On the Dual Nature of Chance
in Evolutionary Biology and
Paleobiology
Gunther J. Eble
SFI WORKING PAPER: 1998-09-085
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SANTA FE INSTITUTE
1
On the dual nature of chance in
evolutionary biology and paleobiology
Gunther J. Eble
Committee on Evolutionary Biology
The University of Chicago
5734 South Ellis Avenue
Chicago IL 60637
In press, Paleobiology 25 (1)
*Present address: Smithsonian Institution, Department of Paleobiology, MRC-121,
Washington, D.C. 20560 and Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501.
Electronic mail: ebleg@nmnh.si.edu, eble@santafe.edu
2
Abstract.- The identification of randomness and nonrandomness is a perennial problem in
evolutionary research. Stochastic thinking in evolutionary biology and paleobiology has
solidified the use of a statistical notion of chance, but the idea of chance in evolutionary
studies goes beyond statistics. A duality arises from the use of a statistical meaning, on the
one hand, and a more strictly evolutionary meaning, on the other. The former implies a
combination of indiscriminate sampling and unpredictability due to multiple causes; the
latter codifies independence from adaptation and the directionality imposed by natural
selection. Often these meanings are kept separate in evolutionary research, used in isolation
according to the empirical situation or the goal of the investigator (recognition of pattern
versus process). I argue that evolutionary studies in general and paleobiological studies in
particular can benefit from the simultaneous application of statistical and evolutionary
notions of chance. Following some background on the notion of chance and its use, I
discuss a series of examples in which insight can be gained by explicit consideration of
both meanings. Thus, typologies of extinction become clearer when phenomena like
wanton extinction are made explicit; exaptive radiations are exposed as an alternative to
adaptive radiations; the possible nonadaptive nature of deterministic chaos becomes
sensible; the nonrandomness of community-assembly is put into question; parallel
taxonomies of sorting rooted in different notions of nonrandomness are suggested as a
means of facilitating understanding of relationships across the hierarchy; developmental
constraints and self-organization are more easily distinguished from selective constraints;
and a new term, “incidentals”, is suggested to refer to both exaptations and nonaptations.
Finally, I point to ways in which the dichotomy between chance and necessity can be
approached in evolutionary theory, by showing that the dual nature of chance in evolution
entails a distinction between functional and structural necessity, and that chance ultimately
becomes a unifying concept for a number of criticisms to neo-Darwinism.
3
Introduction
Stochastic approaches in evolutionary paleobiology are now widely appreciated,
after a controversial beginning (Raup et al. 1973; Van Valen 1973; Kitts 1975; Raup 1975;
Raup et al. 1975; Stanley et al. 1981). Deterministic explanations are still part and parcel of
a search for causes, but consideration of stochastic alternatives has become a desirable
feature of many studies. The view that species in particular and taxa in general may behave
as particles in space and time, and that ensemble properties and nomothetic principles are
worth pursuing through statistical analyses (Raup et al. 1973; Van Valen 1973; Raup and
Gould 1974; Raup 1977a,b; Gould et al. 1977; Gould 1980; Raup and Schopf 1978;
Schopf 1979), has been very influential and encouraged increased rigour in
macroevolutionary studies. Whatever the successes of this approach, it has shaped the way
in which randomness and nonrandomness are perceived in the field by emphasizing the
meaning and importance of null hypotheses in empirical research.
Should rejection of a random null hypothesis exempt the paleontologist from a
concern with issues of chance and luck? Or are the notions of chance and randomness
more subtle than our allegiance to the statistical paradigm would suggest? Similar
questions can be posed to neontologists (say, ecological geneticists or molecular
evolutionists) after rejection of a hypothesis of random drift or selective neutrality. The
centrality of adaptation, the randomness of mutation, and a Darwinian heritage are all
hallmarks of evolutionary studies. They are the very reasons that make the notion of
chance in evolutionary biology and paleobiology more complicated than statistics alone
would suggest.
Currently, two meanings of chance relevant to evolutionary studies exist in relative
isolation, codified in different research traditions. In addition to the established statistical
meaning, associated with sampling error and probabilistic statements (the nuts and bolts of
4
quantitative empirical research), a very peculiar, "evolutionary" meaning of chance exists in
connection with the problem of the causes of variation and their Darwinian interpretation :
"random" is an attribute of phenomena that would appear to occur independently of
adaptation or ultimate utility (Simpson 1953; Mayr 1961; Gould 1982; Ridley 1993;
Marshall 1995; Futuyma 1998). Thus, mutations are random with respect to their effects,
the same being true of exaptation (Gould and Vrba 1982) and relationships across
hierarchical levels (Vrba 1983, 1989; Vrba and Gould 1986), to cite just a few examples.
