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SOLVING THE CIRCULARITY PROBLEM FOR FUNCTIONS
(A RESPONSE TO NANAY)
Bence Nanay1describes a circularity problem that the etiological theory of biological
functions is said to face.2The etiological theory of biological functions holds that a token
traits function depends on what traits of the relevant type were selected for. The
function of a heart is to pump blood only if traits of its type were selected for pumping
blood. Circularity sets in if we then classify traits into the relevant typesi.e., the types
relevant for the purpose of ascribing functions on the basis of their functions. That is,
if a heart is (constitutively) an organ that has the function of pumping blood and if a heart
has the function of pumping blood because (constitutively) it is a heart (and hence an
organ that has the function of pumping blood) then there is a problem of circularity.3
Nanay also holds that a good theory of functions should meet three desiderata: (1)
that a trait be allowed to have more than one function (e.g., feathers can have the function
in flight and also the function of thermoregulation), (2) that the function attributions that
we make be allowed to depend on our explanatory projects (e.g., if we are interested in
how eagles catch mice, we might be more interested in the aerodynamic function of
feathers than their function in thermoregulation) and (3) that malfunction be possible, so
that a token trait can have the function of performing F even though it cannot perform F
(e.g., a broken wing that does not enable flight can nonetheless have the function of
enabling flight).
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We agree with these desiderata,4but they are not exhaustive. Another
desideratum is that the theory gives correct function attributions. It must at least get them
approximately right, if it is to be considered in the running. There is room for some
revision but not for whole scale revision.Similarly, it is not enough to provide for the
possibility of malfunction. Our theory should entail that malfunctioning traits are
malfunctioning and that traits that are functioning normally or properly are not
malfunctioning. The usual view is that the standards for normal or proper functioning
cannot be found by peering at a token trait.
Nanay rejects the usual view in order to solve the circularity problem. He suggests
that we can avoid this problem by offering an account according to which functions
depend on facts pertaining to tokens only. This allows the analysis of functions to remain
silent on the individuation of trait types. Here we offer counterexamples to Nanays
analysis and more important, show how the etiological analysis solves the circularity
problem that it is alleged to face.
I. THE MODAL THEORY
On Nanays modal theory, the functions of token traits do not depend on what is true of
traits of the type. Instead, they depend on counterfactuals that pertain to trait tokens,
regardless of what is true of any other traits of the same type.
On Nanays modal theory, performing F is a function of an organisms (Os) trait
[token], x, at time t if and only if:
1. There is a relatively close possible world, w, where [token] x is doing F at t.
2. Doing F at t contributes to Os inclusive fitness in w.
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3. This world, w, is closer to the actual world than any of the possible worlds in
which x does F at t but this does not contribute to Os inclusive fitness.
Consider how this proposal provides for the possibility of malfunction. An
eagles broken wing is not broken in some relatively close possible world. In that world,
other things being equal, the eagle would be able to fly and its ability to fly would
contribute to its fitness. Further, a world in which this eagle can fly and flying
contributes to its fitness is closer to the actual world than one in which it can fly but this
does not contribute to its fitness. So, on Nanays analysis, this particular wing of this
individual eagle has the function of flight, even though this eagle cannot fly.
Note that, although Nanay leaves the notion of a relatively close world vague,
worlds must count as relatively close if the difference to the actual world is no greater
than any change required to avert malfunction (or else his theory cannot accommodate
actual malfunctions). This, as we will see, is a problem.
Like over half of all humans, one of us (A) is lactose intolerant. On Nanays
analysis, the mechanisms that make A lactose intolerant (unable to digest lactose as an
adult) must have the function of providing lactose tolerance (an ability to digest lactose as
an adult). Why is that, and why is it a problem?
A little human natural history makes clear why. Long ago in mammalian
evolution there was selection for turning off the lactose operon system in adults. It is
normal in mammals, including most humans, to begin producing less of the lactase
enzyme (needed for digesting lactose) after weaning. Quite recently in human evolution
circumstances made it advantageous to change this. It became advantageous to keep the
system on, allowing lactase to be produced in adults. This lactase persistence can give
adults year round access to food in certain regions (i.e., those with access to domesticated
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milk producing animals or their products). Making it advantageous of course is not the
same thing as making it happen. That would be a version of Lamarckian evolution. But
owing to improbable events at the level of the nucleotide (ones that happened twice on
chromosome 2, first about 7-9,000 years ago and again separately in the last 3000 years,
at different nucleotide sites in the region of chromosome 2 in the human genome) the
pathway that shuts down lactose digestion in adults was disabled, and the continuing
ability to digest lactose was enabled in certain (originally Balkan but later) Nordic
populations and certain sub Saharan African ones. Lactase persistence has not yet been
driven to fixation. As yet it is present in only a minority of the human population.
