## 1. Punctuated Equilibria



The classic paper on punctuated equilibria (PE) was co-authored by
Niles Eldredge and Stephen Jay Gould (1972), although Eldredge (1971)
was originally responsible for the idea. Their (1972) presentation of
PE draws heavily on themes from Thomas Kuhn's (1962) work, as
they present PE as a new paradigm to rival traditional Darwinian
gradualism (Turner 2011b: chapter 2). Far from challenging the modern
synthesis in that early work, they present PE as a straightforward
consequence of allopatric speciation (speciation driven by isolating
geographic change). Eldredge and Gould reasoned that if speciation
were typically allopatric, then it would tend to happen fairly rapidly
in geological terms, and new species would be geographically distant
from parent species. Consequently, in the fossil record, one should
not expect to see very many cases where a fossil series documents a
clear and gradual transition from one species to another. Eldredge and
Gould also argued that most morphological change occurs in
(geologically) rapid spurts during speciation events and that new
species, once they arise, typically exhibit stasis.



When Eldredge and Gould (1972) introduced punctuated equilibria, the
received view of expected morphological change via evolution was that
of phyletic gradualism. This is a notion that can be traced all the
way from Darwin's *Origin of Species* to, for instance,
Simpson (1970). In accepting phyletic gradualism, paleontologists had
long had to accept that the incompleteness of the fossil record was
significantly responsible for the lack of (ideally expected)
geological documentation of gradual transformation in the majority of
fossilized speciation events. What Eldredge and Gould uniquely
realized, however, was that the expectation of phyletic gradualism
clashed not only with what the fossil record actually showed, but also
with what the dominant model of (allopatric) speciation predicted. So
they developed PE: an alternative view of morphological evolution (and
stasis) intended to synthesize what allopatric speciation projected
with what the fossil record depicted.



In the *Origin of Species*, Darwin had attributed the lack of
fossil series exhibiting gradual morphological transition to the
incompleteness of the fossil record. He thought of speciation and
morphological evolution as gradual processes (though he did
acknowledge variation in evolutionary rates), and thought that they
often happen under the geological radar. As he put it: "Nature
may almost be said to have guarded against the frequent discovery of
her transitional or linking forms" (Darwin 1859 [1964: 292]).
Eldredge and Gould were, in contrast, proposing that the fossil record
is more complete than it seems, and that we should take the rapid
appearance of new species more or less at face value. As they
eventually put it, in designing the theory of punctuated equilibrium
they were "hoping to validate our profession's primary
data as signal rather than void" (Gould & Eldredge 1993:
223). One epistemological issue in play here is whether the absence of
evidence--in this case, the absence of evidence of gradual
evolutionary change--is really evidence of absence (Sober 2009;
Currie & Turner 2017).



The ensuing debate about punctuated equilibria proved to be complex
and contentious. Gould, in particular, made increasingly bold and
controversial claims on behalf of PE during the 1980s, and he tended
to see PE as the opening wedge for a more ambitious critique of the
modern synthesis. Notoriously, Gould also flirted with non-Darwinian
accounts of speciation, such as Richard Goldschmidt's (1940)
idea that new species arise from "hopeful monsters" (Gould
1977). The stasis claim of PE was also controversial, especially since
Gould took it to mean that cumulative, directional natural selection
is a less significant factor in evolution than many had thought. The
reasoning was that if directional natural selection were the dominant
driver of evolutionary change, then you would not expect to see such
clear patterns of stasis in the fossil record. PE elicited a great
deal of opposition from biologists and philosophers who were more
committed to a selectionist picture of evolution (Dawkins 1986;
Dennett 1995; and see Sterelny 2001a for a useful guide to the
controversy).



In the meantime, PE also inspired new empirical research in
paleontology, with scientists doing statistical analysis of large
fossil samples to try to assess the relative importance of stasis vs.
gradual directional change. This research continues to get more
sophisticated, and as it happens, PE has fared reasonably well,
empirically (J. Jackson & Cheetham 1999; Hunt 2007; Uyeda, Hansen,
Arnold, & Pienaar 2011).



One other philosophical issue raised by PE concerns the explanation of
evolutionary stasis (Turner 2017). Eldredge and Gould never really
offered a very clear account of the mechanism(s) that could maintain
stasis, a fact that critics often seized upon (e.g., Coyne &
Charlesworth 1996). They did propose that species might be homeostatic
systems. To make matters worse, there is a mismatch between the fossil
evidence (where stasis is common) and studies of extant populations
that show lots of directional evolutionary change (Price, Grant,
Gibbs, & Boag 1984; Endler 1986; Lenski & Travisano 1994;
Harshman & Hoffmann 2000), a puzzle sometimes called the
"paradox of stasis" (Hendry 2007). Many evolutionary
biologists (e.g. Estes and Arnold 2007) are quick to attribute stasis
at larger scales to well-known processes such as stabilizing selection
and/or habitat tracking, but Kaplan (2009) raises some critical
questions about that move. And Sterelny (2001b) has pointed out that
cases of "coordinated stasis" in the fossil record pose
special explanatory challenges. Coordinated stasis is a pattern where
a whole ecological assemblage seems to persist without change for
millions of years. Stasis, the idea at the heart of PE, remains an
issue of considerable theoretical and empirical interest in
paleontology (Lidgard & Hopkins 2015; Lidgard & Love
2018).


