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Evolutionary Contingency, Stability, and Biological Laws
Jani Raerinne, Ph.D.
Draft. Journal for General Philosophy of Science (forthcoming)
Department of Philosophy, History, Culture, and Art Studies
University of Helsinki
Address: P.O. Box 24 (Unioninkatu 40 A)
00014 University of Helsinki, Finland
E-mail: [email protected]
Abstract The contingency of biological regularities – and its implications for the
existence of biological laws – has long puzzled biologists and philosophers. The best
argument for the contingency of biological regularities is John Beatty’s evolutionary
contingency thesis, which will be re-analyzed here. First, I argue that in Beatty’s thesis
there are two versions of strong contingency used as arguments against biological laws
that have gone unnoticed by his commentators. Second, Beatty’s two different versions
of strong contingency are analyzed in terms of two different stabilities of regularities.
Third, I argue that Beatty and his commentators have focused on the more ineffective
trajectory stability version of the argument, whereas the constancy stability version
provides a more substantial and applicable argument against the existence of biological
laws. Fourth, I develop a counterexample to Beatty’s thesis. Finally, I discuss the
possibility of evolution producing repeatable and general non-lawlike regularities and
patterns by utilizing the notion of generative entrenchment and by criticizing the thesis
of multiple realizability of biological properties.
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Key words: evolutionary contingency thesis; generative entrenchment; laws;
multiple realization
1 Introduction
Many arguments have been made in support of the claim that there are no biological
laws. One argument in particular has been taken seriously by many, namely, the
argument that charges biological regularities with being contingent rather than necessary
in the sense that laws are supposed to be necessary. The best explication of this
argument is the evolutionary contingency thesis put forth by John Beatty (1995).
To Beatty (1995), there are two kinds of contingency to which his thesis refers.
By weak contingency Beatty means that biological regularities are riddled with
exceptions. By strong contingency Beatty means that biological regularities lack the
necessity associated with laws. Taken together, weak and strong contingency pose a
threat, both to biological regularities’ lawlikeness and to their truth. Biological
regularities’ strong contingency suggests that they are not lawlike, but rather
accidentally true regularities, whereas biological regularities’ weak contingency suggests
that they are not even accidentally true. Instead, they are false regularities, owing to the
regularities’ open-ended sets of exceptions that defy systematic and simple treatment.
My aim is to re-analyze Beatty’s thesis. In section 2, I review previous responses
to his thesis in terms of stability and scope. In section 3, I argue that in fact there are two
versions of strong contingency in Beatty’s thesis raised as arguments against biological
regularities’ lawlikeness and that the two are analyzable by different forms of stability of
regularities, the trajectory stability version and the constancy stability version. In section
4, I argue that Beatty and his commentators have concentrated on the more ineffective
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version of the strong contingency thesis, the trajectory stability version, whereas the
constancy stability version provides a more substantial and applicable argument against
the existence of biological laws. In section 5, I develop a counterexample to Beatty’s
thesis. The final section concludes by describing how my analysis of Beatty’s thesis
differs from previous responses and analyses.
The biological laws debate is an interesting philosophical topic in itself.
Moreover, many authors think of laws as having special or proprietary roles in the
sciences, or they think of lawlikeness as a central concept in analyzing other interesting
concepts, such as induction, confirmation, and theory structure. If Beatty’s thesis is
successful as an argument against biological laws, then the question arises of whether it
applies to other sciences and has similar consequences there. This shows that Beatty’s
thesis is of interest to philosophers other than those of biology alone. Finally, the
evolutionary contingency thesis is connected to other interesting topics, such as whether
evolution is capable of bringing about general and repeatable patterns or whether it
results in idiosyncratic and unique outcomes.
2 Previous Reponses to Beatty: Scope and Stability
As Beatty’s critics have argued, rather than being an explanans of why biological
regularities lack lawlikeness, evolutionary contingency is an explanandum that needs to
be analyzed. Mitchell’s (1997, 2000) idea is that strong contingency should be analyzed
in terms of stability of regularities. Waters (1998) suggested that weak contingency
should be analyzed in terms of scopes or “distributions” of regularities. Beatty’s
evolutionary contingency thesis can thus be analyzed as follows:
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1) Given their weak contingency, biological regularities are riddled with
exceptions, that is, their scope is far from universal.
2) Given their strong contingency, biological regularities lack the stability that
guarantees that they would hold in many or all possible background conditions.
Let me call a regularity stable if it holds in many possible background conditions. Stable
regularities are relatively context insensitive, reliable, or projectable in that they
continue to hold in many possible background conditions.
The connection between strong contingency and stability is that these are concepts
of degree, which are inversely related to one another: the more strongly contingent a
regularity is, the less stability it has and vice versa. Stability considerations of
regularities are evaluated on the basis of how many background conditions there are and
what kinds of background conditions the regularities are dependent or contingent upon.
The more stable a regularity is, the less dependent it is in holding in these background
conditions. Of course, stability depends not only on the number of background
conditions, but also on their nature. Likewise, different sciences and disciplines differ on
the background conditions considered important in evaluating the stability of regularities
(cf. Lange 2005).
The application domain of a regularity in the past or present is described by its
scope. This domain includes those (dis)similar systems to which a regularity applies or
has applied. Biological regularities typically generalize about different taxa, features,
entities, and so forth in different times and/or places. As examples of scope, consider the
following: some of the members of the lineage of horses exhibited the pattern of Cope’s
rule in the Miocene and currently, rain forests are almost exclusively located at low
latitudes. Many biological regularities have narrow or limited scopes (Hairston 1989,
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Alroy 1998, Hecnar 1999), whereas traditionally it has been thought that laws have
unlimited or universal scopes.
Scope furnishes us with information about the application domain of a regularity
to (dis)similar systems rather than about the holding of a regularity in different
background conditions. The scope of Mendel’s rules is (only) all or nearly all sexually-
reproducing taxa. The conditions, like the evolution of mitosis and meiosis, on which
Mendel’s rules depend in their holding are the background conditions in the above sense
of stability. A regularity that has a narrow scope could have a high degree of stability
within this scope. The converse could also be true: a regularity that has a broad scope
could have a low degree of stability within its scope.
