I recently ran across a nice paper on the logic of adaptive
stories (here),
along with a nice short discussion of its main points (here)
(by Massimo Pigliucci (P)). The Olson and Arroyo-Santos paper (OAS) argues that
circularity (or “loopiness”) is characteristic of all adaptive explanations
(indeed, of all non-deductive accounts) but that some forms of loopiness are
virtuous while others are vicious. The goal, then, is to identify the good
circular arguments from the bad ones, and this amounts to distinguishing small
uninteresting circles from big fat wide ones. Good adaptive explanations
distinguish themselves from just-so stories in having independent data afforded
from the three principle kinds of
arguments evolutionary biologists deploy. OAS adumbrates the forms of these arguments and uses this inventory to contrast lousy adaptive accounts from compelling
ones. Of particular interest to me (and I hope FoLers) is the OAS claim that
looking at things in terms of how fat a circular/loopy account is will make it
easy to see why some kinds of adaptive stories are particularly susceptible to
just-soism. What kinds? Well ones like those applied to the evolutions of
language, as it turns out. Put another way, OAS leads to Lewontin like
conclusions (see here)
from a slightly different starting point.
An example of a just-so story helps to illustrate the logic
of adaptation that OAS highlights. Why
do giraffes have long necks? So as to be able to eat leaves from tall trees.
Note, that giraffes eat from tall trees confirms that having long necks is
handy for this activity, and the utility of being able to eat from tall trees
would make having a long neck advantageous. This is the loopiness/circularity
that OAS insists is part of any adaptational account. OAS further insists that
this circularity is not in itself a
problem. The problem is that in the just-so case the circle is very small, so
small as to almost shrink to a point. Why? Because the evidence for the
adaptation and the fact that the adaptation explains is the same: tall necks
are what we want to explain and also constitute the evidence for the
explanation. As OAS puts it:
…the
presence of a given trait in current organisms is used as the sole evidence to
infer heritable variation in the trait in an ancestral population and a
selective regime that favored some variants over others. This unobserved selective
scenario explains the presence of the observed trait, and the only evidence for
the selective scenario is trait presence (168).
In other words, though ‘p implies p’ is unimpeachably true, it is not interestingly so. To get some explanation out of an account that
uses these observations we need a broader circle. We need a big fat
circle/loop, not an anorexic one.
OAS’s main take home message is that fattening circles/loops
is both eminently doable (in some cases at least) and is regularly done. OAS lists
three main kinds of arguments that biologists use to fatten up an adaptation account: comparative arguments, population arguments, and optimality
arguments. Each brings something useful to the table. Each has some
shortcomings. Here’s how OAS describes the comparative method (169):
The
comparative method detects adaptation through convergence (Losos 2011). A basic
version of comparative studies, perhaps the one underpinning most state- ments
about adaptation, is the qualitative observation of similar organismal features
in similar selective contexts.
A second kind of argument focuses on variations in a single
population and sees how this affects “heritability and fitness between
potentially competing individuals” (171). These kinds of studies involve
looking at extant populations and
seeing how their variations tie up
with heritability. Again OAS provides an extensive example involving “the
curvature of floral nectar spurs” in some flowers (171) and shows how variation
and fitness can be precisely measured in such circumstances (i.e. where it is
possible to do studies of “very
geographically and restricted sets of organisms under often unusual
circumstances” (172)).
This method, too, has a problem. The biggest drawback is that the population
method “examines relatively minor characters that have not gone to fixation”
and “extrapolation of results to multiple species and large time scales” is
debatable (171, table 1). In other words, it is not that clear whether the
situation in which population arguments can be fully deployed reveal the
mechanisms that are at play “in generating the patterns of trait distribution
observed over geological time and clades” because it is unclear whether the
“very local population phenomena are…isomporphic with the factors shaping life
on earth at large” (172).
The third type of argument involves optimality thinking. This
aims to provide an outline of the causal mechanisms “behind a given variant
being favored” and rests on a specification of the relevant laws driving the
observed effect (e.g. principles of hydronamics for body contour/sleekness in
aquatic animals). The downside to this mode of reasoning is that it is not
always clear what variables are relevant for optimization.
OAS notes that adaptive explanations are best when one can
provide all three kinds of reasons (as one can in the case, for example, of
aquatic contour and sleekness (see figure 4 and the discussion in P). Accounts
achieve just-so status when none of the three methods can apply and none have been
used to generate relevant data. The OAS discussion of these points is very accessible and
valuable and I urge you take a look.