## 2. Species Selection



If punctuated equilibria (PE) was the first innovative idea about
macroevolution to emerge during the 1970s, species selection followed
close on its heels. Just how the two ideas are related is itself a
topic of some philosophical interest (Turner 2010). According to
Eldredge and Gould (1972), driven or *directional* change is
only part of the theoretical story about the evolution of morphology;
it is not, perhaps, even a geologically common narrative. Directional
selection is selection that changes a population, pushing its
distribution of traits from one pattern of trait expression to
another. More typical, according the Eldredge and Gould, are cases of
either *non-directional* change or even *stasis*. Like
directional selection, nondirectional selection also changes a
population, but not towards any particular distribution of
traits--it just moves a population from one pattern to another.
Stabilizing selection is selection that keeps a population the same,
nudging it back to its trait distribution pattern whenever it starts
to stray away. PE as originally formulated favors stabilizing
selection, suggesting that both individuals and species are inherently
stable homeostatic systems--systems that are "amazingly
well-buffered to resist change and maintain stability in the face of
disturbing influences" (Eldredge & Gould 1972: 114). On this
view, whatever the mechanisms that "resist change and maintain
stability" are, they are in conflict with and must be overcome
by directional natural selection in order for natural selection to
successfully push for or "force" directional change.
Whether natural selection is properly thought of as a force is itself
an interesting philosophical issue (Sober 1984; Walsh 2000; Matthen
& Ariew 2002; Walsh, Lewens, & Ariew 2002; Bouchard &
Rosenberg 2004; Stephens 2004; Brandon 2006; Millstein 2006; Filler
2009). Setting that question aside, however, allows us to consider the
suggestion that species might be homeostatic systems which
"resist change by self-regulation" (Eldredge & Gould
1972: 114)--as this is a suggestion rife with implications for
macroevolutionary theory.



At first, the idea that species are homeostatic systems may have been
a way of addressing critics of PE, who wondered what sort of
biological mechanisms could maintain stasis over long stretches of
geological time. However, it is a short step from thinking of species
as homeostatic systems to thinking of them as the sort of thing that
could be a unit of selection. Eldredge and Gould thought that
variation introduced amongst organisms is typically isolated within
the population and constrained by stabilizing selection, unless
geographic separation creates the conditions for rapid evolutionary
change. During allopatric speciation events, the adaptive forces
negate the stabilizing ones effectively enough to disrupt the stasis
of the species and cause the formation of a new lineage. Eventually
though, the stabilizing forces (re)assert themselves upon the emergent
species and (re)establish stasis. Most biologists think of stabilizing
selection as an ordinary microevolutionary process (Schmalhausen
1949); however, the suggestion that a species is a homeostatic system
makes it look like some form of stabilizing selection might be
operating (in some sense) as a mechanism that maintains equilibrium at
the level of populations.



When modeling how these interactions generate macroevolutionary
patterns, PE presents the dominant (i.e., equilibrium) state as one of
population-level morphological stasis--a state in which
speciation attempts often occur but generally fail to take. These
periods of stasis are occasionally interrupted (i.e., punctuated) by
periods of population-level morphological change--periods of
successful speciation. This bifurcated model (depicting states of both
stasis and change) is one in which species behave a little bit like
organisms: speciation begins to look like reproduction, and extinction
begins to look like death. Crucial to this picture is the claim that
once a new species comes into existence, it mostly exhibits stasis.
The analogy is by no means perfect (Havstad 2016 in Other Internet
Resources), but it did open up a new field of theoretical exploration.
What if the differential speciation and extinction of whole lineages
is a lot like the differential survival and reproduction of
individuals in a population? PE may not logically imply species
selection, but it does point in that direction by suggesting that
species resemble organisms in relevant ways--for instance, by
having comparable integration and cohesion, enough for populations to
block directional change in favor of stasis.



Initially, Eldredge and Gould (1972) were quite explicit that they did
not see their model as entailing a new type of selection. They
straightforwardly wrote "we postulate no 'new' type
of selection" (Eldredge & Gould 1972: 112). But in 1975, the
paleontologist and evolutionary theorist Steven M. Stanley dubbed
Eldredge and Gould's population-level selection process a form
of species selection, and by 1977 the inventors of PE concurred. They
wrote that they nonetheless understood species selection as
representing "no more than the operation of natural selection at
higher levels" (Gould & Eldredge 1977: 139), but still, this
interpretation presents a challenge to one typical way of
instantiating the dominant model of natural selection.