The above analyses help to clarify the implications of the evolutionary
contingency thesis. Waters shows that weak contingency is a serious threat to biological
regularities’ lawlikeness if one holds to an account of laws in which biological
regularities have unlimited or universal scopes. However, only a minority of
philosophers and biologists today think of biological laws in these traditional terms.
Likewise, there are accounts of laws that tolerate exceptions, such as the pragmatic,
paradigmatic, and inference ticket accounts (Lange 1993a, 1993b; Mitchell 1997, 2000).
Consequently, it can be claimed that weak contingency alone is not a serious difficulty
for the lawlike status of biological regularities.
Mitchell’s analysis shows that strong contingency is potentially a more damaging
threat to the existence of biological laws, because if biological regularities lack maximal
or high degrees of stability, then they lack the modal force of laws that is connected to
the laws’ ability to support counterfactuals. Regularities lacking high degrees of stability
would not hold had the background conditions for the regularities changed in space or
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time. Laws of nature are thought to hold in widely different conditions and even under
all the different background conditions.
However, what the previous commentators, including Mitchell, have missed is
that there are two versions of strong contingency in Beatty’s thesis which are used as
arguments against the existence of biological laws.
3 Two Senses of Strong Contingency
The following is the passage usually quoted from Beatty (1995, 46-47; see also Beatty
1995, 45-51, 57 for similar ideas) as a characterization of strong contingency:
All generalizations about the living world:
(a) are just mathematical, physical, or chemical generalizations (or deductive
consequences of mathematical, physical, or chemical generalizations plus initial
conditions),
or
(b) are distinctively biological, in which case they describe contingent outcomes
of evolution.
The first part of this claim is meant to acknowledge that there are generalizations
about the living world whose truth values are not a matter of evolutionary history.
Evolution has not and will not result in any forms of life that are not subject to the
laws of probability, or to Newton’s laws of motion. Nor will evolution result in
any carbon based forms of life that are not subject to the principles of organic
chemistry. But while these sorts of principles are true of the living world, we do
not call them “biological” principles. [Emphasis added.]
The meaning of this passage can be paraphrased as follows: evolution can lead to
different outcomes from the same or similar starting points given the same or similar
selection pressures. Thus, even from the same selection pressures, similar or identical
adaptations need not, and probably do not, follow, even given similar organisms in
similar environments. Regularities concerning biological phenomena are contingent,
accidental, or unique outcomes in that evolution is easily switched to another track or
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disturbed by small changes in its initial conditions. I will call this version of the
evolutionary strong contingency thesis ESCT1.
Beatty (1995, 57-59) gives reasons why biological regularities are strongly
contingent in the sense of ESCT1. First, selection often selects from a set of trait variants
that are similar in fitness, but differ in realization, that is, the multiple realizability thesis
is true of biological properties. Second, there are other evolutionary forces besides
selection, such as genetic drift and mutations, that affect evolution and its consequences.
Secondly and differently, by strong contingency Beatty (1995, 46, 47, 51-53, 63)
also means that even if biological regularities were true, there might be and quite likely
will be other background conditions in the universe or on this planet where these fail to
hold. Thus, biological regularities are at most representable as accidentally true
generalizations that hold because of certain background conditions. I will call this
version of the evolutionary strong contingency thesis ESCT2.
The two different versions of evolutionary strong contingency can be analyzed in
terms of two different stability concepts. With ESCT1, Beatty was concerned with
biological regularities’ sensitivity to changes in initial background conditions or what is
more commonly known as “trajectory stability,” whereas with ESCT2 he was concerned
with biological regularities’ general endurance during changes in the background
conditions or “constancy.”
What the idea of trajectory instability means when used to analyze the strong
contingency of biological regularities ESCT1 is that biological regularities are accidental
outcomes or products of (evolutionary) history. Biological regularities lack the
inevitability associated with laws, because instead of the current and prevailing
regularities, different regularities could have evolved on this planet had the historical or
initial conditions been different. Thus, biological regularities lack the necessity and the
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trajectory stability in the sense that their evolution could easily have been switched to
another track or disturbed by past historical or initial conditions.
The regularity described by Bergmann’s rule could be argued to have a low
degree of trajectory stability in the sense that its validity is dependent on certain initial or
historical conditions (but see the next section).1 Bergmann’s rule is a geographical or
latitudinal gradient in body size, according to which the members of a species of
endothermic animals are larger in their body size in colder regions or at higher latitudes
than members of the same species in warmer regions or at lower latitudes.
Endothermicity arose as an adaptation to fluctuating environmental or climatic
conditions. Now, if the earth’s past climates or environmental conditions had been
constant rather than fluctuating, then it is possible that endothermicity would not have
evolved. If endothermicity had not evolved, then the regularity described by Bergmann’s
rule would not hold, since the regularity applies to endothermic rather than to
ectothermic animals. Past environmental or climate conditions can be seen as an initial
condition that could have disturbed the validity of regularity described by Bergmann’s
rule had these conditions been different.
The same holds for the regularities described by Mendel’s rules and the Hardy-
Weinberg rule. Had there not been mitosis or had there been some equivalently fit or
fitter alternative, then meiosis would not have evolved, because meiosis evolved from
mitosis; thus, Mendel’s rules would not have evolved on this planet, because the holding
of Mendel’s rules is dependent on the operation of meiosis and mitosis. Moreover, had
Mendelian rules not evolved or had the conditions changed so that they no longer hold,
1 Bergmann’s rule is presented by Beatty (1995, 58-59) as an example of an evolutionary contingent
regularity. If the lack of trajectory stability of the rule can be challenged, then this casts doubt both on
Beatty’s reasoning and on his examples of evolutionary contingent regularities.
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then the Hardy-Weinberg rule might not hold, because it seems to be a consequence of
Mendelian rules. The regularities just described are contingent and accidental products
of history, and their evolution could have been switched to another track or disturbed
had some of their initial or historical background conditions been different. In other
words, they display trajectory instability.