This duality in meaning has not existed without unease. Comments on the
contingent and problematic status of the chance concept are often made (e.g., Gould 1982;
Vrba and Gould 1986; Ridley 1993). Previous discussions of macroevolutionary theory
(e.g., Schopf 1979; Gould 1982; Gould and Vrba 1982; Vrba and Gould 1986; Williams
1992) have addressed the issue of chance, but have not attempted to resolve the problem of
multiple meanings. This paper explicitly addresses the topic.
Here I use a series of examples across a range of research areas to highlight the fact
that the multiplicity of evolutionary problems requires somewhat different notions of
chance. Randomness in general is ultimately a relative notion (Gigerenzer et al. 1989;
Dembski 1991). While seemingly unparsimonious, a relativistic treatment of chance
reduces the danger of simplistic reliance on deterministic explanations, and enriches the
realm of empirical evolutionary research. After identifying the roots of the current state of
affairs, I point to its resolution by arguing that an evolutionary, as opposed to statistical,
concept of chance has a proper domain of applicability, and that the relationship between
evolutionary and statistical meanings is one of complementarity, not subordination. The
examples are mostly drawn from macroevolutionary topics but also include issues in
ecology and evolutionary genetics. They are meant to indicate the value of the explicit,
simultaneous use of both meanings -- a more complete appreciation for the causes of
evolutionary phenomena.
5
Background
The concept of chance has changed in meaning before attaining its present, standard
statistical manifestation (see Adler 1952; Beatty 1984). Different meanings sometimes overlap
or can be viewed as derivations of other meanings, but it is instructive to identify them
separately since each contains a different emphasis. I introduce four "non-evolutionary"
notions here. Two classical positions are of limited relevance to current statistical thinking: the
view that something is random when it works in an absolutely uncaused fashion, an
indeterministic position at times associated with quantum mechanics and nonequilibrium
thermodynamics; and the view that something is random when it does not happen by virtue of a
deliberate plan, hence its unexpected character. The latter notion was popularized by natural
theology early in the nineteenth century (Paley 1831). Of more relevance here are two other
notions, which together form what is here referred to as the “statistical meaning”: (1) An event
occurs at random because it is unpredictable, due to our ignorance of causes. This "ignorance"
interpretation (dating back to Laplace), is probably the most frequently used. It is usually
associated with the assumption of probabilistic behavior and indiscriminate sampling (viewed
as a property of independent events in nature, not of experimental design; see Sokal and Rohlf
1995:p.68 for a distinction). (2) An event may be said to be random when it is the result of the
confluence of independent causal chains, as in traffic accidents (this "coincidence"
interpretation stems from Aristotle). Notion (1) is more operational, since the suspension of
search for individual causes allows us to rely on probability statements and predictions. Notion
(2) is perhaps more relevant to the study of natural processes, in its allusion to causal
pathways. In effect, these two versions of chance imply and complement each other, and stand
both as renditions of the statistical meaning of chance.
This statistical meaning is an integral part of the analytical arsenal of every evolutionary
biologist and paleobiologist, permeating parametric and nonparametric approaches, including
6
resampling. It constitutes the basis of much of everyday research related to the identification of
patterns (e.g., inhomogeneities in diversity or disparity time series, phylogenetic structure,
deviations from the molecular clock). Alongside such a notion, in effect the received view of
chance in the natural sciences, another meaning is present in evolutionary biology. It dates
back to Darwin and was initially related to the problem of the origin of variability. In modern
form, it is principally through the interpretation of the phenomenon of mutation, expressed in
the dictum "mutation is random", that this meaning of chance enters evolutionary theory (Sober
1984). By extension, its implications are far-reaching, spanning all sorts of evolutionary
studies. As Beatty (1987: p. 231) puts it, "...the notion of chance variation, as originally
conceived by Darwin, continues to play a conceptually very important role in discussions of
evolutionary change." From Darwin on, including recent usage and textbook ratification, the
gist of the evolutionary notion of chance is that events are independent of an organism's need
and of the directionality provided by natural selection in the process of adaptation. While for
mutations evolutionary randomness is clear (the alternatives are Lamarckism and
predetermination), for most other evolutionary phenomena this is not trivial, given the
directionality implicit in the expectation of selection operating as a constructive, creative force
towards adaptation over many generations (Mayr 1963; Gould 1982). "Directionality" is here
materialistic and has no implication of teleology. Patterns are evolutionarily random whenever
selection and adaptation are not directly involved.
The original development of this notion of chance by Darwin is quite informative. The
emerging fields of probability theory and statistics had little influence (Beatty 1987). Further,
there is enough variability in usage in Darwin's writings to trace his evolutionary notion of
chance to an interplay of ignorance (the problem of prediction), natural theology (the problem
of design) and coincidence (the problem of ultimate sources) interpretations of chance (Beatty
1984; Hodge 1987). This does not detract from the uniqueness of the evolutionary notion -- it
is simply not reducible to the statistical notion because the latter is silent about adaptation.