Regarding the dominant model: classically and summarily described (see
Godfrey-Smith 2007), evolution occurs when individuals in a population
accumulate a change to the degree that this change becomes a feature
in the population. On this view, natural selection requires variation,
fitness, and heritability (Lewontin 1970; though see Brandon 1990).
When all three of these things coincide within populations, and change
resultantly accumulates, evolution occurs. Proponents of species
selection argue that these three conditions are met at the species
level.



But on a typical way of instantiating this rather abstract model of
natural selection, one can identify what plays the role of the
individuals in this model (organisms) and what plays the role of the
populations (species). This popular view treats organisms as the units
of selection and species as the units of evolution. Species evolve,
owing to the differential survival and reproduction of organisms. This
emphasis on organism-level selection and species-level evolution is
another which dates back to Darwin's *Origin of Species*,
although Darwin also countenanced family selection (Darwin 1859),
sexual selection (Darwin 1871) and group selection (Stauffer 1975).
Some early proponents of the modern synthesis (e.g., Mayr 1942, 2001)
also shared this basic picture in which selection acts on organisms,
while species evolve. To propose that selection occurs at the level of
species as opposed to that of organisms, is to suppose that not only
can organisms act as the units of selection but that species can
too.



So, if the proper way to understand natural selection casts organisms
as the units of selection, and species as the unit of evolution, then
Gould and Eldredge's model of PE challenges that view.
Alternatively, if the proper way to understand natural selection
merely names biological individuals as the units of selection and
biological populations as the unit of evolution, and species can be
biological individuals too, then Gould and Eldredge's model of
PE is not really a challenge to the model of natural selection, it is
just a new instance of it. This is because the notion of
 biological individual (see entry)
 is more expansive than that of biological organism; biological
individuals include but are not limited to biological organisms.
Biofilms, coral reefs, insect colonies, obligate symbionts,
siphonophores, slime molds, and viruses are just a few of the many
biological entities that we might want to consider as individuals,
although they are not organisms (see Buss 1983; J. Wilson 1999; Clarke
2010, 2013; and R.A. Wilson & Barker 2013, among others). If
species are themselves individuals (Ghiselin 1974; Hull 1976), then
the traditional evolutionary theorist may be able to accommodate
species-level selection--in this sense, it is just another flavor
of individual-level selection. Note that is it is very easy for this
dispute to become an instance of what the philosopher John Beatty
(1997) calls a "relative significance" debate--one in
which proponents move from arguing about whether something is possible
(species acting as the unit of selection) to whether that thing is
significant (as opposed to a mere oddity).



Many of these issues--such as how to understand stabilizing
mechanisms and selection, what biological individuals are, whether
species are individuals, and whether species-level selection is
reducible to individual-level selection--are ongoing. In the
decades since Eldredge and Gould first outlined the theory of
punctuated equilibria, various biological theorists have attempted to
provide definitive and irreducible examples of species-level
selection--ones that cannot be recreated at lower levels such as
that of the organism. Species-level traits like range size (where a
species may be wide-ranging even though its organisms are not; see
Jablonski 1987) and rate of speciation (where a species may be slow to
"reproduce" although its organisms are not; see Vrba 1987)
are especially promising candidates. In addition to the issue of
providing definitive examples of irreducible species selection, there
is also the issue of providing a satisfactory definition of the
concept itself. The biologist Elisabeth Vrba (1984) posits that all
proposed definitions of species selection ought to identify causes, be
restricted enough for testing, and employ a concept of selection that
is consistent with how selection is understood to act at other levels
as well. See Vrba (1984), Cracraft (1985), Grantham (1995), Jablonski
(2008), and Turner (2011b: chapters 4 and 5) for further discussion of
species selection.


## 3. Hierarchical Theory



It is worth noting that several discussions of species selection
pre-date Eldredge and Gould's (1972) proposal (e.g., Fisher
1929, Wright 1956; Lewontin 1970). However, it is the ensuing
discussion of stabilizing selection prompted by PE--in
combination with a contemporaneous uptick in discussion of kin
selection, group selection, and altruism (Haldane 1932; Wright 1945;
Wynne-Edwards 1962; Hamilton 1963; Maynard Smith 1964; Boorman &
Levitt 1973; E.O. Wilson 1973; Levin & Kilmer 1974; D.S. Wilson
1975; Wade 1978)--which transforms these macroevolutionary sparks
into an ongoing, hierarchical conflagration (see Gould 1980b, 1982,
2002; Vrba & Eldredge 1984; Cracraft 1985; Eldredge 1985; Grantham
1995; and Eldredge, Pievani, Serrelli, & Temkin 2016, among
others). Indeed, some theorists see the paleobiological revolution as
contributing to a "hierarchical expansion" of evolutionary
theory.



