top of page

An Evolutionary Solution

What are natural functions? One well-established tradition uses Darwinian evolutionary theory to solve the problem. The function of a zebra’s stripes or milkweed’s latex is whatever they evolved to do: the effect that was favored by natural selection in the modern organism’s ancestral lineage. In philosophical discussions, this is called the “selected-effects” view of function. Scientists still disagree about the main way that a zebra’s stripes help it to survive – whether by camouflaging it from predators like lions, helping it to avoid overheating, or preventing biting flies from landing on its body. But many (not all!) agree that finding out how the stripes help zebras survive, thrive, and reproduce will answer the question of why zebras evolved to have stripes, and thus tell us what the stripes are for.


What’s the point of thinking in terms of functions and purposes? One of the most common ways to talk about functions is to use them to explain why something is the way it is. Why do zebras have stripes? To help them avoid fly bites and the diseases flies carry. Why does milkweed have latex? To protect it from insects that try to eat its leaves. This is sometimes called “teleological explanation” – explaining something by reference to its purpose.

This kind of explanation is one of the main reasons that science has rejected teleology: it seems to get explanation the wrong way round, explaining why something is the way it is by pointing to an effect that it has. But effects can’t explain their causes since the cause must come before the effect. So, appealing to purposes seems not just to take us beyond what can be observed and measured, but to be drawing on a form of explanation that is incoherent. Or is it?

One of the beauties of the evolutionary interpretation of natural functions is that it makes sense of this sort of explanation. It does this by looking at cause and effect across multiple generations. Today’s zebras have stripes because in past generations, the stripey coats of some zebra-ancestors helped them survive long and well enough to have babies. The effect of those ancestral stripes became the cause of the existence of today’s stripey zebras.

So if zebras evolved their vivid all-over stripes because the (past) ones that had more or bolder stripes, escaped fly-borne illnesses better than their less-stripey fellows, then it seems fair to say that (present) zebras have stripes because stripes protect zebras (past and present) from fly bites.

Photo credit: Clément ROY via Unsplash

The teleological explanation becomes a shorthand for a particular kind of causal explanation, one that features organisms that inherit their ancestors’ most successful traits. No weird teleological “backwards causation” is involved, just a special kind of causal sequence enabled by biology.

Problems, and Alternatives

A huge number of philosophical papers have been published about the selected-effects view of functions, and more are still being written. New problems for this view are constantly being pointed out, and refinements proposed to handle those problems. The core intuition that survives is the idea that the function of something is what it was selected for in its evolutionary past. This evolutionary role means that functions are teleological and normative in some genuine way. Philosophers who support this way of thinking about functions often think that biological functions show how teleology and normativity purpose and value) first come into being in the natural world in a sort of minimal form. The full-fledged kinds of purpose and value found in human life and thought are then built up on this biological foundation.

We won’t try to survey the criticisms that this view has attracted, or its elaborations. (If you want to read up on that debate, we can recommend some good places to start. We do want to note a few important points, though.

The selected-effects view has been used to try to understand a lot of other important ways that teleology and normativity seem to show up in the world. It seems to many philosophers and scientists to shed light on purpose, value, and meaning in many contexts. How can words or thoughts represent or misrepresent the world? And how can we get them right? What is the difference between healthy variation and disease or disorder? The selected-effects perspective has been used to tackle these questions, and many others. Yet there are also many contexts in which we might want to talk about functions or use teleological or normative concepts, where the selected-effects story is hard to apply.

The main reason for this is that this story depends on evolution, on an evolutionary history of selection within a population. So, it applies well to functions in organisms, or maybe to functions in genes, genomes, or cells – all things that compete for survival and for success in adding to new generations of offspring or replicates. But what about other kinds of objects or systems, that might seem to have functions but don’t have the same kind of evolutionary history? Ecologists talk about ecological functions, but ecosystems don’t seem to compete with each other to survive and reproduce, exactly. What about functions of social institutions like money or law? It seems that for cases like these we need another way of thinking about functions, purposes, and how things should be.

Several other influential views have been put forward. Noteworthy among them are the causal-role view, the organizational view, and the persistence view. We’ll look at those in separate posts.

bottom of page