7
The composite nature of the evolutionary notion of chance does allow clarification
of its roots, as well as its implications. First, Darwin always acknowledged that variation,
while not directed, is not uncaused, such that natural laws of variation might be
discoverable. This is important, for it implicitly allows for biases in the production of
variation. Second, there is a clear relationship between evolutionary chance and chance in
natural theology. The absence of a design, plan or purpose is the hallmark of chance as
conceived by both Darwin and the natural theologians. In Darwin's materialistic terms, one
would be talking about the independence of generative causes (of variation) from
adaptation. In the evolutionary paradigm, function and apparent purposefulness continued
to be the primary features demanding explanation in the organic world. Whether adaptation
should have the status of default expectation is an issue in itself (Gould 1989a; Antonovics
and van Tienderen 1991), but adaptation is a central concept in evolutionary theory both
among supporters and critics.
On the Current Use of Chance in Evolutionary Biology
In light of the distinctions discussed above, the contrast between evolutionary and
statistical meanings as currently used is summarized diagrammatically in Figure 1. Each
usage brings its own subtleties, however. Noticeable are the problems of the nature of
variation and of the meaning of null hypotheses, associated with the evolutionary notion
and the statistical notion, respectively.
Evolutionary Usage. - The eclipse of developmental biology through much of the
history of Darwinism and neo-Darwinism was accompanied by a decline of interest in the
sources of variation (Kauffman 1993; Goodwin 1994). The underemphasis on the problem
of the origin of variation in the theory of natural selection has led to some confusion over
the use of the concepts of chance and random variation. Many neo-Darwinists tend to
assume that intrinsic constraints on variability are unimportant, if not absent for all practical
purposes, and accordingly discuss the fate of variation as a result of extrinsic, selective
8
forces (e.g., Charlesworth et al. 1982). Perpetuation of this line of discourse has led
sometimes to a conflation of neo-Darwinism with the assumption of absolutely unbounded
variation, by supporters and critics alike. Ho and Saunders (1984a:
Figure 1. Diagrammatic representation of randomness and nonrandomness in statistical
and evolutionary senses. In (a), the distribution is statistically random because the position
of particular points is not predictable (causal chains corresponding to X and Y are
potentially independent). In (b), the distribution is statistically nonrandom because the
position of particular points is statistically predictable (causal chains corresponding to X
and Y are potentially nonindependent). If X and Y are biological variables, the distribution
in (a) would be evolutionarily random if the lack of association could be shown not to be
grounded on, say, conflicting selection pressures. In (b), the association observed would
be evolutionarily nonrandom if it could be shown to be an adaptation. If the association has
causes other than natural selection (e.g., intrinsic correlations of growth), then it is
evolutionarily random despite its statistical nonrandomness.
(B)
(A)
x
xx
Y
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Y
x
x
x
x x
x x
X
x
x
x
x x
x
x
x
x x
x x x
x
x xx x
x
x xx
x x
x x x
xxx
X
p.5), for example, who criticize neo-Darwinism from a structuralist perspective, have
asserted: " The neo-Darwinian concept of random variation carries with it the major fallacy
that everything conceivable is possible." The same ambiguous interpretation of the
evolutionary notion of chance is present in the writings of scientists with similar
backgrounds (e.g., Schoffeniels 1976; Fox 1984; Matsuno 1984; Wicken 1984; Lima-deFaria 1988), and even in the mainstream of evolutionary theorizing (e.g., Rosen 1982;
Brundin 1986). This situation is in part a result of the sometimes exaggerated neo-
9
Darwinian emphasis on variability as a given, as indicated. But it also reveals the tacit
assumption that the "standard", statistical notion of chance is our only guide to pattern in
nature. A more flexible approach is needed, as discussed below.
Criticisms of neo-Darwinism that attempt to refute the notion of chance in evolution
altogether are misdirected. The evolutionary notion of chance is not in the realm of neoDarwinism alone (notwithstanding its use ever since Darwin), but of evolutionary biology
as a whole (which is not exclusively neo-Darwinian). As long as biases in the production
of variation are acknowledged, this notion can continue to be used fruitfully, for its ability
to underscore the centrality of the debate over the importance of adaptation. Consider for
example Wright (1967: p.117): " The meaning of 'random' ... is that the variations are, as a
group, not correlated with the course subsequently taken by evolution (which is determined
by selection). The variations are, of course, severely limited in kind by the accumulated
results of past evolution." Or Gould (1982: p.386): "By 'random' in this context,
evolutionists mean only that variation is not inherently directed towards adaptation, not that
all mutational changes are equally likely." It is important to note that this evolutionary
meaning is acknowledged and discussed in other academic circles as well (e.g., Sober
1984; Popper 1992).