Evolutionary theorists often distinguish between the unit of selection
(Williams 1966; Franklin & Lewontin 1970; Lewontin 1970) and the
unit of evolution (Williams 1966; Hull 1978; Ereshefsky 1991).
Expanding the category of what can act as a unit of selection leads
directly into a corresponding expansion of what can act as a unit of
evolution. This is how a hierarchical theory of evolution is
theoretically generated: when theoreticians consider not just
organisms or individuals as the
 unit of selection (see entry)
 but also species and (kin) groups, and genes (for instance), they are
also and correspondingly considering not just species or populations
as the unit of evolution but also clades, and cultures, and cells or
perhaps organisms (respectively). Each proposed unit of selection
incurs another, higher-level candidate for the unit of evolution.
Hierarchical evolutionary theory is just the name for theories of
evolution that admit of different units of selection and evolution
than that of (merely and respectively) organisms or individuals and
species or populations. Hierarchical theorists see evolutionary
processes (especially selection processes) unfolding simultaneously at
different biological levels and temporal scales.



Hierarchical thinking about evolution is perhaps best illustrated with
an example. It has already been noted that one way of explaining
morphological stasis is stabilizing selection. Stabilizing selection
is a well-understood Darwinian mechanism that operates on organisms in
a population. Here, organisms are the units of selection while species
are the units of evolution. But Sheldon (1996) introduced another
potential explanation of stasis, which he called the "plus
ca change" model. Sheldon observed that in times of
environmental turmoil, ecological generalists usually have lower
extinction risk, while ecological specialists are more prone to
extinction. Perhaps surprisingly, then, a pattern of stasis in the
fossil record might be due to the persistence of ecological
generalists during times of environmental upheaval. This model appeals
to species selection, or to the differential extinction and
persistence of whole species, rather than to stabilizing selection
processes occurring within those species. Hierarchical theorists hold
that evolutionary patterns (like stasis) can be generated and
sustained by processes operating at different biological levels.



Altogether, hierarchical evolutionary theory is potentially inclusive
of macroevolutionary processes (such as species selection), along with
paradigmatic microevolutionary processes (such as organism-level
selection), sub-microevolutionary processes (such as genic selection),
and even super-macroevolutionary processes (like clade selection).
Lewontin (1970) considers the possibility of the following acting as
units of selection: molecules, cell organelles, cells themselves,
gametes, individuals (this is the term he uses), kin groups,
populations, species, communities of species, and phyla. Williams
(1966) and Dawkins (1976) are often credited with developing the
notion of genic selectionism. Debate over genic selectionism--a
theory within which genes are both the units of heredity and the units
of selection--has led to further philosophical discussion
regarding what counts as a replicator as opposed to an interactor
(Hull 1980; Brandon 1988, 1990; Lloyd 1988; Griffiths & Gray 1994,
1997; Sterelny 1996; Sterelny, Smith, & Dickison 1996;
Godfrey-Smith 2000; Griesemer 2000; Nanay 2002). Various biologists
(e.g., Mayr 1963; Gould 1980a; Lewontin 1991) have ardently disputed
the claim that genes act as direct targets of selection. Philosophers
of biology have taken positions both pro (e.g., Sterelny & Kitcher
1988; Waters 1991) and con (e.g., Sober & D.S. Wilson 1994; Lloyd
2005) on the topic of genic selectionism.



Philosophical discussions of genic selection, clade selection, other
units of selection, and the hierarchical theory of evolution are
extensive and ongoing (in addition to works already cited, see Wimsatt
1980; Kitcher, Sterelny, & Waters 1990; Sober 1990; Godfrey-Smith
& Lewontin 1993; Stanford 2001; Glennan 2002; R.A. Wilson 2003;
Walsh 2004; Waters 2005; and Okasha 2006, among others). Another
interesting, relatively recent, and somewhat related development is
the proposal of a further, even-more-extended evolutionary synthesis,
or EES (Jablonka & Lamb 2005; Pigliucci & Muller 2010;
Laland et al. 2014; though see Wray et al. 2014). Proponents of EES
contend that phenomena such as developmental bias (Arthur 2004), niche
construction (Odling-Smee, Laland, and Feldman 2003), phenotypic
plasticity (West-Eberhard 2003), and non-genetic inheritance processes
like social learning and cultural transmission (Hoppitt & Laland
2013) challenge evolutionary theory as standardly understood.
Opponents contend that the now-traditional theoretical framework
constructed via the modern synthesis can accommodate all these
phenomena and more. See Pigliucci and Finkelman (2014) for a
discussion of the philosophical implications of the EES debate.