This succession of events confers on the mechanisms responsible for As lactose
intolerance the function of making him lactose tolerant (i.e., the function of enabling him
to digest lactose as an adult) on Nanays theory, for the mechanism satisfies all three of
Nanays modal requirements.
1. There is a possible world, w, in which A is able (as an adult) to digest lactose. It
is a world in which there was a point mutation in a cell from which A (or one of
his ancestors) developed. It is a relatively close world.
2. In w, digesting lactose contributes to As inclusive fitness.
3. World wis closer to the actual world than other worlds in which A digests
lactose but this does not contribute to As fitness.
Note that if the point mutations occurrence is an event governed by probabilistic laws,
then wdiffers from the actual world, not in any laws, nor in any initial conditions, nor
even in its history, up until the indeterministic point mutation that made A lactose
tolerant in adulthood in w. So it is a relatively close possible world. The closeness of
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worlds is notoriously hard to measure but this is about as close as possible worlds can
get.
Certainly, wis closer than a world where A is lactose tolerant in adulthood but
where that is a selectively neutral or a fitness reducing fact about A. For such worlds are
ones in which the laws are different or there are more differences in history or in our
current environment. Accordingly, if Nanay is right, the function of the mechanisms
responsible for As adult lactose intolerance is adult lactose tolerance and the relevant
mechanisms in A, in the actual world, are malfunctioning. This is not the right outcome.
It is normal for A, given his ethnicity, to have adult lactose intolerance.
For every malfunctioning token trait, Nanays theory requires that there be a
relatively close possible world in which it is not malfunctioning (e.g., a world in which
the eagle did not break its wing). At the same time, for every token trait that is
functioning normally (e.g., the shut-down of As lactose tolerance) the modal theory
requires that there be no relatively close possible world in which it is improved in some
fitness enhancing way (e.g., As lactose operon system does not shut down).
The problem is that relatively close with respect to possible worlds remains
hopelessly ill defined in Nanays theory and it is hard to see how any more precise
definition would help. One can of course deploy it in a circular fashion as follows. If the
possible world is just like ours, except that a trait that is actually malfunctioning in our
world is functioning properly in that world and all else is otherwise as similar as it can be
(consistent with this) then count that world as relatively close. But if the possible
world is just like ours, except that a trait that is functioning normally in our world is
improved upon in a fitness enhancing way and all else is otherwise as similar as it can be
(consistent with this) then count that world as too far away. However, thisis circular.
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This is a very general problem for the modal theory. Consider every beneficial
mutation that will occur in every species on Earth in the near future (in evolutionary
terms). For each such beneficial mutation, there is something beneficial, F*, that a token
trait will one day do, which traits in its lineage are presently unable to do. They can now
do F, but not F*. Many of these beneficial things that these future traits will do would be
beneficial if done now in the current environment. Many would contribute to fitness if
done now. And the possible worlds in which these traits do these beneficial things now
are relatively close possible worlds. They are as alike to our world as can be, except for
the fact that a beneficial mutation that will actually occur soon will in these worlds have
occurred sooner. These worlds are no further away than any world in which a damaging
mutation that actually occurs and that actually leads to malfunction does not occur.
Moreover, the worlds in which these improved traits do these beneficial things that
contribute to fitness is closer to the actual world than any world in which they do these
beneficial things and this does not contribute to fitness. On Nanays theory of functions,
it follows that many present day traits, which have the function to do F, also have the
function to do F*, even though no trait of the type can yet do it. By implication, on his
theory, all such traits are malfunctioning by virtue of their current inability to do F*.
Further, there is no reason to limit our concern to mutations that will soon occur.
Some beneficial mutations that could soon occur will not soon occur, and perhaps will
never occur. However, the same in principle problem applies. On Nanays theory, traits
malfunction now, by virtue of their inability to do the improved things that would be
done were such mutations to occur.