What the idea of constancy instability means when used to analyze the strong
contingency of biological regularities ESCT2 is that biological regularities depend on
different background conditions in order to hold. If some of these background conditions
were to change, then the biological regularities would fail to hold. For instance, there are
non-Mendelian mechanisms of inheritance on our planet (Crow 1979). If the
environment changes so that these become as fit as or fitter than Mendelian mechanisms,
then they might become as omnipresent as Mendelian mechanisms are at present. Thus,
Mendel’s rules seem to have a low or moderate degree of constancy stability.
Regularities that hold owing to certain background conditions lack the necessity
associated with laws and are at most accidentally true. Laws of nature are thought to
hold in all or at least in most of the possible background conditions rather than holding
only because of certain accidental background conditions.
4 ESCT1 and ESCT2 and the Existence of Biological Laws
The trajectory stability version, ESCT1, is the most common interpretation of Beatty’s
strong contingency (Carrier 1995, 84, 89-90; Schaffner 1995, 102-103; Mitchell 1997,
S470-S472, 2000, 250-251; Powell 2009, 394). In fact, it seems to be the only
interpretation of strong contingency offered by previous authors. I will deal with it first
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by arguing that the trajectory stability version provides a weaker argument against the
existence of biological laws than does the constancy stability version, ESCT2.
Considerations of trajectory stability are important when we are explaining,
predicting, or manipulating. We have to be careful and sensitive in setting up and
studying the initial conditions of a process or a system with a low degree of trajectory
stability. However, the connection between trajectory stability and lawlikeness is a moot
point.
We should not confuse the process or mechanism with its results or outcomes.
The process or mechanism, such as natural selection, could be lawlike, although varying
in results if its initial conditions were varied (see Carrier 1995 for a similar view).
Although natural selection might produce alien-looking life forms on a planet that has
antecedent conditions different from ours, this difference in results does not provide
reasons to doubt the putative lawlike status of natural selection itself. Sensitivity or
variance of results or outcomes to initial conditions does not imply much about the
lawlike nature of the process or mechanism that produced the results. In other words,
one should not confuse contingent evolutionary history as a result with the putative
lawlike mechanisms or processes responsible for this history (Hempel 1965, 370, Ruse
1973, 211-212).
Beatty’s thesis was directed against the existence of “distinctively” biological
laws. A low degree of trajectory stability is not at all distinctive of biological
phenomena, processes, mechanisms, or systems. The same goes for the regularities that
describe the behavior or dynamics displayed by phenomena, mechanisms, processes, or
systems. There are non-biological systems in physics, economy, engineering, and
meteorology that exhibit low degrees of trajectory stability, such as chaotic systems in
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general. Thus, ESCT1 might fail to meet its objective if it does not target anything
distinctively special or unique about the status of biological regularities.
One reason why Beatty believes that biological regularities, in contrast to physical
ones, are strongly contingent in the sense of displaying trajectory instability is his idea
that evolution can lead to different outcomes from the same or similar starting points,
given the same or similar selection pressures, since natural selection often selects from a
set of trait variants that are similar in fitness, but different in realization. Thus, even from
the same selection pressures, similar or identical adaptations need not follow, even given
similar organisms in similar environments. However, this argument is based on a
concept of multiple realizability that rests on unreliable intuitions concerning what
counts as a different realization of the same property.
The traditional explanation for Bergmann’s rule is that the larger body size of an
endothermic animal is an adaptation to cold climates, which results from the fact that the
ratio of “surface area to volume” is smaller in animals with a larger body size than in
animals with a smaller body size. The reason is that, when body size becomes larger, the
surface area of the body increases as the square of the mass, whereas its volume
increases as the cubic of the mass. This allows larger-sized endothermic animals to
conserve their metabolized heat more effectively, because the heat is dissipated through
the surface area in cold environments. Accordingly, if the temperature of an environment
(during the cold season) is a critical factor for an organism’s survival, then selection
should generally and even universally favor and select for different body sizes — and
this is seen in the pattern of Bergmann’s rule, in which the members of a species of
endothermic animals in cold climates or at higher latitudes have larger body sizes than
their relatives in warmer climates or at lower latitudes.
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There are other means for reducing the dissipation of heat in cold climates.
Thicker and heavier fur or plumage and modifications of animals’ blood circulation
might have effects similar to heat dissipation as the increase in body size (Scholander
1955, Irving 1959). These enhance an animal’s insulation without affecting its surface
area to volume ratio or body size. This suggests that there are functionally equivalent,
but physically different means or realizations for achieving the same level of fitness in
cold climates. Hence, the “dissipation of heat” appears to be a multiply realizable
property. Had we “replayed life’s tape,” the latter traits might have been selected as
adaptations to cold climates instead of differences in body size. Thus, it is not necessary
for Bergmann’s rule to be valid as a result of evolutionary history, because there are
alternative trait variants or multiple realizations of the dissipation of heat that could have
led to the same result or effect and which do not affect the ratio of “surface area to
volume.” Thus, given the multiple realizability thesis of biological properties, the
regularity described by Bergmann’s rule seems to lack trajectory stability.
However, the claim that there are or could be different means for reducing heat
dissipation in cold climates is not valid. It is true that a larger body size results in smaller
“surface area to volume ratio” and that modifications of animals’ fur or plumage and
changes in blood circulation result in better insulation. But interestingly, there are not
many other realizers of the dissipation of heat. Thus, the realizers of the “dissipation of
heat” are not diverse and heterogeneous as far as different mechanisms for reducing heat
dissipation in cold climates are concerned. Furthermore, the idea of larger body size
reducing heat dissipation in cold climates is not accurate, since differences in body size
do not have an effect on an animal’s heat dissipation per se. Rather, body size functions
only via the reduction of the “surface area to mass ratio,” which has the effect of a
relatively smaller amount of metabolized heat being dissipated through the surface area
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of larger-sized animals than is dissipated in similar animals with smaller body sizes. The
same applies to such behavioral adaptations as dwelling underground and basking in the
sun, both of which may affect an animal’s fitness in cold climates, but which
nevertheless do not affect its heat dissipation per se. In fact, it could be argued that there
is only one or a few general mechanisms that affect heat dissipation per se in animals,
namely, such changes as modifications of animals’ fur or plumage and changes in blood
circulation that result in changes in the animals’ insulation.
