Although Darwin’s paradigmatic book (necessarily) covers a broad range of topics alongside his theory of natural selection, it is only the latter I will focus on here, in order to spare us from devolving (nudge nudge) into discussions of how to distinguish plant species from varieties or regarding the domestication of pigeons. The composition of On the Origin of Species makes it evident that Darwin is, at heart, an eager student of Nature, and his love and admiration of her work sustains his own. Darwin’s theory of natural selection has been applied far beyond what he likely ever imagined, and what I want to consider here are the philosophical questions presented by the theory.
“Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relationship to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.”
Natural selection is the mechanism by which Darwin explained evolution in the natural world, and his claim is a causal one. There are three conditions which are necessary for natural selection: 1) variability, 2) heritability 3) a causal connection between variability and the ability to survive, resulting in differential reproduction or differential mortality, i.e. success surviving and/or reproducing as a result of the variable, hereditary traits. When we explain the success, of reproducing or survival, by the term natural selection, we rely on the concept of Darwinian fitness, whether a particular trait is more fit (or better adapted) than its competitors. Differential fitness of traits results in differential reproduction or differential mortality in common environments.
Here rises one of the first philosophical problems of natural selection. How are we to understand what is meant by fitness? I will admit from the outset that I don’t think I can answer this question, and it remains an area of mystery for many philosophers and biologists. The general idea, I gather, of Darwinian fitness, seems to have to to do with how well an organism “fits” within its environment. An organism is a collection of variants, and these variable traits are determined internal to the organism’s ontogeny. The environment in which the organism lives is not structured in any way that is causally determined by the organism; thus the organisms that “fit” best within this common selective environment – as a result of their variable, hereditary traits – are the best adapted to successful survival and reproduction. Fitness, then, very simply speaking, is the success (through survival and/or reproduction) of particular traits in particular environments. This definition of ecological fitness, however, does not seem to have solved the problem: in fact, this explanation of Darwinian fitness – on which the definition of natural selection depends (differential reproduction or mortality based on differential fitness in a common environment) – seems to at best create an explanatory circle.*
Before moving on, I want to briefly note that where many might say it is only differential reproduction that ought to be considered in natural selection – and not differential mortality – I am remained unconvinced, and therefore refer to both. Those who would say differential reproduction is the only significant result of fitness are arguing that nature selects those that survive only insofar as it increases their chances of reproducing. Although a less popular view, those who say differential mortality is the only significant result of fitness argue that nature selects those who reproduce more successfully only insofar as such reproduction increases the chances of survival; in such a case, differential persistence is necessary for natural selection, but differential reproduction is not. I would tend to agree (due to being exposed to the Red Queen at a young age) with those who argue for differential reproduction as being necessary for natural selection; however, I am not entirely convinced of the falsity of the opposing claim, nor have I heard a convincing argument that it is not some combination of the two that is necessary in natural selection. I will leave it at this.
An early philosophical problem with Darwin’s theory that is more or less ignored now, due to science’s acceptance of ‘statistical’ theories, or theories whose core principles rely on chance and probability and that thus cannot make absolute claims, is the fact that natural selection is a probabilistic explanation. It is obvious that the theory of evolution or natural selection does not eliminate the possibility that every organism with better adapted variable x will more successfully reproduce and/or survive those organisms without x, nor that an environment will never be able to sustain all organisms within it such that there is no longer any natural competition. Competition, selection, differential reproduction and mortality, with success resulting from differential fitness, allows one to make frequently accurate predictions about trends, though it stops far short of allowing one to make absolute claims. This problem hardly seems to survive today, except insofar as there are those who remain skeptical that we can have a proper philosophical conception of such probability-reliant theories.
A most obvious problem when confronting the theory of natural selection concerns teleology. A Darwinian understanding of selection has two components: variation occurring independently of an organism’s adaptive needs; and the differential perpetuation of those variations based on their success in serving an organism’s adaptive requirements. Does natural selection erase or save teleology in the natural world? Are the causal processes of Darwinian selection aimless, but veiled by a teleological appearance in that they work for ‘the good of populations’? If Darwin’s explanation of natural selection is teleological, the functions served by perpetuated variants are an essential part of that explanation.
It is clear the first part of natural selection’s theory is not teleological. The variation we see in species occurs randomly within an array of possible alternatives, and one or several of those variations are perpetuated. Why? To me there seems to be only one substantive and logical explanation for the perpetuation of some variants over others, and that is the relative value of their consequences. Take the example of Endler’s guppies.** Endler studied the color patterns of male guppies in rivers flowing into the Caribbean. The color selection had two important factors: mate selection and predator selection. Bright colors attract mates, but they also attract predators. Male guppies with bright red spots had great reproductive and survival success because one of their predators, prawns, are color blind to red. The red spots did not put them at a disadvantage in avoiding this predator, and it did succeed in attracting mates. Selection in this case appears to result from value differences, which benefit the organism and, to return to this troublesome concept, increase its fitness. That one variant v has a higher fitness value among a range of possibilities has to do with the value of this trend relative to the survival and reproductive success of organisms that posses variant v. This would appear to be a specific form of teleological explanation: in which that for the sake of which a variant is possessed, it’s value-consequences, account for that variant’s perpetuation in a population.***
Relying on value-consequences does seem to me to be a problematic way to grant Darwin’s selection theory a kind of teleology. Admittedly I’ve never been a fan of teleology in nature, nor am I entirely convinced that we can call natural selection a teleological explanation. How would one determine the tension between survival-value and reproductive-value, among other values, if they exist? – a serious problem for those who, like me, are unconvinced of the priority of differential reproduction or differential mortality. As I am failing to think of a decent example off the top of my head that couldn’t be refuted with quantitative value-consequence relativity (anyone?), I will leave this discussion here for now.
As ever, there is a wealth of things I have ignored. Perhaps I will write again on Darwin if I ever visit the ‘poor man’s galapagos’ (the real Galapagos cost an arm and a leg to visit… one of my Ecuadorian dreams crushed).
* This mainstream understanding of fitness would have us understand it as reproductive success; but what is natural selection but variation casually connected to reproductive success?
**(Endler 1983, 173–190)
***See James Lennox’s SEP article on Darwinism.