In an 1802 book titled Natural Theology, the English clergyman William Paley laid out an argument for the existence of God founded on observations of the natural world. He pointed out that if we were to come across a stone lying in a field, we wouldn’t necessarily try to find any special explanation for how it came to be there. However, if we were to discover a watch in the same place, we would infer that it must have been made by a watchmaker. Unlike the stone, the watch clearly exists for a purpose – telling the time – which is evident from the way it’s constructed. Its design cannot simply be explained as the effect of random chance or the operation of some generic law of nature – it has to be explained in terms of the intention of a designer.
Living organisms, for Paley, were similar to the watch in the field. An organ like the eye was not simply a random mass of cells that coincidentally happened to be capable of vision – it was clearly a precise mechanism designed for vision. Its existence, like that of the watch, could not have come about by mere chance. It must have been designed by an intelligent being, and this being was God.
Paley was not the originator of this argument. It had been part of Christian theology for centuries, articulated by St. Thomas Aquinas in his Summa Theologica. Aquinas in turn had been influenced by Aristotle, who held the view that everything in nature had a telos, or end, for the sake of which it existed and that caused it to behave the way it did. Aristotle called this a “final cause” and distinguished it from other types of causes – material, formal and efficient causes – that explained a phenomenon or process with reference to its mechanics rather than its ultimate goal.
For Aristotle, teleology – that is, explanation involving final causes – was not limited to phenomena that gave the appearance of design, nor was it evidence of a creator or designer. Final causes were a type of cause that existed in some metaphysical sense in nature. Although not every phenomenon had a final cause – some were just side-effects of other phenomena – there were regularities in nature that could not be explained without invoking final causes. This was especially apparent in biology, where it made far more sense to explain the form of an organ in terms of its function than in terms of the process of embryonic development. After all, there was no obvious reason why, for example, a chicken embryo should develop into a chicken, unless it had the goal of developing into one.
Although neither Aristotelian teleology nor Christian theology is used as an explanatory paradigm in science today, the philosophical point of these arguments still stands. If the blind, purposeless motion of matter is all there is, if nothing other than mechanical determinism or randomness is at work, then how is it that everything in the living world appears to have goals and purposes that it seems precisely designed to fulfil? How can purpose arise out of randomness? Why do living creatures have the purposes they do, and no others?
As an undergraduate at Cambridge, Charles Darwin read and deeply admired Paley’s arguments in Natural Theology. At the time, species were widely believed to be immutable and created separately, so the idea that each species had a unique design and was suited to a unique purpose made sense. The transmutation of species – the idea that a species could transform into another species over time – was a theory held by some scientists, but there was no agreement on how this transformation might take place.
One of the problems with transformism was that it offered no explanation for apparent design in biology. If species could transform into other species, then their design and purpose could not be innate. In order to explain this, most transformists posited some kind of vital force or principle directing the process of transformation, that caused life forms to evolve from ‘lower’ to ‘higher’ and from simple to complex.
The French naturalist Jean-Baptiste Lamarck, for instance, held that there were two principles that directed evolution – a complexifying force, that caused organisms to progress up the ladder of complexity, and an adaptive force, that caused organisms at the same level of complexity to branch out horizontally and become adapted to diverse environments. The mechanism of adaptive change consisted of the law of use and disuse, according to which organs were enlarged and strengthened, or reduced and weakened, in proportion to their frequency of use, and the law of the inheritance of acquired characteristics, according to which changes accumulated during an organism’s lifetime could be passed on to its offspring. (Lamarck’s name is today mainly associated with this latter idea.)
In Lamarck’s view, adaptation occurred as a result of an organism’s response to the needs created in it by its environment. Giraffes had developed long necks because they had attempted to stretch their necks to reach the highest leaves, and then passed these elongated necks down to their offspring. Paley’s intelligent designer was replaced by the organism itself, modifying its own form and that of its descendants to become better adapted to its habitat.
These ideas were related to another nineteenth-century idea about evolution – orthogenesis, or the notion that evolution proceeded along a definite trajectory in a particular direction. Some variants of orthogenetic views were deeply embedded in the contemporary worldview of social and cultural progress – evolution, like history, was a narrative of progress toward a state of perfection. Teleology was not found in the organism or in the mind of its creator, but in the evolutionary process.
Darwin’s theory of evolution by natural selection offered an alternative to all of these ideas. Humans, he pointed out, could create different varieties of plants and animals with desired characteristics by selecting the individuals that showed those characteristics in a greater degree and breeding them with each other – a kind of artificial selection. Could an analogous process in nature – call it natural selection – similarly create a diversity of species by selecting the individuals whose characteristics best fitted them for survival and reproduction in a given environment, and causing them to breed with each other?
There was no need, in this account, for any kind of intentional design or goal-directed progress – for a watchmaker, or a telos, or a vital force. All that was required was for there to be some degree of heritable variation in traits among the individuals of a given species, and for some of these traits to give their possessors a relative advantage in the number of offspring they left behind. The traits that ended up being ‘selected’ over several generations would be the ones that proved advantageous in a particular environment, while the ones that proved disadvantageous would tend to die out.