Statistical Usage. - The statistical meaning of chance usually applies to the
identification, as opposed to explanation, of pattern over evolutionary time. Beatty (1984,
1992) gives a complete account of the issue by reference to population genetics and random
drift. In the search for patterns, it is a statistical notion of chance, associated with the null
hypothesis of indiscriminate sampling (under strict independence or Markovian semiindependence of events), that underlies accounts of drift, and for that matter, a gamut of
other evolutionary and biological problems in ecology, macroevolution, paleobiology,
systematics and much of experimental biology.
This statistical notion of chance is routinely used in most of evolutionary biology,
but this is not equivalent to the use of null models, for null models are simplified
10
hypotheses that do not incorporate causes of special interest and are used to test a particular
interpretation of putative orderliness in the empirical data (see Wimsatt 1987). As such,
null models can sometimes be deterministic. For example, nonstochastic exponential and
logistic growth models have been successfully tested against diversity data (Sepkoski 1991;
Eble 1998a). Of relevance here are stochastic null hypotheses about the behaviour of a set
of data, that invariably rely on a notion of randomness akin to the ignorance interpretation
of chance. This means that objective predictions in terms of probability or probability
distributions (e.g., normal, binomial, exponential) are possible (see Raup 1977, 1991), even
if proximal causes are not known and the shapes of probability distributions have uncertain
meaning.
Science as hypothesis testing is not immune to methodological criticism (e.g., Van
Valen 1976, 1985). It also faces the fact that testing protocols are inevitably dependent on
scales of perception -- of researchers and evolutionary units (Lewontin 1966; Raup and
Schopf 1978; Schopf 1979; Wimsatt 1980; Dembski 1991). This is especially relevant in
paleontology: a nonrandom pattern on a microevolutionary scale (e.g., selection within a
species) may be inconsequential in macroevolution, when clades behave as particles (e.g.,
clade drift).
Caveats aside, a common goal in evolutionary research is to show that an apparent
pattern, which is often intuitively obvious, is indeed concrete beyond an expectation based
on some theory (see Kitts 1975; Raup et al. 1975). If a random null hypothesis is not
rejected, it is the assumption of a specific pattern that is challenged, not the possibility that
some pattern may indeed be present. Parsimony leads us to acknowledge randomness as
conceivable, nothing more.
On the other hand, the possibility remains that support of random models may be
really telling us something about the way nature is organized (or disorganized). Stochastic
models commonly applied in paleontology, for example, incorporate certain constraints and
assumptions like stasis and constancy of speciation and extinction probabilities that are
11
themselves indicative of a particular kind of pattern (e.g., Raup 1977). Neutral models of
molecular evolution, if found to fit the data, naturally lead to claims that selective neutrality
is the "cause" underlying the behaviour of the system. Random models in evolutionary
developmental biology, like Kauffman's (1989, 1993) "genomic regulatory networks", are
constructed under the constraint that N genes are regulated by K other genes, entailing a
series of self-organized "generic properties" to the modelled systems. The important point
is that the use of a probability-based, statistical notion of chance can lead to perceptions of
pattern whether or not the random null hypothesis is rejected.
The Simultaneous Use of Both Meanings of Chance: Examples
As implied above, in evolutionary studies it is almost always true that the
evolutionary meaning of chance is methodologically isolated in issues surrounding process,
while pattern-oriented research goes on as usual in its use of a statistical meaning. The
explicit analysis of empirical situations using both meanings, regardless of whether one is
primarily dealing with pattern or process, is a possibility rarely appreciated. Rather than
creating confusion, it may actually enrich understanding. The following examples (see
Table 1) are designed to help put this claim in focus. Unless otherwise noted, they refer to
only one focal level in the hierarchy.
(1) Extinction Studies. - Raup (1991) categorized three different types of
extinction: (1) random extinction in a statistical sense, with no selectivity
(nonconstructive); (2) selective extinction in a Darwinian (constructive) sense, with
survival of the better adapted individuals; and (3) wanton extinction, selective extinction
independent of adaptation in normal times (nonconstructive). These alternatives are
necessary to avoid tautological formulations of the extinction problem (cf. Schopf 1979).
In terms of a non-evolutionary notion of chance, the main distinction here is between
complete randomness (1) and selectivity in general (2, 3). By contrast, an evolutionary
notion of chance clearly separates selective Darwinian extinction (2) from completely
12
random extinction and wanton extinction (1, 3), i.e., constructive from nonconstructive
extinction. Wanton extinction is, in the important evolutionary sense, random (Table 1).
Pronounced physical perturbation is outside the experience of organisms and their
Table 1. Examples of simultaneous use of statistical and evolutionary meanings of chance
in evolutionary biology. Each evolutionary phenomenon may be random or not under both
meanings. Question marks indicate dependence on particular empirical situations. See text
for discussion.
13
Evolutionary
phenomenon
Chance concept
Statistically
random
Evolutionarily
random
Darwinian
extinction
no
no
Wanton
extinction
no
yes
Onshore ordinal
origination bias
no
?