## 4. Theory Accommodation and Change



One meta-scientific issue raised by these macroevolutionary disputes
is just how much a scientific theory can flex and change before it
either becomes a different theory altogether, or it becomes a bad
scientific theory (via too much of what Popper [1974] would call
*ad hoc* revisionism).



As an illustrative example, consider punctuated equilibrium, which has
certainty been articulated in many and varying ways. The two initial
publications by Eldredge (1971) and Eldredge and Gould (1972) are
undoubtedly the founding ones, but there are other crucial moments in
the development of the theory (e.g., Gould & Eldredge 1977, 1983,
1986, 1993; Eldredge & Gould 1988; Gould 2002). At times the
shifting nature of PE has been a target of criticism (e.g., Levinton
1986; Coyne & Charlesworth 1997; see Eckhardt 1986 for a
particularly entertaining visit from "the shade of Francis
Galton").



Another dynamic theoretical dimension of PE, hierarchical theory, and
the like is how radically these scientific notions are
perceived--even by their own creators. In 1982, for instance,
Gould first asked and then answered: 



> 
> 
> What would a fully elaborated, hierarchically based evolutionary
> theory be called? It would neither be Darwinism, as usually
> understood, nor a smoothly continuous extension of Darwinism, for it
> violates directly the fundamental reductionist tradition embodied in
> Darwin's focus on organisms as units of selection. (1982: 386)
> 
> 
> 
> 



But in 2002, Gould wrote 



> 
> 
> I do believe that the Darwinian framework, and not just the
> foundation, persists in the emerging structure of a more adequate
> evolutionary theory. (2002:3) 
> 
> 
> 



He devoted much of the extensive efforts of his *Structure of
Evolutionary Theory* to defending an interpretation of Darwinian
evolutionary theory as resting on a revisable "tripod of
necessary support" (Gould 2002: 586)--one whose three
critical supports were challenged by proposed expansions to
evolutionary theory, but which nonetheless could be buttressed via
integration with these challenges, rather than replacement by them. In
short, the later Gould argues that Darwinian evolution traditionally
emphasizes organismic selection, adaptationism, and extrapolationism;
that punctuated equilibria, stabilizing selection, and species
selection challenge the first of these supports, developmental and
other constraints challenge the second, and major evolutionary
transitions challenge the third; but that all three of these
challenges can be accommodated by a revised but still Darwinian
conception of evolutionary theory.



Note that efforts to retain (or reject) the Darwinian label show up in
other contexts as well--for instance, in discussions of
adaptationism (e.g., Gould & Lewontin 1979) and the neutral theory
of molecular evolution (Dietrich 1994). Darwin himself declared that
"He who rejects these views [his] on the nature of the
geological record, will rightly reject my whole theory" (1859
[1964: 342]). Whether or not Darwin's emphatic claim is correct
is still an open question. Additional philosophical resources on
theoretical and conceptual change in science include but are by no
means limited to Kuhn (1962), LaPorte (2004), and M. Wilson
(2006).


## 5. Issues of Rhetoric and Risk



From October 16th to 19th of 1980, a conference
on the subject of macroevolution was held at the Field Museum of
Natural History in Chicago, Illinois. Shortly thereafter--in the
November 21, 1980 issue of *Science*--an exciting report
on some events of the conference was published (Lewin 1980). The
report was entitled "Evolutionary Theory under Fire", and
it prompted a cascade of responses in the Letters section of
*Science*--an exchange of scientific correspondence that
continued for almost two decades. Whatever the motives behind the
headlining of the report, it is empirically evident that the chosen
title was inflammatory.



The February 21, 1981 issue of *Science* included no fewer than
5 separate contributions (in order of printing: Futuyma et al. 1981;
Templeton & Giddings 1981; Carson 1981; Olson 1981; Armstrong
& Drummond 1981). The February 4, 1983 issue of *Science*
contained a letter by critics of PE (Schopf & Hoffman 1983) and a
reply (Gould 1983). In the March 11 issue of that same year both prior
parties were accused of prolonging a fruitless discussion of a
"philosophically intractable" and dubious
"pseudoquestion" (Grant 1983: 1170). In April, one
correspondent wrote in agreement with Grant (Schoch 1983) and another
chided him for failing "to explore and appreciate the latest
blooming in the desert of dogma" (Maderson 1983: 360). To put it
mildly, and purely descriptively: many of the responses elicited by
the original report were impassioned and emotive.



In the February 14, 1986 issue of *Science*, the original
reporter on the macroevolutionary conference fanned the flames with
another piece, one entitled "Punctuated Equilibrium is Now Old
Hat" (Lewin 1986). In March, Jeffrey Levinton registered an
ardent objection to Lewin's characterization of the theory as
widely accepted, as well as to PE in general (Levinton 1986). In
April, Gould responded (Gould 1986), and in May, Robert Eckhardt
responded to Gould's response (Eckhard 1986). Throughout these
exchanges and related ones, many interesting meta-scientific charges
were laid and lobbed. Questions of "where is the
evidence?" as well as accusations of dogma, religionism,
revisionism, sloganeering, and vagueness abounded.