It is thus debatable whether the “dissipation of heat” is a multiply realizable
biological property. Although larger body size may have positive effects on the fitness
of organisms in cold climates, larger body size is not a realization of lesser “dissipation
of heat,” because it does not affect dissipation of heat as do modifications of animals’ fur
or plumage and changes in blood circulation, which result in better insulation. The
former and the latter proceed via different mechanisms to different properties, effects, or
functions; thus, they are not “trait variants that are similar in fitness but differ in
realization.”
Conversely, the presupposition that biological properties are always or often
diverse and heterogeneous in realization can be challenged (Bechtel & Mundale 1999,
Batterman 2000, Shapiro 2000, Raerinne & Eronen 2012). The common theme in these
papers is that when the cases of multiple realizations are scrutinized, it appears that the
realizations of higher-level properties are not as heterogeneous as many have supposed.
Differences that exist between realizations turn out to be unimportant, while explanatory
or mechanistically important things are shared by realizations. A case of this kind was
already discussed above: modifications of animals’ fur or plumage and changes in blood
circulation have an effect on organisms’ heat dissipation. However, these different
realizations of “dissipation of heat” proceed via the same mechanism, namely, by
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improving the insulation of organisms, which suggests that the realizations share
functionally and mechanistically important things. Thus, they are not different
realizations of “dissipation of heat,” but mechanistically and causally similar, from
which it follows that they are not trait variants similar in fitness, but differing in
realization.
The above suggests that one of Beatty’s main reasons for the presence of
trajectory instability in biology can be questioned, namely, the idea that selection selects
from a set of trait variants that are similar in fitness, but differ in realization. If the traits
with similar effects are not so different in terms of the mechanisms that produce the
effects and/or if the set of different realizations is not diverse and heterogeneous after
all, then it follows – contra Beatty – that from the same selection pressures the same or
similar adaptations follow, especially given similar organisms in similar environments.
In other words, the evolution of Bergmann’s rule is not perhaps so easily switched to
another track, but the rule could have some inevitability and necessity after all in the
sense of not displaying trajectory instability.
Moreover, there are evolutionary forces or factors, such as generative
entrenchment (Schank & Wimsatt 1987, Wimsatt & Schank 1988, Wimsatt 1999, 2001),
that foster trajectory stability, despite the fact that a regularity, trait, characteristic, or
mechanism originally had a low degree of trajectory stability and/or constancy.
Although on this planet it is a true generalization that hereditary information is
carried by nucleic acids, this seems to be “an accident” – a conditional fact of our
planet’s and life’s history. Had the historical conditions or other initial background
conditions been different, then other materials could have evolved to do the same thing.2
2 For the sake of argument, I presume that multiple realizability holds in the case of heredity information.
Note that the argument presented here for the stability of generatively entrenched traits is independent of
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Thus, the ubiquity of the genetic code is an accident whose evolution could have been
disturbed or switched to another track. Yet in the course of the history of life on earth,
this code has become so generatively entrenched as to be nearly impossible to change
because so many other things depend on it, i.e., it now represents a functional necessity
or a “frozen accident.” A similar argument can be made in the case of the generative
entrenchment of endothermicity and the holding of Bergmann’s rule. Much of what was
just said above is probably also true of many other basic biological mechanisms, the
presence and functioning of many ancestral or primitive characters or traits,
developmental constraints, and so on, such as Mendel’s rules or mitosis and meiosis,
diplobiontic life cycle, cell respiration, the Krebs cycle, and the use of ATP in metabolic
processes, eukaryoticity, homeobox genes, the mechanism of photosynthesis, bilateral
symmetry, dorsal-ventral polarity, initiator and terminator codons, DNA ligase and DNA
methylation, morphogenesis and organonesis in general, and, for instance, the specific
mechanisms of apoptosis in ontogeny.
In addition to multiple realizability of biological properties, Beatty mentions
mutation and drift as the second source for biological regularities’ strong contingency in
the sense of ESCT1. Generatively entrenched traits are functional necessities for
organisms’ development, survival, and/or reproduction, which resist evolutionary
change, even if we suppose that evolution proceeds via other “forces” than selection,
such as mutation and drift. For a trait to become generatively entrenched, it does not
matter how or under what evolutionary forces it has evolved or will evolve; what matters
the truth of the multiple realizability thesis in the sense that, even if the thesis is presumed to be true,
generatively entrenched traits are still capable of displaying trajectory stability. At the same time, if the
multiple realizability thesis can be questioned, then generatively entrenched traits become even more
stable in the sense of displaying high degrees of trajectory stability, because the functional necessities or
generatively entrenched traits become realization “necessities” as well.
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is how many other traits come to depend on the functioning or development of the trait
in question. A change in a deeply generatively entrenched trait has serious, deleterious
effects on other traits in the development or functioning of an organism. This is why
deeply generatively entrenched traits are preserved and resist evolutionary change,
regardless of whether the trait was a consequence of drift, mutation, or selection.
Generative entrenchment thus provides a counterexample to the second of Beatty’s
reasons for why trajectory instability is present in biology or evolution.
The points just outlined question the validity or applicability of the trajectory
stability version ESCT1 of strong contingency as an argument against biological laws.3
We obtain a more general, substantial, or applicable version of the thesis if we focus on
the lack of stability of biological regularities in the sense of constancy ESCT2 rather
than focusing on trajectory stability ESCT1. Regularities with low and moderate degrees
of constancy stability hold in special or limited background conditions, whereas laws are
3 Beatty developed his evolutionary contingency thesis as an elaboration of Gould’s ideas of the
contingency of macroevolution (for an excellent review of Gould’s ideas, see Powell 2012). Beatty
(2006) develops Gould’s evolutionary contingency further, distinguishing between the unpredictability
notion and the causal dependency notion of evolutionary contingency. According to the unpredictability
notion, evolutionary contingency means that unpredictable outcomes arise from the same or
indistinguishable prior states. According to the causal dependency notion, evolutionary contingency
means that a particular evolutionary outcome depends on which particular states preceded it. Both notions
capture some elements of my trajectory stability version of strong contingency, yet neither is identical to
it. I will not discuss Beatty’s two notions here; Powell (2009, 2012) and Turner (2010) already provide
criticisms of them as meaningful interpretations of evolutionary contingency in the context of
macroevolution. It suffices to point out that Powell’s (2009, 2012) idea of radical contingency, a notion
that is meant both to unify the two Beattyan contingency concepts above and to give them a fair
Gouldian reading, comes very close to my meaning of trajectory stability.
