The popular conception of the history of biology is that Charles Darwin’s theories settled the argument over the origins and development of life on earth. In fact, central tenets of Darwinism continue to be debated and discussed by philosophers of science today. This article focuses on several of those discussions.
It begins with a discussion of the stability and fluidity of evolutionary theory, posing the question of how far Darwinism as a whole is still in question, as opposed to certain discrete elements of it. It then moves on to discuss variation and the question of chance and randomness in evolutionary theory. The relationship between speciation and teleology is discussed next, before a conclusion focusing on the question of species as such.
The Structure of Charles Darwin’s Evolutionary Theory
Stephen Jay Gould, a contemporary philosopher of science, once said that “The structure of evolutionary theory combines enough stability for coherence with enough change to keep any keen mind in a perpetual mode of search and challenge.” What does this mean?
Partly, the quote illustrates that in a broad sense Darwin’s theories have comprehensively won the argument, and further discussions to be had are occurring within a fundamentally Darwinian framework. However, in contemporary debates in the philosophy of biology and the philosophy of science more generally a more specific position than this can also be characterized as ‘Darwinism’, because those who hold it side with Darwin on several important issues which are still up for debate.
Get the latest articles delivered to your inboxSign up to our Free Weekly Newsletter
James Lennox summarizes some of these issues as follows: “[i] the role of chance as a factor in evolutionary theory and the theory’s apparently probabilistic nature; [ii] the nature of selection; [iii] the question of whether selection/adaptation explanations are teleological; [iv] the ontological status of species and the epistemological status of species concepts; and [v] the implications of Darwin’s insistence on the slow and gradual nature of evolutionary change”.
The first element in Darwinian theory to address has to do with the generation of variation (which is random), and the perpetuation of advantageous variations (which is a matter of probability, but not random). There is a contrast to be drawn here with Jean-Baptiste Lamarck, Darwin’s predecessor, who had things the other way around – variations are fitness based, but their perpetuation is not.
Darwin said in the Origins of Species:
“Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?”
There is an element of ‘chance’ in the iteration of variations – largely due to Darwin’s professed inability to explain where variations come from. “Whatever the cause may be of each slight difference in the offspring from their parents—and a cause of each must exist—it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure…”.
In a certain sense, the cause of variation matters less than the point, which is that the process of natural selection aggregates beneficial variations. “Chance” here means that while there are numerous possible outcomes with a certain assignable probability or ‘chance’ of occurring, variations are themselves neutral with respect to the suitability of adaptations. Chance relates not just to instantiation of variations, but the perpetuation of variations.
Calling Ourselves Darwinians
The biologist Motoo Kimura has argued that “the great majority of evolutionary changes at the molecular (DNA) level do not result from Darwinian natural selection acting on advantageous mutants but, rather, from random fixation of selectively neutral or very nearly neutral mutants through random genetic drift, which is caused by random sampling of gametes in finite populations” – what came to be known in the 1960s as ‘neutralism’.
The question of random drift is the question of whether this process plays a big role in the development of life on earth, and it matters greatly as to whether we should call ourselves Darwinians. Nothing was more central to Darwin’s theory than the view that the generation of genetic variation was random, but its perpetuation was a matter of natural selection for advantageous characteristics. Darwin claimed that his theory would be “unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and co-adaptation which most justly excites our admiration”.
Evolution and Teleology
Darwin’s theoretical self-conception gets at a really important question in the philosophy of biology: although life on earth appears to be orientated towards a certain kind of purpose, how far is the attribution of purpose post hoc? In other words – what are the real underlying ‘motivations’ (in the broadest possible sense) behind these causal processes?
The most basic Darwinian answer is that the appearance of purpose is derived from the fact that certain variations are advantageous. The purpose of variation is the survival and reproduction of members of those who possess it. One question raised by modern philosophers is whether this does or does not constitute a thorough teleology – whether we should speak of things being ‘good’ for animals, in the sense given above.
Darwin stressed the potential fluidity between the boundaries of species and subspecies categorizations. The question which follows naturally from this is how far the concept of species corresponds to anything concrete, or is it primarily a taxonomic convenience. To draw this distinction at all, we must hold that a useful distinction from the point of view of a scientist need not correspond to some real difference.
Ernst Mayr, who was both an evolutionary biologist and a philosopher of science, takes up this question in the following way, claiming that: “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups”. The putative distinction between species here makes quite a lot of sense – this is no longer merely a species concept.
Constructing the Species Posited by Darwin
At the same time, as Mayr himself was well aware of, this definition of what species are appears to work best as a characterization of the construction of species, rather than a definition of the species itself.
It does, however, relate to a definition of the species which has proven extremely influential: some living thing is of the same species as another living thing if and only one member of each kind of thing has the capacity to breed and produce productive offspring with each other. That is how we know that dogs are the same species, in spite of the marked apparent differences between a Doberman and a Dachshund.
From a philosophical point of view, this discussion about the meaning of the term species is interesting partly because it illustrates an old tension between two schools of thought. On the one hand, essentialism is a way of looking at the world – especially things we might designate as ‘natural’ – as composed of essences. On the other hand, nominalism expresses the opposite view – that compositional or grouping concepts are basically imposed on top of a world of individual, unique objects or things. Mayr’s stance can be understood as an attempt to re-introduce the objectivity implied by essentialism without positing essences:
“…species are relationally defined. The word species corresponds very closely to other relational terms such as, for instance, the word brother. … To be a different species is not a matter of degree of difference but of relational distinctness.”