The conclusion to draw is that the functions of traits do not depend on distinctive
modal facts about token traits alone. More important, the alleged circularity problem that
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motivated this approach is one that the etiological theory can dispose of without
difficulty.
2. HOW THE ETIOLOGICAL THEORY SOLVES THE CIRCULARITY PROBLEM
Traits do not need to be typed first, prior to the identification of their functions. Instead,
trait individuation and function attributions supervene on the same underlying facts about
trait lineages and the selection pressures operating on them. They co-supervene.
Traits are located in lineages and a lineage of traits is segmented (or as we say,
parsed) both for the purpose of classifying traits andfor the purpose of ascribing
functions by changes in the selection pressures that have operated on it. No circularity is
involved. There is only a superficial appearance of circularity, as we will explain. In
what follows, we explain (1) how token traits are located in lineages, (2) how a lineage is
then parsed by changes in the selection pressures that operated on it, (3) how the
functions of trait tokens are then fixed by their locations in the lineage, and (4) how this
resolves the circularity problem.
(1) How are token traits located in lineages? The mechanisms and processes that
are responsible for inheritance link traits in ancestral and descendent relations and token
traits are located within lineages after their ancestors and before their descendants. These
are the relations that biologists attempt to trace when they trace relations of homology.5
Suppose, for example, that we are interested in a particular eagles forelimb. Its forelimb
(its wing) is located in a lineage of forelimbs, after those of its parents and before those of
its offspring, if it has any. This lineage includes ancient fish fins, ancient amphibian legs,
and the forelimbs of flightless dinosaurs, as well as the proto-wings of dinosaurs that
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could lift a little above the ground or soar some distance between the trees. More recent
are the forelimbs of ancient flying birds, more modern birds, and the wings of ancestral
eagles. Our eagles forelimbs are located in this lineage by virtue of their actual history.
(2) How is a lineage of traits parsed by changes in the selection pressures
operating on it? Suppose that we are interested in whether a particular eagle forelimb has
the function of flight. We locate the relevant lineage of vertebrate forelimbs, the one that
evolved into this eagle forelimb. Now we draw imaginary lines in this lineage whenever
selection for flight starts or stops. This segments the lineage with respect to selection for
flight. (Note that we did not assume that the forelimb had the function of flight in order
to do this. We only asked if the forelimb has the function of flight.) If a capacity for
flight evolved only once in the lineage leading to the modern eagles forelimbs, and
selection for flight did not stop once it started, there is just one line to be drawn in this
lineage in this case.
(3) How are the functions of token traits fixed? If our question is whether our
particular eagle forelimb has the function of flight, first ask to which segment of its
lineage (parsed with respect to selection for flight) it belongs. Then ask whether
selection for flight operated on ancestral homologs within that lineage. Since the answer
is affirmative, the eagles forelimb has the function of flight and it is also therefore a
wing (a forelimb that has the function of flight).6
Where lines are to be drawn in lineages will often be subject to a Sorties-like
vagueness. A capacity for flight probably evolved gradually and there can be no
selection for flight until there is flight.7But this vagueness is not a problem.
8The
vagueness of when in time there was or was not selection for flight corresponds to the
vagueness of whether certain ancestral forelimbs did or did not have the function of
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flight. In fact the vagueness of the etiological theory captures the Darwinian gradualism
of the phenomena. It is in fact vague when dinosaur forelimbs were first selected for
flight and first acquired the function of flight.
Note too how well this approach deals with the case of lactase persistence. Well
under half of all humans belong to lineages in which there has been selection for lactase
persistence. According to the etiological theory, the mechanisms that permit lactose
digestion in adulthood in this minority of people have the function of doing so. Why?
There was selection for the adult digestion of lactose operating on ancestral mechanisms
in the relevant segments of the lineages to which the mechanisms of this minority belong.
Now recall A, who is lactose intolerant. To discern the functions of the mechanism(s)
responsible for As lactose intolerance, we need to identify the lineage (or lineages)
leading to As mechanism(s). We need to parse it (or them) with respect to selection for
the adult digestion of lactose. The relevant mechanisms in A do not belong to segments
of a lineage (or segments of lineages) on which selection for the adult digestion of lactose
has operated on ancestral mechanisms. So his mechanisms are not malfunctioning in
virtue of their inability to provide for the adult digestion of lactose. On our account, one
does not need to be in the evolutionary vanguard to function properly; one need not have
all of the actually imminent or only possible improvements on our current species design
in order to function properly.9
As sketched above, this version of the etiological theory does not say that we
must type traits first and then ask what selection recently operated on that type of trait.
