Why Do Animals Have the Parts They Have? The Debate About Adaptationism

Imagine you’re walking through a forest and you find a bird’s nest. It’s woven together with such precision—twigs curved just so, moss tucked in the gaps, the whole thing shaped like a perfect cup. You think: this nest was designed for holding eggs and baby birds. That’s what it does, and it does it well. So it makes sense to say that natural selection shaped it, step by step, generation after generation, until it became this good at its job.

Now imagine you’re looking at the same forest and you notice the spiral pattern on a pinecone. Those scales are arranged in a Fibonacci sequence—a mathematical pattern that shows up all over nature, in sunflowers and pineapples and cauliflower. Did natural selection put those numbers there because they’re somehow optimal? Or does that pattern just come from the way plants grow, like a constraint built into their development, whether it’s useful or not?

This is the kind of question that starts the debate about adaptationism. It’s a fight among biologists (and philosophers who think about biology) about how much we should explain by saying “natural selection made it that way because it works well” versus saying “it’s that way because of something else—history, physics, development, or just random chance.”

A Strange Thing Philosophers Noticed

The debate got its modern start in 1979, when two biologists named Stephen Gould and Richard Lewontin wrote a paper called “The Spandrels of San Marco and the Panglossian Paradigm.” The title is a bit of a mouthful, but here’s what it means.

First, “spandrels.” If you’ve ever been inside an old church or cathedral, you might have seen a big archway that supports a dome. The spaces between the arch and the dome are called spandrels—they’re the triangular gaps that are just there because of how arches work. In the Cathedral of San Marco in Venice, those spandrels are covered in beautiful mosaics of the four evangelists. But here’s the thing: if you only looked at the mosaics, you might think the spandrels were designed for those pictures. But actually, the spandrels exist because of the architecture. The pictures came after, making use of a space that was already there for structural reasons.

Gould and Lewontin argued that biologists make the same mistake all the time. They see a trait—like the pattern on a shell, or a behavior in an animal—and immediately ask “what’s it for?” They assume it must be an adaptation, shaped by natural selection for some purpose. But maybe it’s just a spandrel: a side effect of something else, a byproduct of how the organism grows, or a leftover from evolutionary history.

And “Panglossian” comes from Dr. Pangloss, a character in Voltaire’s novel Candide who insists that “everything is for the best in this best of all possible worlds.” Even when terrible things happen, Dr. Pangloss finds a way to explain that they’re actually optimal. Gould and Lewontin thought many biologists were Panglossians—always finding a reason why every trait is perfectly adapted, even when they should be considering other possibilities.

Three Ways of Being an Adaptationist

Philosophers have since sorted out that “adaptationism” isn’t really one single view. It comes in three flavors, and you can believe one without believing the others.

First: Empirical adaptationism. This is a claim about how nature actually works. It says that natural selection is incredibly powerful—so powerful that it overcomes most other influences. Most traits, the argument goes, are locally optimal. That means given the organism’s situation, you can’t improve the trait without basically redesigning the whole creature. If you believe this, then when you see a bird’s beak, you assume it’s the best possible beak for the food that bird eats, shaped purely by selection.

Second: Explanatory adaptationism. This is a claim about what’s important to explain. It says that the really amazing thing about living things is how well they’re suited to their environments—how they seem designed. Natural selection is the answer to that big question. So even if other forces also matter, explaining adaptation is the central mission of evolutionary biology. Some biologists think this is a scientific claim; others think it’s more like an aesthetic preference, a choice about what kind of story is most satisfying.

Third: Methodological adaptationism. This is a claim about how to do science. It says that even if adaptations aren’t that common, the best strategy is still to look for them first. Start by asking “what’s this for?” and assume natural selection has an answer. Only if that fails should you consider other explanations. This could be true even if most traits turn out not to be adaptations—it’s a tool, not a prediction.

Most adaptationists believe all three. But some biologists are methodological adaptationists while rejecting empirical adaptationism. They think “look for adaptation first” is a good research strategy even though they know that many traits aren’t optimal. And plenty of biologists reject all three flavors entirely.

What Would It Take to Settle This?

You might think: just count up all the traits in nature and see how many are optimal. Simple, right?

Not so simple. First, you need a way to tell whether a trait is optimal. That means building a model that says “if natural selection were the only force at work, what would the trait look like?” Then you compare that prediction to reality. If they match, you might conclude selection is sufficient. If they don’t, you need to figure out whether your model was wrong or selection wasn’t the main force.

A pair of philosophers and biologists named Steven Orzack and Elliott Sober proposed something they called the “Adaptationism Project.” Their idea was to test empirical adaptationism by assembling a huge collection of studies, all done the same way. Each study would test a specific trait: an optimality model (selection only) versus a model that includes other forces like random drift or developmental constraints. If the selection-only model fit perfectly, and the more complex model didn’t add anything, that’s evidence for adaptation. If the complex model did better, that’s evidence against it.

Here’s the problem: very few studies have actually been done this way. Orzack and Sober searched the scientific literature and found only four analyses that met their standards. After decades of argument about adaptationism, almost nobody had actually done the work to test it properly. This is a little like two teams arguing about whether a basketball player is the greatest of all time, but nobody has bothered to keep track of his stats.

Are Constraints Competitors or Background?

One of the trickiest parts of this debate is figuring out what counts as a genuine alternative to an adaptation explanation.

Suppose you think the Fibonacci pattern in pinecones comes from how plants grow—a developmental constraint. And suppose your colleague thinks it’s an adaptation because the pattern allows the most seeds to fit in the smallest space. Are these competing explanations? Or is the constraint just part of the background, like a rule of the game that selection has to play within?