Evolutionary
radiations
no
?
Deterministic
chaos
no
?
Community-level
properties
no
?
Incidental effect
no
yes
Clade selection
no
no
Sorting
?
?
Genetic
drift
yes
yes
Fluctuation in
selection pressure
yes
?
Developmental
constraint
no
yes
Exaptation/
non-aptation
?
yes
Self-organization
no
yes
adaptations to local environments during background times, and in that sense is
unpredictable (Jablonski 1989). Whatever preferential survival occurs through wanton
extinction, it occurs because of fortunate cooptations of organismic features that happened
to be advantageous in the extremely perturbed environments thought to be associated with
14
extinction events in the fossil record. No direct adaptation is involved. While differential
higher-level adaptation is always a plausible explanation for extinction selectivity, the dual
meaning of chance exposes wanton extinction as an equally plausible alternative. This
duality might facilitate refinement of other typologies of extinction as well, such as when
the sources of selectivity or hierarchical lines of causation are specified.
(2) Origins of Higher Taxa. - Jablonski and Bottjer (1991) demonstrated the
existence of a statistically nonrandom pattern of higher ordinal origination in onshore
environments, as opposed to offshore ones. The use of a statistical notion of chance here
shows a very intriguing pattern, and opens the way for a challenging question under an
evolutionary notion: is the pattern evolutionarily random (e.g., if driven by environmental
stresses affecting development) or nonrandom (e.g., if there is preferential survival of
novelties onshore)?
Similarly, major evolutionary radiations, such as in the Cambrian and Ordovician,
are definitely nonrandom when compared with background times, but much of the debate in
paleobiology and systematics surrounds the evolutionary randomness (intrinsic constraints)
or nonrandomness (extrinsic, selective constraints) of patterns of radiation. In other words,
the debate is about whether particular radiations are exaptive, based on developmental
flexibility or on the incidental cooptation of a conserved trait that happens to increase the
chances of speciation, or whether they are adaptive, based on true key innovations that
enhance the occupation of empty ecospace (see Eble 1998b). Notice that independence
from adaptation (evolutionary randomness) does not need to entail independence from
fitness: both exaptive and adaptive radiations imply higher emergent fitnesses. It is worth
noting that a number of "adaptive" radiations might well turn out to be exaptive, if this latter
category were more fully incorporated in our interpretive schemes.
(3) Determinism in Ecology and Paleoecology. - Population growth dynamics,
competition, and predator-prey interactions have been increasingly modelled as a chaotic
process, where a simple underlying structure (usually associated with the nature of the
15
equations used to describe the process), leads to the identification of strange attractors that
justify the expression "deterministic chaos" (Schaffer 1985; May 1987). Less attention has
been paid to the evolutionary status of the attractors themselves. To the extent that the
attractors result from the inherent properties of nonlinear dynamics and its equations, and
not because of adaptation, one can say that ecological attractors are random in an
evolutionary sense. However, it has been argued that in some instances chaotic regimes
may be favoured by selection (Ferriere and Fox 1995). In this case, a claim for
evolutionary nonrandomness would be justified. More research is needed on the
relationship between chaotic patterns and evolutionary processes, but it is apparent that the
use of both statistical and evolutionary meanings of chance helps puts into focus the larger
evolutionary implications of chaos.
In turn, community-assembly and community-disassembly (Drake 1991; Pimm
1991; Mikkelson 1993) have been shown to follow certain rules, which are nonrandom
under a statistical notion of chance. The exact nature of these rules is still a matter of
debate, but in several cases they hinge on ecological processes such as competition and
predation. This suggests that they are nonrandom in an evolutionary sense as well, given
the importance that Darwinian mechanisms of adaptation and natural selection are likely to
have. If, however, what is seen as a community is in fact an assemblage of species
behaving individualistically (Underwood 1986; Jablonski and Valentine 1993; Jablonski
and Sepkoski 1996), then the whole problem collapses, and statistical and evolutionary
characterizations in terms of community behavior are not called for. The claim for
generality of paleoecological stasis (e.g., Brett and Baird 1995; Morris et al. 1995), for
example, faces such a challenge, since explanations that rely on nonrandom evolutionary
phenomena, like ecological couplings based on selection, are only meaningful if "static"
assemblages over geological time are more than filtered individualism written large. This is
still a matter of contention, for the extent to which patterns of compositional stability in the
fossil record depart from null expectations has only now begun to be more rigorously
16
explored (Baumiller 1996; Bennington and Bambach 1996; Holland 1996). Claims for
determinism in ecology and paleoecology, and the application of chance notions, must be
tied to the identification of ecological and evolutionary units at the appropriate hierarchical
level.