So did references to creationism, specifically. The very first batch
of letters in response to Lewin's initial piece included one
objecting strongly to the choice of title (Armstrong & Drummond
1981). The correspondents wrote that "this article is
undoubtedly destined to enter the out-of-context arsenal that has
become a mainstay of recent creationist literature" (Armstrong
& Drummond 1981: 774). Incidentally, the correspondents were
correct about this (e.g., Meyer 1994; Parker 2006).



And in the March 10, 1995 issue, a different journalist for
*Science* published another piece, this one entitled "Did
Darwin Get It All Right?" (Kerr 1995). By May 5, a scientist had
written in to protest the use of such an inflammatory headline
(McInerney 1995), referencing Lewin's original piece (from
1980). McInerney wrote that Lewin's earlier piece had
"provided a gold mine for subsequent creationist
propaganda" (1995: 624). He bemoaned the fact that another such
headline had just been published by *Science*, and recommended
that "a journal representing some 130,000 scientists be a bit
more judicious in its choice of headlines" (1995: 624).



These references to a wider social context--the one in which such
ardent macroevolutionary debate is being held--raise interesting
issues that have perhaps been somewhat neglected by philosophers of
science (though see the entry on
 the social dimensions of scientific knowledge).
 Do the scientists involved in this dispute, and the
scientific journalists reporting on it, have a responsibility to
consider the (unintended yet foreseeable and potentially avoidable)
uses to which their headlines might be put (by creationists and other
parties)? Some scientific correspondents evidently think they do. And
if such responsibility exists, does it fall upon the shoulders of the
scientist *qua* scientist--or upon the shoulders of the
scientist *qua* citizen, or scientist *qua* educator?
Philosophers focusing on the relation between science and value have
done some relevant work on questions like these, usually while
focusing on other scientific domains--work which could be applied
to macroevolutionary instances and issues (e.g., Rudner 1953; Hempel
1960; Longino 1990; Douglas 2000, 2009; Kourany 2010).



These are not the only issues of provocative linguistic expression
awaiting further exploration by philosophers of macroevolution.
Questions of what it means to sit "at the high table"
(Maynard Smith 1984; Sepkoski 2014) or to "replay life's
tape" (Gould 1989; Beatty 2006; Sepkoski 2016) are perhaps more
familiar to philosophers of biology, but still worth exploring.


## 6. Historical Contingency



Stephen Jay Gould (1989) famously argued that evolutionary history is
contingent. George Gaylord Simpson (1963) foreshadowed that claim when
he suggested that paleontology is a distinctively historical science
that seeks to understand "configurational change". Gould
claimed that if we could rewind the tape of history to some point in
the deep past and play it back again, the outcome would probably be
different.



Gould's thinking about contingency has had significant impacts
in both philosophy and biology. In philosophy, for instance, John
Beatty (1995) argued that the contingency of evolutionary history
implies that there are no distinctively biological laws, an argument
that served as a touchstone for much of the subsequent debate about
laws in biology (Brandon 1997; Mitchell 1997, 2000, 2002; Sober 1997;
Giere 1999; Shapiro 2000; Woodward 2000, 2001, 2004; Elgin 2003, 2006;
and Hamilton 2007; for important precursors to Beatty 1995, see Smart
1963; Ruse 1970; Cartwright 1983; van Fraassen 1989; and Rosenberg
1994). More recently, Beatty (2016) has argued that the contingency of
evolutionary history is closely connected to narrative modes of
explanation. His suggestion is that narrative explanations are
especially well suited to historically contingent series of events.
McConwell (2017) argues that the contingency of evolutionary history
explains why we should be pluralists about biological individuality.
In biology, on the other hand, Gould's argument about
contingency, particularly his thought experiment concerning rewinding
and playing back the tape of history, helped to inspire research in
long-term experimental evolution (e.g., Travisano, Mangold, Bennett,
& Lenski 1995). In addition to fueling interest in understanding
the Cambrian explosion, Gould's work also instigated something
of a backlash from other scientists convinced that selection-driven
convergence is the hallmark of evolutionary history (e.g., Conway
Morris 2003). This debate concerning contingency and convergence
sometimes also has theological overtones. Conway Morris has at least
hinted that he sees convergentism about evolution as cohering
especially well with theism. This contrasts with Gould's
well-known view that science and religion should be regarded as
"non-overlapping magisteria" (Gould 1997).