As Darwin observed in the Origin of Species, one of the strengths of this account was that it explained not only apparent design and adaptation, but also imperfection and maladaptation. Natural selection did not adapt organisms to their environment perfectly and precisely, but only to the extent that the adaptations enabled them to out-compete their neighbours in the struggle for existence. Adaptation was also not instantaneous – it occurred by slow, gradual steps – so that if a species migrated to a different environment or if the environment it was in changed rapidly, it would take many generations for it to become adapted to its new conditions. In addition, natural selection had no effect on traits that were neither advantageous nor disadvantageous – these simply varied randomly.
The theory of natural selection explained how apparent goal-directedness could arise out of a process that had no goal or purpose of its own. Design and purpose seemed to be inherent in organisms of those species that had been acted upon by natural selection for a very long time in a relatively unchanging environment, but if those organisms were moved to a different environment, the bodies of their surviving descendants might become repurposed – so to speak – for an entirely different way of life.
Does this mean, then, that we can no longer speak of the eye as though it had a purpose? We seem to be back to square one here – if teleology has been eliminated from biology by Darwinian evolution, then how do we talk about functions and goals and purposes in a discipline that seems to be teeming with functions and goals and purposes everywhere we look?
The debate over when and how to use teleological explanation in biology has had no clear resolution for the last century or so. There are several issues with such explanation – for one, it might imply some kind of vitalistic or non-physical causal force, if we aren’t careful to only use it metaphorically. We may fall into teleological thinking of the Aristotelian or Paleyan or Lamarckian kind without even realizing it – for instance, if we say that evolution has given us eyes so that we can see, or that the evolutionary purpose of sex is reproduction, this is not much different from saying that God gave us eyes so that we could see, and intended us to have sex in order to reproduce. If “evolution” simply becomes another name for God or the vital principle in our explanation, then we’re not actually using Darwin’s theory of evolution at all.
It does seem, in some sense, that adaptation is the “goal” of natural selection. After all, that’s why Darwin’s analogy with artificial selection works – because natural selection acts in the same way that breeders do. Natural selection is to evolution what Adam Smith’s “invisible hand” is to the free market – a mechanism that leads to the emergence of spontaneous order from undirected processes. But this can easily be mistaken for a kind of Providence guiding the universe, when it’s really only a causal process. The term ‘teleonomy’ has been proposed to describe such apparent teleology that’s not actually teleology in a metaphysical sense, but so far, it hasn’t been used very widely.
Another problem is that explaining things in terms of evolutionary adaptation can lead us to see adaptation and design in every feature of the biological world, and construct unfalsifiable just-so stories to account for its existence. Gould and Lewontin (1979) refer to this tendency in evolutionary thought as the “Panglossian paradigm” – one that views every feature of an organism as having been optimized by natural selection, incapable of any further improvement – the best of all possible worlds. They suggest a set of alternatives to this paradigm, including decoupling adaptation from selection, allowing for the possibility that neither might have occurred in any given instance, and viewing some features as byproducts of other adaptations (“spandrels”) rather than adaptations in themselves.
A third problem is that sometimes teleological language can muddy the distinction between description and explanation (Thompson 1987). This can lead to vacuous and circular statements – for instance, we might say that an animal displays a certain behaviour because it has an instinct to do so, when by “instinct” we simply mean that the animal is motivated to behave in a certain way. Or we might say that evolution works toward the goal of increasing fitness, when by “fitness” we simply mean evolutionary success. Such language can create an illusion of explanation while not actually explaining anything at all.
For these reasons, some biologists prefer to avoid teleological language as far as possible, and to focus on how biological phenomena work and how they came to exist, rather than why. This is not always possible, though, especially in fields like behavioural ecology where the phenomena to be studied (patterns in the animal’s behaviour) can’t always be directly observed in the way that anatomy can, and have to be inferred based on hypotheses about what behavioural patterns might be evolutionarily adaptive or maladaptive for the animal.
Teleological or functional explanation clearly has a unique place in biology that it doesn’t have in physics or chemistry or any of the other sciences. But it’s also a uniquely dangerous kind of explanation, one that can lead us into all sorts of traps if we’re not extremely cautious with it. There doesn’t seem to be any easy way out, except to say that in evolutionary biology, a discipline that has been encroaching since its inception upon the territory once occupied by religion and metaphysics, an understanding of philosophy is perhaps just as important as an understanding of the discipline itself.
Acknowledgments I'd like to thank Charu Mehta and Vanya Bisht for their suggestions and especially for agreeing to review this article last-minute - without which I'd probably never have gotten it done at all.
1. Darwin, C. On the Origin of Species, Project Gutenberg, 1859.
2. Paley, W. Natural Theology or Evidences of the Existence and Attributes of the Deity, 1879. https://archive.org/details/naturaltheologyo00pale
3. Aristotle, On the Parts of Animals. Translated by William Ogle. https://penelope.uchicago.edu/aristotle/parts1.html
4. Gould, S. J. and Lewontin, R. C. “The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme”, Proc. R. Soc. Lond. B. 205:581–598, 1979.
5. Thompson N.S. “The Misappropriation of Teleonomy” in: Bateson P.P.G., Klopfer P.H. (eds) Perspectives in Ethology. Springer, Boston, MA. 1987. https://doi.org/10.1007/978-1-4613-1815-6_10
6. Dobzhansky, T. “Nothing in Biology Makes Sense Except in the Light of Evolution”, The American Biology Teacher 35 (3): 125–129. 1973.
9. Ruse, M. Monad to Man: The Concept of Progress in Evolutionary Biology. Harvard University Press, 2009.
10. Ruse, M. The Evolution-Creation Struggle. Harvard University Press, 2009.