Instead, it tells us that token traits are located in lineages, which are parsed in accord with
stops and starts in selection for F, when it is the function to do F that is at issue. If the
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trait in question belongs to a segment of the lineage in which selection for doing F
operated on its ancestors, the token trait has the function to do F.
(4) How does this resolve the circularity problem? Function and trait typology
both depend on past selection history and on changes in selection pressures operating on
lineages. Traits are typed inascribing functions to them, not before ascribing functions to
them.10
It is important to stress that, in explaining how a particular forelimb is located in
a lineage, in explaining how the lineage is parsed with respect to selection for flight, and
in explaining how we determined on that basis if a particular forelimb has the function of
flight, we do not assume in advance that the forelimb in question had (or did not have)
the function of flight. We do not assume in advance that the eagles wing has the
function of flight.11
Rather, we ask ifit has the function of flight. Then we parse the
relevant lineage at starts and stops in selection for flight and ask if selection for flight had
operated on the relevant segment of the lineage, the one to which the token trait in
question belongs. We ask whether ancestral traits, within that segment, had been selected
for flight. In none of this do we need to know in advance whether the token trait in
question had the function of flight. We also do not need to know if it was a wing (or a
flipper, or a leg or an arm). We locate it in its lineage and go on from there.
There is only a superficial appearance of circularity on the etiological theory. The
superficial appearance of circularity derives from the common basis for both trait
typology and functions. As Neander has elsewhere remarked, Function ascriptions and
these particular kinds of trait classifications go hand-in-hand, hence the superficial
appearance of circularity. One cannot get a wife or a husband unless one marries and one
cannot get married unless one gets oneself a wife or a husband. But there is no Catch 22
here. We do them both together.12
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3. CONCLUSION
The etiological theory meets all of Nanays desiderata. Besides avoiding circularity, he
lists three. A token trait can have more than one function, our function attributions
depend on our explanatory projects, and there is the possibility of malfunction.
A token trait,x, can have more than one function. It can have the functions, F
and G. If you are interested in whether a token trait,x, has the function to F, locatexin
its lineage. Draw a line in the lineage whenever there is a stop or start in selection for
performing F. If there was selection for performing F operating on traits ancestral tox
within its segment of the lineage, then x has the function to F. Now, to see if the same
trait,x, has the function to G, again locatexin its lineage. But now draw a line in the
lineage whenever there is a stop or start in selection for performing G. If there was
selection for performing G operating on traits ancestral to x within its segment of the
lineage, thenxhas the function to G. Since there can be selection for both doing F and
doing G operating on a lineage of traits at the same time, one and the same trait can have
the function to F and to G.
The etiological theory also recognizes that function attributions depend on the
explanatory project: that is to say that the attributions we make depend on our
explanatory projects. We can ask whetherxhas the function to F, or whether it has the
function to G, depending on these explanatory projects. However, the functions
themselves are not dependent on our explanatory projects. Whetherxhas the function to
F or to G (or both) depends on history, not on which questions we ask or which questions
are relevant to our projects.
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The way this version of the etiological theory draws lines in lineages also allows
for the possibility of malfunction. A trait that cannot do F can nonetheless belong to a
segment of a lineage of traits in which selection for F-ing operated on traits ancestral tox.
A broken wing of an eagle belongs to a segment of a lineage in which selection for flight
operated on ancestral forelimbs.
The etiological theory of functions is epistemically demanding. Drawing the lines
in lineages that it employs requires a lot of phylogenetic information along with a lot of
ecological information. But the issue between Nanay and us is metaphysical: what
constitutes a function. We have expounded the metaphysical issues in terms of questions
that need to be answered so as to show that there is no circularity in the constitutive
conditions. We do not imagine that biologists will, in practice, perform these steps each
time they attribute a function to a trait, no more than we assume that we always establish
the chemical composition of water before we drink it.
KAREN NEANDER
Duke University
ALEX ROSENBERG
Duke University
1A Modal Theory of Function, THIS JOURNAL, 107 (8): 412-431.