Adaptationists tend to say: of course there are constraints. Every model has to include some. When you build an optimality model, you specify what the possible variations are—that’s your constraint set. Then selection chooses the best option from that set. Constraints aren’t rivals; they’re just the starting point.

But critics point out that this treats constraints as passive—like walls that limit where you can build. What if constraints are more active? What if they push evolution in particular directions, or make certain outcomes more likely even when they aren’t adaptive? The spiral pattern isn’t just a limit on what’s possible; it might be a positive bias, a tendency that comes from how plants grow and that shows up over and over, whether or not it helps.

This connects to a deep philosophical question: What counts as a cause? If a constraint prevents something from happening, is that a cause? If a developmental system makes certain changes more probable, is that a cause? Philosophers disagree about this, and the biology debate inherits that disagreement.

The Female Orgasm: A Concrete Example

Here’s a case that makes the debate real. Why do human females have orgasms? One obvious adaptationist answer: it feels good, so it encourages sex, which leads to reproduction. The female orgasm might be an adaptation for bonding with a partner or for encouraging repeated mating.

But a biologist named Donald Symons proposed a different explanation. He pointed out that male orgasm is essential for reproduction—if males can’t ejaculate, there are no babies. And the tissue that produces orgasm in males develops very early, before the embryo’s sex is determined. That tissue is present in both male and female embryos. So maybe female orgasm isn’t an adaptation at all. Maybe it’s a spandrel—a byproduct of strong selection on males, carried along in females because of how development works.

This is a genuine competition between an adaptive hypothesis (“it was selected because it helped females”) and a constraint hypothesis (“it’s there because of developmental history and selection on males”). Both are testable, at least in principle. And which one you find more convincing says something about where you stand in the adaptationism debate.

Why This Matters

You might think this is a narrow fight among biologists about how to study snail shells and bird beaks. But the stakes are bigger.

First, it’s about how we understand the living world. Are organisms perfect machines shaped by relentless optimization, or are they messy historical accidents constrained by physics and development? The answer changes how we see everything from human anatomy to ecosystem function.

Second, it’s about how to do science. Methodological adaptationism says “assume it’s an adaptation and test that first.” Critics say this biases the results—if you only look for one kind of explanation, you’ll find it, even when it’s wrong. This isn’t just a biology problem. It’s a general question about scientific method: should we always start with the most exciting hypothesis, even if it’s often wrong?

And third, it connects to how we think about ourselves. Some of the most vocal adaptationists, like Richard Dawkins and Daniel Dennett, argue that seeing the world through the lens of adaptation is fundamentally important. It counters the old argument from design—the idea that only a creator could produce such perfect fit between organism and environment. If natural selection can explain it all, we don’t need God. That’s a big claim, and it only works if natural selection can explain it all.

The adaptationism debate isn’t settled. Most biologists probably lean adaptationist in practice—they ask “what’s it for?” when they study a new trait. But most also acknowledge that not everything is an adaptation. The question is where to draw the line, and how to know when you’ve crossed it.

---

Key Terms

Term	What it does in the debate
Adaptation	A trait shaped by natural selection for a specific function
Adaptationism	The view (in one of three flavors) that natural selection is the main or best explanation for most traits
Spandrel	A trait that exists not because it was selected for, but as a byproduct of something else
Constraint	A limitation or bias on what variations are possible, coming from development, genetics, or physics
Optimality model	A mathematical model that predicts what a trait would look like if natural selection were the only force at work
Local optimality	The claim that a trait is the best possible option given the organism’s situation and constraints

Key People

• Stephen Jay Gould — A paleontologist who loved pointing out that not everything in evolution is adaptive; co-wrote the famous “Spandrels” paper in 1979
• Richard Lewontin — A geneticist who worked with Gould on the “Spandrels” paper; argued that biologists need to take non-adaptive explanations seriously
• Elliott Sober — A philosopher of biology who helped clarify the different flavors of adaptationism and proposed ways to test it
• Steven Orzack — A biologist who worked with Sober on the “Adaptationism Project,” trying to actually test whether most traits are optimal
• Donald Symons — An anthropologist who argued that female orgasm might be a spandrel, not an adaptation

Things to Think About

1. If you had to design a study to test whether a particular trait was an adaptation, what would you need to know? How would you rule out the possibility that it’s just a byproduct of something else?

2. The adaptationism debate is partly about how much credit to give natural selection versus other forces. But is this really an either/or question? Could a trait be shaped by selection and by constraints at the same time?

3. Think about something humans do—maybe laughing, or blushing, or dreaming. Is it an adaptation? A spandrel? How would you even begin to decide?

4. Methodological adaptationism says “look for adaptation first” even if adaptations are rare. Is that good science, or does it guarantee you’ll find what you’re looking for even when it’s not there?

Where This Shows Up

• Medicine: When doctors assume the body is well-designed, they sometimes miss “spandrels”—features that aren’t adaptive and can cause problems. (Why do we have appendixes? Why do wisdom teeth get impacted?)
• Technology: Engineers who design robots or AI sometimes debate whether they should copy nature’s “optimal” designs or whether nature’s solutions are just one possibility among many.
• Psychology: Evolutionary psychologists often assume that human behaviors are adaptations—but critics say they’re too quick to tell “just-so stories” without evidence.
• Your own sense of the world: When you catch yourself thinking “that must be there for a reason,” you’re being a little bit of an adaptationist. The question is whether you’re right.