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traditionally understood to be very stable or maximally stable regularities that hold in
most, if not in all, possible background conditions.
Low degrees of constancy stability deprive regularities of their necessity and
nomic force. With low degrees of constancy stability goes the support of counterfactuals,
the distinctive property of laws, according to which laws govern not just what actually
happens, but what would have happened under various background conditions that did
not actually happen. Nor are unstable regularities projectable or extrapolable in the sense
that laws are sometimes thought to be.
One reason why this latter form of stability deprives regularities of their lawlike
status is that with degrees of constancy other than maximal or very high degrees come
restrictions on where and under what background conditions the regularity holds, i.e.,
such regularities are accidentally true rather than lawlike. Another reason is that
biological regularities in general seem to have low or moderate degrees of constancy. By
contrast, a maximal or very high degree of constancy is usually attributed to physical
regularities.
Consequently, Beatty’s strong contingency, when interpreted along the lines of
ESCT2, represents a problem for biological regularities’ lawlikeness, because laws are
commonly understood to be maximally or highly stable regularities in the sense of
constancy. And if biological regularities display low or moderate degrees of this form of
stability, then they are accidental regularities.
Finally, when ESCT1 has force as an argument against biological laws, it rides
piggyback on ESCT2. A low degree of trajectory stability of a regularity is a
consequence of its low degree of constancy. Similarly, a high degree of constancy
results in a high degree of trajectory stability. Regularities that hold in many or all of the
different background conditions, i.e., the regularities that show maximal or high degrees
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of constancy stability, are the ones that quite likely would have held had the initial or
past historical conditions been different, i.e., the regularities also have high degrees of
trajectory stability. Thus, if evolution or evolutionary history is law-governed in the
sense of displaying high degrees of constancy, then we should expect it also to show a
high degree of trajectory stability.
However, regularities displaying high degrees of trajectory stability are not
necessarily regularities that display high degrees of constancy, that is, the two forms of
stability de-couple (see Figure 1). Generative entrenchment and lack of multiple
realizability of biological properties can foster trajectory stability despite the lack of a
high degree of constancy. Here we have a mechanism begetting stability to evolution
without lawlike regularities or mechanisms in the background.4 Thus, a lack of high
degree of trajectory stability is indicative of non-lawlikeness of biological regularities
(this is what Beatty got right with his ESCT1), because a low degree of trajectory
stability follows from a low degree of constancy. But a high degree of trajectory stability
is not necessarily indicative of the lawlikeness of biological regularities by contrast to
what Beatty and his commentators believe, because this does not imply a high degree of
constancy of biological regularities, given that functional necessities and realization
necessities can resist evolutionary change.
4 Generative entrenchment, such as a developmental constraint, is interpreted even by the proponents of
the evolutionary contingency thesis as a mechanism begetting stability to biological systems (Powell
2012, 357-358; see also Wimsatt 1999, 139-152).
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5 A Counterexample to the Evolutionary Contingency Thesis
According to the competitive exclusion principle (CEP), species with similar niches, not
to mention those with identical niches, cannot stably coexist in the same habitat for long
periods of time: n number of sympatric competing species cannot coexist in equilibrium
indefinitely on fewer than n number of common limiting resources. The magnitude or
intensity of competition between ecologically similar species is thought to be
proportional to the degree of overlap in their niches or use of resources. Species that
stably coexist do so because there are “important differences” in their niches or in their
use of resources.
(-)
Deg
ree
of
stab
ilit
y (
+)
Fig 1. Connections between constancy stability (solid arrows), trajectory stability (broken
arrows), generative entrenchment (GE), and multiple realizability (MR).
The presence of GE
and/or the lack of
MR
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I will argue that CEP is not riddled with genuine exceptions, only apparent ones.
This suggests that the principle might be universally true. I will also show how
ecologists recognize two kinds of coexistence mechanisms capable of producing
apparent exceptions to CEP. The two mechanisms specify the relevant background
conditions under which the principle does not necessarily hold. Moreover, I argue that
CEP is a strict law rather than a ceteris paribus law, because the law can be strictly
complemented or its proviso clause can be eliminated. Furthermore, I argue that CEP is
distinctively biological or autonomous as a law, because ecologists are capable of
accounting for the exceptions to CEP in terms of proprietary or distinctively ecological
terminology rather than using the mechanisms, causes, and so on of a lower-level
science to correct the principle. The competitive exclusion principle is thus presented as
a counterexample to Beatty’s thesis.5
There are many “exceptions” to CEP. For instance, Hutchinson (1961) described a
situation in which there are many sympatric phytoplankton species living in bodies of
water, despite the fact that the common limiting resources of species are both scarce and
fewer in number than the number of coexisting species. In such a situation, one would
expect that the competitive dominant species would drive other species into local
extinction by outcompeting competitively less superior species from their common
habitat.
When faced with a situation that seems to be an exception to CEP, for instance, a
guild of birds of prey that hunt for the same food, such as the voles of the genus
Microtus, ecologists make a closer investigation of the situation and the ecology of the
species. Often, competing species are found to be less similar in their niches or use of
5 The strict lawlike nature of CEP and its “same-level” explanations to exceptions are discussed by the
author in detail in two other articles (under review).
21
resources than it first appears (Heatwole & Davis 1965). Thus, what seemed to be a
genuine exception to the principle is found to be an apparent exception, since the
principle does not apply to cases where there are “important” differences between the
niches of competing species.
An apparent exception is a prima facie exception to a generalization, which is
found to be an instance to which the generalization does not apply. An instance in which
a generalization does not hold is not an instance that could show that the generalization
is false. Genuine exceptions are instances in which the generalization applies or holds,
and, as exceptions, they represent falsifying or disconfirming instances of the
generalization.
What makes an exception to a generalization apparent rather than genuine? Many
philosophers and scientists maintain that if an exception can be explained independently
and in a non-ad-hoc way, then the exception is apparent, not genuine (Fodor 1991,
Pietroski & Rey 1995, Carrier 1998).