There is some question about just what Gould meant by the term
'contingency'. Some philosophers read him as meaning that
downstream outcomes are sensitive to small changes in upstream
conditions (Ben-Menahem 1997; Inkpen & Turner 2012). On this
reading, contingency comes in degrees, and in contrast with historical
*in*sensitivity to changes in upstream conditions, which we
could think of as historical convergence or robustness. When, for
example, Sterelny (1995, crediting F. Jackson & Pettit 1992) talks
about "robust process explanations", he means explanations
that show how an outcome would have resulted from a wide range of
upstream conditions.



Beatty (2006), however, has shown that there are two different senses
of 'contingency' in play in Gould's work. In
addition to what Beatty calls contingency as causal
dependence--basically, sensitivity to initial
conditions--there is a second form of contingency that Beatty
initially called contingency as unpredictability, but now calls
contingency *per se* (Beatty 2016). These two senses of
contingency correspond with two versions of the famous thought
experiment that Gould (1989) deployed. Sometimes, Gould imagines
rewinding the tape of history, tweaking an upstream variable, and then
playing the tape back. On other occasions, he talks about playing the
tape back from the same initial conditions. Beatty (2016) thinks that
both senses of 'contingency' are important, and he takes
it that the second sense--contingency *per se*--must
commit us to some sort of causal indeterminism. On the other hand,
Turner (2011a) has tried to give an account of this second sense of
contingency that is neutral with respect to determinism. His
suggestion is that what Gould really cared about was random or
unbiased macroevolutionary sorting. Processes such as coin tosses, or
random genetic drift, can be random or unbiased (in a sense) without
violating causal determinism. One way to think about this is by
adopting a frequentist conception of probability: the outcome of a
coin toss could be causally determined by small-scale physical
influences, but the outcome is still random or unbiased in the sense
that over a long series of trials, the ratio of heads to tails will
approximate 50:50. This account is closely allied with
Sepkoski's (2016) suggestion that Gould's metaphor of
replaying the tape of history was rooted in his work on simulations of
macroevolution in the 1970s. Turner (2015) has also argued that there
is a connection between contingency, thus understood, and the notion
of a passive evolutionary trend. 



McConwell and Currie (2017) have added further complexity to the
discussion by arguing that there is an important difference between
abstract, *source-independent* accounts of contingency and
*source-dependent* accounts. The notion of sensitivity to
initial conditions is a good example of what they mean by a
source-independent account: it gives an abstract characterization of
historical or biological processes without saying what the source of
contingency is. Does the source have something to do with genetic
drift? With mutational ordering? Or something else entirely? McConwell
and Currie argue that in order to establish the autonomy of
macroevolutionary theory, a source-dependent account is required.



Some commentators have noted that Gould's defense of
evolutionary contingency sits in tension with his arguments, in the
1970s and early 1980s, for a more "nomothetic"
paleobiology (e.g., Raup & Gould 1974). A more nomothetic
paleobiology would seem to place the emphasis on laws and
predictability. However, contingency is thought by some to mean that
there are no distinctively biological laws (Beatty 1995). Haufe (2015)
addresses this puzzle by arguing that Gould was interested in
developing a "nomothetic" science of macroevolution
involving generations at large spatial and (especially) temporal
scales, but that this is compatible with saying that contingency
reigns at other scales. One other larger theme that may help to unify
some of Gould's thinking is his skepticism about the power of
natural selection, which is the classic example of a
micro-evolutionary process (see also Gould & Lewontin 1979). One
way to limit the importance of natural selection is to insist on the
autonomy of macroevolutionary theory, and to show that natural
selection is insufficient to explain certain larger scale patterns.
Another way is to insist that evolutionary history is contingent,
since one would expect selection to produce convergent outcomes.



Thus, philosophers have done a lot of work on challenging conceptual
questions about the various senses of 'contingency' and
their relationship to other macroevolutionary ideas. One problem
lurking in the background of this discussion is empirical: how can
scientists even tell whether evolutionary history is contingent or
(say) convergent? Conway Morris (2003) assembles a whole catalog of
examples of evolutionary convergence in an effort to provide
counterevidence against Gould's contingency thesis. One
important issue here is to find a principled way of distinguishing
cases of convergent evolution (where unrelated lineages subject to
similar selection pressures evolve similar traits) from parallel
evolution (where lineages with similar traits to start with follow
similar evolutionary trajectories) (Pearce 2012). Another issue is
that one can multiply convergences by giving coarser-grained
descriptions of the traits in question--for example, by counting
agriculture in humans and some ant species as the same trait (Sterelny
2005). Powell & Mariscal (2015) argue that not all of the putative
examples of convergence really tell against the contingency thesis,
and they try to specify more clearly what would actually count as
evidence against contingency.