The OAS framing also carries an important moral, one that
both OAS and P note: if going from just-so to serious requires fattening with
comparative, population and optimization arguments then some fashionable
domains of evolutionary speculation relying on adaptive consideration are likely
to be very just-soish. Under what circumstances will getting beyond hand waving
prove challenging? Here’s OAS (184, my emphasis):
Maximally
supported adaptationist explanations require evidence from comparative,
populational, and optimality approaches. This requirement highlights from the
outset which adaptationist studies are likely to have fewer layers of direct
evidence available. Studies of single species or unique structures are important examples.
Such traits cannot be studied using comparative approaches, because the
putatively adaptive states are unique (cf. Maddison and FitzJohn 2015). When
the traits
are fixed within populations, the typical tools of populational studies
are unavailable. In humans, experimental methods such as
surgical intervention or selective breeding are unethical (Ruse
1979). As a result, many aspects of humans continue to be debated, such as the
female orgasm, human language, or rape (Travis 2003; Lloyd 2005; Nielsen
2009; MacColl 2011). To the extent that less information is available, in many
cases it will continue to be hard to distinguish between different alternative
explanations to decide which is the likeliest (Forber 2009).
Let’s apply these OAS observations to a favorite of FoLers,
the capacity for human language. First, human language capacity is, so far as
we can tell, unique to humans. And it involves at least one feature (e.g.
hierarchical recursion) that, so far as we can tell, emerges nowhere else in
biological cognition. Hence, this capacity cannot be studied using comparative
methods. Second, it cannot be studied using population methods, as, modulo
pathology, the trait appears (at least at the gross level) fixed and uniform in
the human species (any kid can learn any language in more or less the same
way). Experimental methods, which could in principle be used (for there
probably is some variation across individuals in phenomena that might bear on
the structure of the fixed capacity (e.g. differences in language proficiency
and acquisition across individuals) will, if pursued, rightly land you in jail
or at the World Court in the Hague. Last, optimization methods also appear
useless for it is not clear what function language is optimized for and so the
dimensions along which it might be optimized are very obscure. The obvious ones relating to efficient
information transmission are too fluffy to be serious.[1]
P makes effectively the same point, but for evo-psych in
general, not just evo-lang. In this he reiterates Lewontin’s earlier
conclusions. Here is P:
If you ponder the above for a
minute you will realize why this shift from vicious circularity to virtuous
loopiness is particularly hard to come by in the case of our species, and
therefore why evolutionary psychology is, in my book, a quasi-science. Most
human behaviors of interest to evolutionary psychologists do not leave fossil
records (i); we can estimate their heritability (ii) in only what is called the
“broad” sense, but the “narrow” one would be better (see here); while it is
possible to link human behaviors with fitness in a modern environment (iii),
the point is often made that our ancestral environment, both physical and
especially social, was radically different from the current one (which is not
the case for giraffes and lots of other organisms); therefore to make
inferences about adaptation (iv) is to, say the least, problematic. Evopsych
has a tendency to get stuck near the vicious circularity end of Olson and
Arroyo-Santos’ continuum.
There is more, much more, in the OAS paper and P's remarks are also very
helpful. So those interested in evolang should take a look. The conclusion both
pieces draw regarding the likely triviality/just-soness of such speculations is
a timely re-re-re-reminder of Lewontin and the French academy’s earlier
prescient warnings. Some questions, no matter how interesting, are likely to be
beyond our power to interestingly investigate given the tools at hand.
One last point, added to annoy many of you. Chomsky’s
speculations, IMO, have been suitably modest in this regard. He is not giving
an evolang account so much as noting that if
there is to be one then some features will not be adaptively explicable. The
one that Chomsky points to is hierarchical recursion. Given the OAS discussion
it should be clear that Chomsky is right in thinking that this will not be a
feature liable to an adaptive explanation. What would “variation” wrt Merge be?
Somewhat recursive/hierarchical? What would this be and how would the existence
of 1-merge and 2-merge systems get you to unbounded Merge? It won’t, which is
Chomsky’s (and Dawkins’) point (see here for
discussion and references). So, there will be no variation and no other animals
have it and it doesn’t optimize anything. So there will be no available
adaptive account. And that is Chomsky’s point! The emergence of FL whenever it
occurred was not selected for. Its emergence must be traced to other non adaptive factors. This conclusion,
so far as I can tell, fits perfectly with OAS’s excellent discussion. What
Chomsky delivers is all the non-trivial evolang we are likely to get our hands
on given current methods, and this is just what OAS, P and Lewontin should lead
us to expect.
[1]
Note that Chomsky’s conception of optimal and the one discussed by OAS are
unrelated. For Chomsky, FL is not optimized for
any phenotypic function. There is nothing that FL is for such that we can say that it does whatever better than
something else might. For example structure dependence has no function so that
Gs that didn’t have it would be worse in some way than ones (like ours) that
do.