There are background conditions under which CEP is not expected to hold,
producing apparent exceptions to the principle. For instance, environmental
heterogeneity can alter the competitive rankings between the species so that the species
do not reach their competitive equilibrium, namely, the local extinction of competitively
inferior species by competitively superior species from their common habitat. This was
the explanation that Hutchinson (1961) gave for his paradox of the plankton discussed
above. In high latitudes, for instance, there are temporal variations of common limiting
resources of many species and guilds: nesting sites, amount of sunlight, minerals,
nutrients, etc. Roughly speaking, if temporal variation of resources is faster – yet not too
fast – than the time it takes for competitive systems to reach their equilibriums, then the
temporal variation of resources can prevent competitive systems from reaching their
22
competitive equilibriums. Thus, one (or more) species is locally not outcompeted and
displaced by another, ecologically more efficient species.
Spatial environmental heterogeneity works differently. There might be poor-
quality habitat patches that provide “refugee camps” for a competitively inferior species
and from which the species recolonizes the high-quality patches from which it was
previously displaced by a competitively superior species. If vagility or the intrinsic
growth rate of the competitively inferior species is sufficiently high, and/or it has a
broader environmental tolerance than the competitively superior species, then this can
lead to the coexistence of both species, despite the fact that one is superior insofar as
competition and the use of resources are concerned. Therefore, different instances of
environmental heterogeneity have a common result: they create apparent exceptions to
CEP and thus make coexistence possible, or at least they considerably slow down the
rate at which competitive exclusion occurs.
In rocky intertidal marine habitats, the top predators (e.g., starfish and gastropod
species of the genera Thais and Pisaster) prey on different consumer species (the
different mussels, barnacles, and other species of many genera). In such habitats, there is
severe and intense competition among the consumer species for living space, which is
the major common limiting resource and a critical one. The consumer species compete
for living space, because, as sessile organisms, they are permanently attached to a solid
substrate at their mature stage. There is surprisingly high alpha or local diversity of the
consumer species in many such habitats. It is surprisingly high, because, on the basis of
CEP, one would expect only a few competing species to be sympatric and coexisting,
given the keen competition for living space.
Paine (1966) discovered experimentally that removing one or more of the top
predator species from such a habitat had the effect of reducing the alpha diversity of the
23
consumer species: without a common predator species, a few and perhaps even one
competitively dominant consumer species came to monopolize the living space by
outcompeting other consumer species. Paine’s explanation was that the top or keystone
predators can mediate the coexistence of their prey and maintain the local diversity of a
community at a high level by keeping the density and abundance of competing prey
populations below a level at which the competition would become so severe as to cause
local extinctions.
These and similar experimental findings and explanations of exceptions to CEP
were later generalized and labeled as the intermediate disturbance rule. According to this
rule, intermediate levels of disturbance, such as predators or pathogens, are capable of
mediating the coexistence of competitor species and thus maintain the local diversity of
a community at a high level. Too small or infrequent disturbances lead to local
extinctions of competitively inferior species by competitively dominant ones (CEP thus
holds), whereas too intense or too frequent disturbances allow for the few species that
are the most stress-tolerant to exclude other species from a habitat (this is another
apparent exception to CEP, but without a high level of local diversity). Intermediate
disturbances have the same effect, for instance, through reduction of population densities
of competitor species, which counter the strong competitive effects between species and
provide apparent exceptions to CEP.
Many other empirical and theoretical studies of various so-called coexistence
mechanisms have understood exceptions to CEP as being apparent, not genuine. I
suggest that these coexistence mechanisms are of two kinds, intervening and
constitutive. Both kinds are used to explain the exceptions to CEP as apparent rather
than genuine. This suggests that ecologists know the relevant background conditions
24
under which the principle is not expected to hold, and they have theoretical resources to
explain away the exceptions to CEP as apparent exceptions.
By a constitutive coexistence mechanism, I mean explanations of exceptions to
CEP that show that a certain system does not realize or constitute a competitive system
despite initial appearances. The coexistence of competitors is possible and explained
because the competitive system is only an apparent, not a genuine one, since there are
important ecological differences between the species that allow for coexistence. An
explanation showing that the niches of competing species are less similar than they
originally appeared to be is an instance of a constitutive coexistence mechanism,
because there are ecological differences between species that promote the coexistence of
apparent competitors (Heatwole & Davis 1965). For n species competing for less than n
number of limiting resources, variance in resource levels, such as an unpredictable
amount of rainfall in arid environments, can itself work as a new resource (Tilman
1986). In this case, we are again dealing with an apparent competitive system, because
the number of species is not less than their common limiting resources. Alternatively,
the situation can be described as one in which there are important ecological differences
between species’ niches, with the effect that some species are more capable of utilizing
variance in resource levels than others.
An intervening coexistence mechanism is one that interferes with a system which
constitutes a real competitive system, but prevents competitive exclusion from
occurring. Such an interference affects the system in such a way that its capacity for
competitive exclusion is not manifested because the triggering conditions of the
system’s capacity is affected or the competitive system is prevented from reaching its
equilibrium. A common predator or parasite on a competitor species can rarefy the
population densities of competing species so that the densities remain within the
25
carrying capacity of their environment (Utida 1953, Slobodkin 1964, Paine 1966). In
such a situation, there is a luxury or hoarding competition between the species: A real
competitive system exists with the capacity for competitive exclusion, although the
capacity does not necessarily manifest itself. The reason is that the common resources of
species are not limiting due to an interference coming from outside the competitive
system. The interference rarefies the populations of competing species so that there is an
abundance of common resources allowing coexistence. Environmental heterogeneity
and intermediate disturbance, discussed above, are other examples of intervening
coexistence mechanisms accounting for apparent exceptions to CEP.
I have no argument that CEP lacks any genuine exceptions. Besides, the issue is
empirical rather than philosophical. However, I do have an argument that shows that the
principle is not riddled with genuine and unsystematic exceptions, but rather has
systematic apparent exceptions, which are explainable by using proprietary and
distinctively ecological terminology. This at least is evidence for the claim that the
principle might be universally true and a strict law.