2Others have also addressed the problem. See for instance, Paul S. Davies,Norms of
Nature(MIT: Cambridge, 2001), Paul Griffiths, In what sense does nothing in biology
make sense except in the light of evolution? inActa Biotheoretica, 57 (2009): 11-32,
and Karen Neander, How are traits typed for the purpose of ascribing functions to
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them? in Jean Gayon (ed.)La Notion de Fonction: des sciences de la vie a la
technologie (2010).
3
Nanay points out that most other theories of function potentially face a similar problem.
They all seem to need an independent function-free way to individuate trait-types or else
to abandon the idea that functions belong to tokens in virtue of their types. However, the
alternatives to typing traits by their functions do not look promising, or so Nanay argues.
For example, if we use homology as the basis for the classification of traits, the resulting
classifications are insufficiently fine-grained for many purposes (as we have previously
argued ourselves in Rosenberg and Neander, Are Homologies (Selected Effect or Causal
Role) Function Free? inPhilosophy of Science, 76, 3 (2009): 307-334). Two traits can
be homologous and yet can have very different functions and so homology does not seem
to provide the fine-grained types needed for ascribing functions to traits. For instance,
the forelimbs of vertebrates are all homologous and yet different vertebrate forelimbs
have different functions. Some have the function of assisting in flight, some in paddling
or swimming, some in grasping the branches of trees, some in walking and running and
so on. So it cannot be that we type traits for the purpose of ascribing functions to them
on the basis of homologous relations alone. But add function to the mix and oneseemsto
have a problem of circularity. The appearance is deceiving we shall argue.
4We understand (2) in such a way that biological functions do not depend on our
interests, although the attributions that we make do so depend.
5There are difficulties in providing a full treatment of historical homology and we do not
slight those difficulties but nor do we treat them here. These difficulties do not include
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the present problem of circularity and nor does Nanay seem to take issue with a notion of
historical homology.
6
A few other examples might help. Consider a modern penguins forelimb (one of its
flippers) and a modern emus forelimb (one of its vestigial wings). First, locate the
lineage of forelimbs for the penguin. Now draw imaginary lines in the lineage whenever
selection for flight starts or stops. There is thought to be at least one time when selection
for flight starts and at least one time when selection for flight stops. As long as our
penguins forelimb belongs to a segment of this lineage in which there was no selection
for flight operating on ancestral forelimbs, it does not have the function of flight. Now
consider the emus forelimb. Again, we can segment its lineage by drawing lines
whenever selection for flight stops or starts. Then we can ask whether this emus
forelimb belongs to a segment of the lineage in which ancestral forelimbs were selected
for flight. It does not. It belongs to a segment of the lineage in which there was no
selection for flight. So it is vestigial with respect to flight.
7The capacity for flight seems to have developed gradually, either from an enhancement
of an ability to soar from tree to tree in arboreal creatures or from an enhancement of an
ability to leap from the ground in ground-dwelling creatures.
8Vagueness is a problem for the modal theory of functions because, in order to apply the
theory, the vagueness of the notion of relatively close possible worlds must be resolved
in an ad hocway, after peeking at the functions of traits.
9Although it remains true on our proposal that much relevant selection is recent, relative
to a token trait, this proposal differs from the modern history version of the etiological
theory of functions that Nanay considers. In some lineages, traits can gain and lose and
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can again gain other functions fast. So it is hard to see how a non-circular answer can be
given to the question, How recent is recent enough? On our approach, we do not need
to answer that question. If selection for F-ing starts and stops in short order, there might
be more than one segment of the lineage vis a visF that is recent, in evolutionary terms.
But this does not matter for whether that trait has the function to perform F on the present
proposal. Instead, it matters only which segment the token trait belongs to and whether,
within that segment, there was selection for performing F operating on ancestral traits.
10Nanay touches on something like this strategy (see Nanay,op. cit., note 1, p. 421) but
he fails to see how it solves the circularity problem. This is presumably because Nanay
treats any appeal to what there was selection for as an appeal to function. According to
the etiological theory of functions, functions are (roughly) what traits were selected for.
So Nanay presumably thinks that, if we appeal to what there was selection for in our
proposed solution to the problem, we appeal to function and return to running around the
same old circle.
11Nor do we assume in advance that the penguins flipper or the emus vestigial wing did
not have the function of flight. See note 6.
12This is in the original English, from Neander op. cit., note 2 (p XXX), which offers an
earlier version of this solution to the circularity problem.