(4) Hierarchy Theory. - In the context of hierarchy theory, relationships across
levels can become more explicit when both notions of chance are used. To follow up on
the example above, if community-level properties were the blind by-product of processes at
lower levels, one would have evolutionary randomness even in the face of nonrandomness
at lower levels. However, if it could be shown that community-level selection is taking
place (Wilson 1988), those properties would not be random with respect to adaptation at
that level.
In macroevolution, clade sorting, being just differential origination or extinction,
can be random or not in both senses (Table 1). Vrba and Gould (1986) classified
nonrandom causes of sorting in a statistical sense only, arguing that a hierarchical approach
based solely on a statistical notion of chance avoids the relegation of deterministic causes of
phenomena above or below the focal level of putative selection to the black box of
randomness (e.g., species selection above and segregation distortion or substitution bias
below the focal level of populations) . While this stance highlights deterministic controls
underlying multilevel interactions, it may obscure the often incidental nature of such
interactions. Incidental effects (Vrba 1983, 1989) are random in an evolutionary sense, and
this is a dimension that deserves equal consideration in hierarchical studies. From a
mechanistic perspective, underscoring independence from adaptation (Darwinian
indeterminism) or the existence of deterministic controls beyond selection and adaptation
has the same heuristic value -- they are complementary facets of an empirical problem. The
investigation of particular causes behind multilevel processes need not commit to a
statistical notion of nonrandom sorting, if only because it is the relative importance of
hierarchical selection (and thus adaptation) what we want to determine. The simultaneous
17
use of both notions of chance can be instrumental in achieving this objective, by making
more explicit the ways in which selection may or may not be involved in hierarchical
phenomena. This by itself reduces the risk of underemphasis on particular aspects of
biological causality, and enriches the realm of hierarchical explanations.
Thus, when clade sorting is categorized either in terms of incidental effects (Vrba
1983, 1989) or clade selection, a distinction arises, for while neither is statistically random
in the absence of clade drift, the former is evolutionarily random but the latter is not (Table
1). By the same token, in the generative aspect of clade sorting, origination may be
statistically random (no birth bias) or not, but in both instances it will be evolutionarily
random as a reflection of Wright's rule (Wright 1967): origination is blind with respect to
potential long-term directionality supposedly provided by natural selection. In Williams'
(1992) terms, gene pool divergence occurs without regard to its significance for the
persistence of the changed gene pool. Even so, origination may be evolutionarily
nonrandom below the focal level, if its immediate cause happens to be natural selection.
Thus, the scale of the problem at hand may determine what is random or not, and in what
sense. Generally speaking, upward or downward effects that lead to increased fitness at the
focal level will tend to be evolutionarily random, and only emergent features at a given focal
level will be subject to a claim for evolutionary nonrandomness, regardless of how one
defines processes (e.g. species selection) at the focal level. Species selection may be
defined in terms of emergent fitnesses instead of emergent characters (Lloyd and Gould
1993; Grantham 1995), but whenever emergent fitnesses are incidental consequences of
processes at different levels, "selection" will be random with respect to adaptive value at the
focal level. The complexity of multilevel relationships (see Vrba 1989) should benefit from
the mutually informative character of parallel taxonomies of sorting rooted in different
perceptions of chance and, by extension, nonrandomness.
(5) Evolutionary Genetics. - How random is genetic drift? The notion of drift
simultaneously incorporates both meanings of chance. Indiscriminate sampling can arise
18
by either sampling error or selective neutrality (which occurs even in the absence of small
sample sizes). In either case, there is also no role played by adaptation. The evolutionary
definition of chance is tacitly accepted here, which helps explain why genetic drift has
become a paradigm of randomness in evolution. At the limit, "drift would be stochastic and
selection deterministic even if quantum mechanics were someday discarded and replaced by
a deterministic theory" (Sober 1984: p.115).
Another situation in which the use of an evolutionary notion of chance helps in
understanding process is the nature of random fluctuations in selection pressure. By
including fluctuations in systematic pressures under the category of (stochastically) random
fluctuations, Wright (1949 and later; see Beatty 1992) has blurred the distinction between
genetic drift and natural selection. Regardless of how one views this dilemma, random
variation in selective pressures is nonrandom from an evolutionary perspective, since it is
coupled with variation in expected optimal adaptations at the focal level. However,
fluctuations in selective pressures may often be random with respect to higher-level
adaptation, as a result of Wright's rule.