There is a potentially large and heterogeneous set of interfering conditions, such
as earthquakes, celestial bodies hitting the earth, human interventions, fires, and storms,
that are capable of producing exceptions to CEP. Some theoretical arguments have
shown that interfering conditions do not decrease competitive interactions per se, but
affect only the rate at which competitive exclusion is reached (Chesson & Huntly 1997,
Chesson 2000). Thus, interfering conditions are not sufficient to be considered genuine
exceptions to CEP. This suggests that the principle is valid and perhaps even universally
so, because there is no coexistence without “important” niche differences between
competing species. Let us, however, suppose that this is not the case.
26
Lange (2005) has argued that biologists do not need to be worried about all the
counterfactual background conditions that are physically possible and that would violate
biological laws. This suggests that many of the above kinds of interfering conditions are
outside the domain of counterfactual scenarios about which ecologists should be
worried. However, let us suppose that this, too, is not the case.
Let me mark with an “I” conditions interfering with CEP that are physical,
chemical, geological, and so on in nature, that is, conditions that come from the lower
levels. In fact, it does not matter whether the I’s come from levels higher than biology,
and be cultural, social, economic, and so on in nature. Thus, the I’s are interfering
conditions that appear to be outside the domain of biology. The I’s could have various
kinds of effects on competitive systems and the components of such systems. However,
the I’s do not produce unsystematic genuine exceptions to CEP, but are systematically
explainable as apparent exceptions by means of coexistence and intervening coexistence
mechanisms; and we can provide the explanations in terms of proprietary ecological
terms. Below, I will use a cursive typeface to highlight the explanations of exceptions to
CEP given in distinctively ecological explanantia.
The I’s might affect the limiting resources of species and result in the coexistence
of competitors that would not otherwise have coexisted. The I’s might make certain
limiting resources superabundant (human interventions/geological factors that, for
instance, result in eutrophication of water). This would be explained, however, as a case
in which there is a luxury or hoarding competition, that is, the resources were no longer
limiting. The exception to CEP by the I’s is thus an apparent one. The I’s might
negatively affect population densities of competing species (celestial bodies falling to
earth, earthquakes, etc.) and result in coexistence that would not have happened without
the I’s. This would be a case that could be explained as an apparent exception to the
27
principle: the species are not at their environment’s carrying capacity and/or the
resources are not limiting. The I’s could destroy some of the species from the system
(earthquakes, storms, etc.) and a coexistence of competitors might follow. If, as a
consequence, the number of species in the competitive system is fewer in number than
the common limiting resources of the species, the exception would be explainable: the
competitive system is only an apparent one.
The I’s could affect the variance in resource levels (climatic and weather
conditions, storms, fires, etc.) and result in competitive coexistence that would not have
happened without the I’s. I have already given reasons above how this case can be
explained as an apparent exception to the principle: coexistence happened, for instance,
because the niches of the species are different and/or the variance in resource levels
counts as a new resource itself. The I’s could affect the environment in such a way that
it becomes spatially or temporally heterogeneous (earthquakes, storms, fires, climatic
and weather conditions, human interventions, etc.) and result in coexistence that would
not have happened without the I’s. Again, this can be explained, for instance, by
claiming that the competitive rankings between species change so that a competitive
equilibrium is not reached. Thus, there seem to be good reasons to believe that all sorts
of I’s – lower- or higher-level ones along with those within the domain of biology and
those that are not – can be explained as producing systematic apparent exceptions to
CEP by using proprietary and distinctive ecological terminology.
Proponents of ceteris paribus laws typically hold that the explanations for
exceptions to regularities in the special sciences and the specification of their application
domains come from lower levels of mechanisms and/or are stated in the vocabulary of
the lower-level sciences (Fodor 1991 and Carrier 1998). However, in the case of CEP,
ecologists are capable of explaining exceptions using distinctive and proprietary
28
ecological concepts, such as common predators, pathogens, differences in niches or
resource use, resources not being limiting, and so on, which produce apparent exceptions
to the principle. There is no need to refer to the lower-level sciences when ecologists
employ intervening and constitutive coexistence mechanisms as explanatia to the
exceptions to CEP and specifying under which background conditions the principle
should hold.
The problem of ceteris paribus laws is providing truth conditions to the proviso
clause “when some other unknown background conditions remain absent and/or the
same” that prescribes the domain of a law outside of which the law does not necessarily
hold. If the interfering background conditions were known in advance, there would be
no need for ceteris paribus clauses nor any problem of semantics for ceteris paribus
laws; in other words, there would be only an epistemic problem of finding out what
known background conditions could interfere with a law. And with regard to CEP, there
is an epistemic, not a semantic, problem, because we are familiar with different kinds of
mechanisms producing apparent exceptions to the principle, namely, intervening and
constitutive coexistence mechanisms, as discussed above.
The above points suggest that CEP should be interpreted as a strict law rather than
as a ceteris paribus law. There is no semantic problem of ceteris paribus clauses in the
case of CEP, but rather an epistemic problem of knowing which of the known
coexistence mechanisms are present when there is a prima facie exception to the
principle. The competitive exclusion principle has only the appearance of a ceteris
paribus law, while in fact it is a strict law, because it can be strictly complemented: CEP
holds except when intervening and constitutive coexistence mechanisms are in place.
There is no empirical vacuity, semantic inaccuracy, ad hocness, or open-endness in the
proviso clause of CEP, there is only the epistemic problem of figuring out which of the
29
different instances of known exception-making mechanisms is responsible for
exceptional instances to the principle, not a semantic problem of spelling out in detail
the proviso clause of the principle in terms of ceteris paribus clauses. The proviso clause
of CEP is eliminable by a known, complete, and finite list of external factors capable of
producing apparent exceptions to the principle. Moreover, the conditions under which
CEP holds can be explained by utilizing distinctively biological or ecological
terminology, suggesting that CEP is autonomous or distinctively biological as a law.
The above concludes the argument that CEP is neither weakly nor strongly
contingent in the sense of ESCT2 as a regularity, but rather is a strict and distinctive and
autonomous biological law that might be universally true and holds in all the relevant
background conditions.
