do this.

The first thing to note — which Dennett does in his book — is that the passage contains a series of ‘if…then…” statements (or conditional statements). As every first-year philosophy student knows, ‘if…then…’ statements are the building blocks of simple deductive arguments, such as:

(1) If X, then Y

(2) X

(3) Therefore, Y

 

Darwin’s argument consists of a chain of two “if…then…” arguments that build to his conclusion in favour of natural selection. Admittedly, some of the ‘if…then…’ statements that make up those two arguments are complex, and contain asides that are distracting, but it’s easy to see them in the text.

The first one is actually a double conditional statement contained in the first sentence. Here it is with the key bits highlighted:

If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man.

 

To put this a bit more simply:

• (1) If there is variation in organic beings, and if there is a severe struggle for life, then there must be some variations that are useful to surviving that struggle.

I have changed the bit after the ‘then’ in order to capture the essence of what Darwin is trying to say. If I had my druthers I would amend it even further to match modern terminology (e.g. “variations will be fitness enhancing”). The asides in the text are the claims that both of the conditions (variation and struggle) are met in reality. So the first part of Darwin’s argument, with the logical inferences filled in, works like this:

• (1) If there is variation in organic beings, and if there is a severe struggle for life, then there must be some variations that are useful to surviving that struggle.

• (2) There is variation in organic beings.

• (3) There is a severe struggle for life.

• (4) Therefore, there must be some variations that are useful to surviving that struggle (from 1, 2 and 3).

This brings us to the second part of Darwin’s argument, which occurs in the next two sentences of the quoted passage. Here they are with the relevant bits highlighted:

But 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.

 

Okay, I highlighted a lot of that section because it is slightly less convoluted than the first sentence. But there is still a lot going on here. Tidying it up, here is what we get:

• (5) If some variations are useful to surviving the struggle, and if there is a strong principle of inheritance, then useful variations will be preserved.

• (6) There is a strong principle of inheritance (i.e. offspring are likely to resemble their parents) [implied not stated in the quoted passage]

• (7) Therefore, useful variations will be preserved (from 4, 5 and 6).

And the preservation of useful variations is simply what Darwin calls ‘natural selection’.

In full, then, Darwin’s logical argument for natural selection, taken from the quoted passage, looks like this:

• (1) If there is variation in organic beings, and if there is a severe struggle for life, then there must be some variations that are useful to surviving that struggle.

• (2) There is variation in organic beings.

• (3) There is a severe struggle for life.

• (4) Therefore, there must be some variations that are useful to surviving that struggle (from 1, 2 and 3).

• (5) If some variations are useful to surviving the struggle, and if there is a strong principle of inheritance, then useful variations will be preserved.

• (6) There is a strong principle of inheritance (i.e. offspring are likely to resemble their parents) [implied not stated in the quoted passage]

• (7) Therefore, useful variations will be preserved (from 4, 5 and 6).

There is a lot of detail packed into this argument. I have called it the ‘logical argument’ since no empirical evidence is adduced in the quoted passage in support of the key empirical claims (2, 3 and 6). The rest of the Origin of Species provides a lot of evidence in support of those claims. Darwin meticulously documents variation and inheritance in species and gives many examples of the struggle for life. Since Darwin’s time, the field of evolutionary biology has provided reams and reams of evidence in support of those claims, identifying, in much greater detail, the mechanisms of inheritance. In fact, one of Darwin's famous blindspots was the mechanism of inheritance: he knew it happened but didn't know why because he knew nothing about genetics. The amassing of evidence since the time of Darwin is one reason why the argument still holds up to this day.

If I were to make one amendment to the argument it would be to insist that the first premise include the phrase ‘if there is [a lot of] variation…”. Why? Because it seems obvious to me that if organisms vary only in one or two ways, an insufficient volume of variation will be produced to allow variations useful to the wide diversity of struggles for existence to arise. Fortunately, we know that there is a lot of variation in reality so this amendment is easily made.

Anyway, that's all I wanted to say in this post. I hope this logical reconstruction of Darwin's argument is of interest to some people.

Subscribe to the newsletter