(6) Evolutionary Developmental Biology. - Developmental constraints (biases in
the production of variation caused by inherent properties of the developmental system -- see
Maynard Smith et al. 1985) exist in relative independence from the environment. It is thus
clear how the notion is distinct from selective constraint (stabilizing selection), regardless of
temporal scale and past history: developmental constraints are random in the sense of
prospective independence from adaptation. Putative developmental constraints must be
recognized as instances of either exaptation or nonaptation, following Gould and Vrba's
(1982) terminology. If the resulting structures are functional, they are exaptations; if
nonfunctional, they are nonaptations. In any case, "...all exaptations originate randomly
with respect to their effects" (Gould and Vrba 1982: p.12); the same is true for
nonaptations. To the extent that exaptations and nonaptations are conceptually alike, one is
faced with the possibility of finding a term that can refer to both (see Gould and Vrba
19
[1982: p.12] on the need for terms unburdened by "convictions about the supremacy of
adaptation"). I suggest that, granted the insight of an evolutionary meaning of chance, both
exaptations and nonaptations can be called biological incidentals. The term "incidentals" is
preferable over "fortuities" or "accidents" because it avoids some of their positive and
negative connotations, respectively. An incidental is a feature that appeared for reasons
unrelated to current functionality, and therefore independently of current adaptation,
whether or not a function exists. "Spandrels" (Gould and Lewontin 1979; Gould 1997)
represents a related term, but it refers only to architectural by-products, which could be
viewed as a subset of incidentals. Thus, all developmental constraints are in principle
biological incidentals. This realization may throw light on the supposed "chaos of
constraint terminology" (Antonovics and van Tienderen 1991), by unambiguously
conveying a distinction from selective constraints and reducing the overlap among different
classes of constraint.
Self-organization in ontogeny is another concept that has to be viewed in different
light under an evolutionary meaning of chance. Kauffman (1993, 1995) and Goodwin
(1994) clearly distinguish between self-organization and selection. While both are involved
in generating order, the distinction holds because self-organization implies spontaneous
order, arising from inherent properties conducive to ahistorical universals. The distinction
is made even clearer when one recognizes that self-organized order is logically independent
of adaptation (although it may and probably usually does interact with it), and therefore
random in evolutionary terms. Oxymoronic as it may sound, to speak here of "random
order" (in this case, order from intrinsic properties) is clarifying for it marks a distinction
from "selective order" (externally determined order). The origins of evolutionary order and
the hopes for a marriage of self-organization and selection are thus put in proper
perspective.
20
Although some of these distinctions are self-evident, many are rarely explicitly
recognized, with the potential for misunderstanding of the basic issues that surround
several ongoing controversies in evolutionary biology. Randomness and chance are
relative notions, i.e., they are dependent on the particular meaning used. Different
meanings are context-specific, and failure of specification can easily lead to contextdependent ambiguities. This is also true of fitness (Sober 1984). Beyond the clarification
of individual topics, the simultaneous use of both meanings of chance in a systematic way
could be instrumental in addressing the more general problem of the role of chance in
evolution, with consequences for the structure of evolutionary theory.
Chance and Necessity
More than twenty-five years ago, Jacques Monod (1972) surprised the scientific
community with his treatment of the old Democritan distinction between chance and
necessity: chance was turned into effectively unbounded random variation, necessity into
selection (Fig. 2, topology #1). "Drawn out of the realm of pure chance, the accident enters
into that of necessity, of the most implacable certainties. For [then] natural selection
operates ..." (Monod 1972: p. 118; emphasis added). Although the distinction was clearly
within the Darwinian framework, it was considered simplistic (e.g., Dobzhansky 1974),
and it failed to underscore the difference between evolutionary and non-evolutionary
notions of chance, as discussed in this paper (although the distinction was mentioned for no
effect in passing). Also, it failed to account for the difference between two kinds of
necessity: functional necessity (adaptation) and structural necessity (natural or physical
necessity, but with no implication of absolute inevitability; cf. Popper 1959, Beatty 1995).
Figure 2. A simplified logical scheme for the status of chance in evolutionary theory.
Different positions are represented in a transformational series, going from Monod's
distinction between chance (mutation) and necessity (selection) (topology #1), to the
recognition of two different kinds of necessity, with structural necessity clustered with
statistical chance (topology #2), to a simple dichotomy of functional necessity, representing
the neo-Darwinian core of adaptation and natural selection, as opposed to chance under
both meanings, whereby structural (natural) necessity is rendered random in an
evolutionary sense and chance thus stands as a unifying concept for a variety of
21
developments lying at the periphery of neo-Darwinism (topology #3). See text for further
discussion.
N ECES SIT Y
C H ANC E
TOPOLOGY #1
EVOLUTION
FUN CT ION AL
N ECES SIT Y
ST RU CT UR AL
NE C ESSIT Y
ST AT IST ICAL
CH ANC E
DETERMINISM
ADAPTATION
INDETERMINISM
INCIDENTALS (EXAPTAT ION, NONAPTATION)
TO POLOGY #2
EVOLUTION
FUN CT ION AL
N ECES SIT Y
NEO-DARWINIAN CORE
CH ANC E
NEO-DARWINIAN PERIPHERY
TOPOLOGY # 3
EVOLUTION
Functional necessity is adaptation-oriented. Structural necessity is blind,
spontaneous, automatic. Since Darwin, the essence of the evolutionary perception of
chance versus necessity has been the opposition of adaptation and the rest (Hodge 1987).