Sober (1997) responded to Beatty’s thesis by claiming that Beatty failed to
recognize that laws do not hold outside their domain of application. When this domain is
added to the statement of a law, the result is a statement that is not contingent, but
instead is necessary. The Hardy-Weinberg rule is restricted to situations in which “there
are no evolutionary forces present.” Let “I” denote this applicability clause, which
defines a domain of application. When there are evolutionary forces present, the Hardy-
Weinberg rule does not apply. Let this rule be written as a statement of a universal form,
x(FxGx); the probabilistic formulation would not affect the point being made here.
According to Sober, we get a non-contingent statement if we add the clause “I” as an
antecedent to the statement of the Hardy-Weinberg rule. In other words, the statement “I
(x(FxGx))” as a whole is not contingent, although both of its components “I” and
“x(FxGx)” could be. In this way, the necessity of biological generalizations is
redeemed.
30
Sober’s response could be criticized as being a trivial logical exercise. It is similar
to another reply to Beatty’s thesis: the idea that biological regularities are ceteris paribus
laws (Carrier 1995). However, Sober seemed to distinguish his reply from that of the
ceteris paribus law. In this section, I have shown how Sober’s response can be fleshed
out and how CEP can be necessitated without referring to ceteris paribus clauses, which
many think of as being problematic because of empirical, semantic, and epistemic issues
involved in ceteris paribus clauses.
6 Conclusions
This article has provided several new results insofar as contingency and its implications
for the existence of biological laws are concerned.
Beatty was correct in arguing for the contingency of biological regularities in the
sense that such regularities lack maximal or high degrees of constancy ESCT2, but he
was less correct about trajectory stability aspects of this contingency ESCT1, as far as
the implications of contingency for the existence of lawlikeness of biological regularities
are concerned. ESCT2 provides a more general, applicable, and substantial argument
against biological laws than ESCT1. ESCT2, in contrast to ESCT1, does not presuppose
the truth of the multiple realizability thesis of biological properties. I have suggested
above and many others have argued elsewhere that the truth of this thesis, if not
debatable, is at least such as that its scope is more restricted than was traditionally
supposed. In other words, instead of there being widely heterogeneous realization bases
for biological properties at lower levels, there might in many cases be realization
necessities showing no multiple realizability.
31
Beatty’s other reason why trajectory instability ESCT1 holds for biological
regularities is that drift and mutations affect evolution by making it a contingent
outcome or result. This claim is true to a certain degree. However, there are functional
necessities or generatively entrenched traits that resist evolutionary change. The
existence of generatively entrenched traits along with the questioning of the truth of the
multiple realizability thesis suggest that, for certain traits and features of organisms,
replaying life’s tape might result in a similar evolutionary history, because functional
necessities might be realization necessities as well; that is, evolution and biological
phenomena might display trajectory stability. Finally, it was argued that regularities that
have a high degree of constancy are those that display a high degree of trajectory
stability. Regularities that hold in many or all the different background conditions are the
regularities that would have held had the initial historical background conditions been
different. In other words, a high degree of trajectory stability follows from a high degree
of constancy. As was argued above, however, the converse does not necessarily hold.
Generatively entrenched traits (and realization necessities) are capable of showing high
degrees of trajectory stability despite being instable in the sense of constancy.
There is thus the possibility that evolutionary history might show repeatable and
generalizable regularities and patterns and yet be non-lawlike, in contrast to what Beatty,
some of his commentators, and some paleobiologists have suggested. An example of a
regularity displaying a high or moderate degree of trajectory stability, but with a low
degree of constancy was suggested above, namely, Bergmann’s rule. Other putative
examples of non-lawlike frozen accidents include the Krebs cycle, Mendel’s rules,
hereditary information, and regularities concerning other functional necessities
mentioned above.
32
A counterexample to Beatty’s thesis was also suggested by the competitive
exclusion principle (CEP). CEP is not riddled with genuine exceptions, only apparent
exceptions, and it might even be universally true. Thus, CEP is not weakly contingent.
Ecologists recognize two types of mechanisms, intervening and constitutive, capable of
producing apparent exceptions to the principle. The two mechanisms specify the relevant
background or constancy stability conditions under which the principle does not
necessarily hold. When the two coexistence mechanisms are not in place, the principle
has a maximal or a very high degree of constancy stability. Thus, CEP is not strongly
contingent. Finally, the principle is distinctively biological or autonomous and strict as a
law, because ecologists are capable of explaining away its exceptions and specifying its
domain by using proprietary ecological explanantia.
Of course, the presence of one counterexample does not prove that Beatty’s thesis
does not apply to biology in general, meaning that biology is deficient in laws. There are
other responses to Beatty’s thesis, whereby a putative biological law is presented as a
counterexample to the thesis. Morgan (2009) suggested that the Caspar-Klug theory of
virus structure – a hypothesis concerning the structure and organization of capsids in
viruses – is such a counterexample. There are problems with Morgan’s suggestion. It
could be argued that the example is a biochemical regularity, lacking constancy, but
having a high or moderate degree of trajectory stability, and that its exceptions cannot be
explained in terms of of proprietary or distinctively biological explanantia. This
suggests that Morgan’s example is not distinctively biological or a strict law, in contrast
to the competitive exclusion principle discussed above.
Finally, Mitchell’s (1997, 2000) analysis of the evolutionary contingency thesis is
different from the above analysis. Mitchell treats stability as a monolithic thing, and
consequently thinks that stability has general or uniform implications for the existence of
33
biological laws. I argued that not only are there distinct forms of stability, but also that
distinct forms of stability have different implications for the existence of biological laws.
Acknowledgments The work was supported financially by the Academy of Finland as a
part of the project Causal and Mechanistic Explanations in the Environmental Sciences
(project no. 1258020). I am grateful to the anonymous referees and to the editor of this
journal, Helmut Pulte, who provided helpful comments and suggestions. N. Emrah
Aydinonat, Markus Eronen, Till Grüne-Yanoff, Andrew Hamilton, Tomi Kokkonen,
Jaakko Kuorikoski, Aki Lehtinen, Caterina Marchionni, Uskali Mäki, Anna-Mari
Rusanen, Petri Ylikoski, Tero Ijäs, Petri Turunen, Rami Koskinen, Miles MacLeod, and
Ilkka Pättiniemi all provided helpful comments, discussions, and suggestions on
previous drafts of this paper.
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