It is the evolutionary notion of chance that makes such opposition sensible, by allowing
22
structural necessity to be lumped with statistical chance (Fig. 2, topology #2). This clearly
demarcates the realm of biological incidentals (exaptations, nonaptations) in terms of
nondarwinian evolutionary mechanisms (deterministic or not). At the limit, the distinction
that evolutionists really face is between chance (under both meanings) and functional
necessity (Fig. 2, topology #3). This constitutes a simple way of summarizing the variety
of debates that relate to the adequacy of neo-Darwinism. The emphasis on functional
necessity is the core of neo-Darwinism. Most challenges to this core, which can be said to
lie in the periphery of neo-Darwinism, are inextricably linked to the perception that chance
(under either meaning) is bound to be important in evolution. There is thus a connection,
through the dual nature of chance, among the various criticisms to neo-Darwinism, a
suggestion that deserves further scrutiny.
Needless to say, the dichotomies here discussed are not intended to encapsulate all
aspects of a supposedly binary reality. They are just instruments for further thought. In
particular, the account of chance in this paper is distinct from the so called "evolutionary
contingency thesis" (Beatty 1995). Contingency is a subtle concept, at the border of chance
and necessity (Gould 1989b), and with obvious importance to the understanding of
evolution. Contingency is the confluence of all causal chains; individual causes may at
times reflect functional necessity, but because of Markovian constraint, accidental events
and boundary conditions (Bambach 1996), the historical network of causation may be more
an expression of chance (under both meanings) than functional necessity. Still, due to its
hybrid character, it is unclear where exactly contingency lies in between the nomothetic
(search for laws) and idiographic (uncovering history) halves of evolutionary (paleo)
biology. Its status in evolutionary theory should be further refined.
Conclusions
The use of a statistical notion of chance helps us identify pattern. The use of the
evolutionary notion, in turn, helps us interpret pattern in evolutionary fashion. Evolutionary
phenomena can often be rendered random by application of the latter. It has been seen that
23
this is not unfair. In a paradoxical but meaningful way, "random order" is fully possible in
evolutionary biology. In other words, one has "order for free" (Kauffman 1993, 1995),
without purpose (Goodwin 1994).
Ironically, it is still possible to see such an evolutionary indeterminism being similar
to the more common notion of indeterminism in the nonbiological sciences, that is, chance
as unpredictability. In evolutionary terms, one can say that a phenomenon is unexpected
when it does not occur in conformity with the prediction of orientation (via natural
selection) toward adaptation. In addition, evolutionarily random or nonrandom events can
certainly be redescribed in terms of the confluence or not of independent causal chains. The
intent here, however, is not to deny an in principle reducibility of chance concepts to a
single notion, whatever it may be. It is instead to highlight the empirical value of duality in
meaning, at least for evolutionary purposes. Evolutionary biology and paleobiology deal
with mechanisms that are absent in other natural sciences, and our conceptual schemes
should reflect this fact as much as possible. If selection did not exist, there would be no
need for an evolutionary meaning of chance. Electrons have no fitness. Biological entities
potentially have some fitness, and an important goal of evolutionary research is to
determine the relative importance of selection and adaptation in producing such entities.
Conceptual refinement is a step towards this goal.
Ultimately, one could argue, as Dembski (1991) does, that there is no absolute
notion of randomness: something is random by virtue of deviation from a theoretical
collection of patterns specified beforehand. It is the pattern of interest that will determine
what is random and what is not. This relativistic attitude is more flexible and heuristically
more appropriate in light of the complexity of evolutionary problems.
In evolution, chance means simply that: no adaptation, no selection at the focal level of
causation of evolutionary phenomena. But evolutionary studies also engage in a nonevolutionary, statistical usage. One should view such usages as complementary, and not in
terms of subordination or as pertaining to separate empirical domains. Together they
24
sharpen our thinking, and end up giving more insight and reducing confusion in a field as
conceptually fluid as evolutionary biology. Much as with "survival of the fittest", "survival
of the luckiest" (Kimura 1989) may be reasonable in more than one sense.
Acknowledgements
I thank J. Alroy, R. K. Bambach, D. H. Erwin, M. Foote, J. Huss, D. Jablonski, R.
Lupia, D. McShea, G. Mikkelson, K. Saalfeld, J. J. Sepkoski, Jr., S. Suter, L. Van Valen,
G. P. Wagner and W. Wimsatt for discussion and/or comments on different versions of the
manuscript. I also thank R. B. Sansom and an anonymous reviewer for additional
comments and suggestions. This research benefited from CNPq grant 201542/91-9, a
William Rainey Harper Fellowship (University of Chicago), the Division of Biological
Sciences of the University of Chicago, and postdoctoral fellowships from the Smithsonian
Institution and the Santa Fe Institute.
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