In developing his initial claim about evolutionary contingency, Gould
(1989) made much of the Burgess Shale fauna, exquisite Cambrian
fossils from western Canada first described by the American
paleontologist Charles Doolittle Walcott in the early 1900s. Gould was
writing in the wake of what some call the "first
reclassification" of the Burgess fossils, when work by British
paleontologist Harry Blackmore Whittington and students Derek Briggs
and Simon Conway Morris had placed many of the strange forms in their
own phyla, with the idea that each phylum represented a unique body
plan. Most of those Burgess phyla went extinct, but some (including a
likely ancestor of chordates) persisted. But what if that sorting had
happened differently? Since 1989, scientists Graham Budd, Allison
Daley, and others using cladistic methods of classification (which
were slow to be adopted by paleontologists) have reclassified the
Burgess fauna yet again, placing many of the strangest ones in stem
groups of existing clades (such as arthropods). So one further
philosophical question is whether and to what extent this change in
taxonomic practice matters to the argument that Gould tried to make
about contingency (Brysse 2008).



There is a closely related debate about the status of phyla. Some
cladists see phyla as being nothing terribly special. Like any higher
taxa, phyla must be monophyletic, but from a cladistic perspective
there is nothing about the taxonomic rank of phylum that has any
particular importance to macroevolutionary theory. Others have argued
that lower ranks like order and class are evolutionarily meaningful in
a way that phyla are not (e.g., Holman 1989). Gould (1989), however,
drawing upon German paleontological tradition, saw phyla as
corresponding to stable morphological body plans (or
*Bauplane*). This connects closely with his argument about
contingency: if different Cambrian phyla had persisted, downstream
evolutionary history would have involved modification of totally
different body plans. Some scientists have recently tried to revive
the idea that there is something special about the role of phyla in
macroevolution (Levin, et al. 2016; though see Hejnol & Dunn
2016).



Finally, historical contingency is a counterfactual notion, and
although this issue has not gotten as much attention as it deserves,
there is a nascent philosophical literature on historical
counterfactuals (Tucker 2004: 227ff; Nolan 2013; Radick 2016; Zhao
2017 in Other Internet Resources). The debate about historical
contingency can be construed as a disagreement about the truth of
various historical counterfactuals. Gould claimed that if things in
the Cambrian had been slightly different, there would be no
vertebrates today, let alone humans, while other convergentists claim
that humanlike cognitive abilities, language, tool use, and sociality
would have evolved even if other things had been different in the
past--for example, if the non-avian dinosaurs had not gone
extinct.


## 7. Passive vs. Driven Trends



Paleontologists have long been interested in documenting and
explaining larger-scale evolutionary trends in the fossil record
(McShea 1998). Perhaps the classic example of a macroevolutionary
trend, and one that paleontologists have studied extensively, is
Cope's rule of size increase. Paleontologists routinely
distinguish within-lineage trends (e.g., average size increase within
a single lineage) from among-lineage trends (e.g., increase in the
average size of mammals over the Cenozoic). The latter are
distinctively macro-level patterns. While it might be tempting to
think that trends such as evolutionary size increase are generally
attributable to natural selection, Stanley (1973) pointed out that
these trends could be merely statistical phenomena. Suppose that a
clade, such as mammals, starts out at or near a fixed lower boundary
on body size. And suppose that evolutionary size increases and
decreases are equally probable. As the clade evolves, the mean body
size might do a "random walk" away from the fixed
boundary--an idea that Gould (1996) later popularlized. Many
scientists also think of this as a passive diffusion model of
evolutionary change.



The notion of a passive trend is closely related to other themes of
macroevolutionary theory. Gould, in particular, was enthusiastic about
the possibility that Cope's rule might be a passive trend. If
that were the case, then it would mean that evolutionary size increase
is not attributable to natural selection. What's more, if we
explain size increase by supposing it to be a passive trend, then the
explanation need not reference any micro-level causes. The fixed
boundary in the state space could be maintained by developmental
constraints--another favorite theme of Gould's. And
finally, a trend resulting from a random walk away from a fixed
boundary would not seem to be progressive at all. To the extent that
larger-scale evolutionary trends are passive, chance looks like a more
significant factor in evolutionary history, and selection looks like a
less significant one.



McShea (1994) gave one widely-cited formulation of the distinction
between passive and driven trends, and also suggested several
approaches for determining, empirically, whether a trend is passive or
driven. One such approach is the stable minimum test: track, say, the
body size of the smallest members of the clade. If that increases
steadily over time, it is a signal that the trend is driven.
Paleobiologists now routinely try to determine whether trends are
passive or driven, though this work can sometimes encounter
underdetermination issues (Turner 2009). Body size has received a lot
of empirical attention, in part because it is easy to estimate from
fragmentary fossil remains, and partly for the simple reason that it
is a trait that every animal has.



In addition to its role in the development of macroevolutionary theory
and its influence on scientific research, the passive/driven
distinction raises a number of philosophical issues. As noted already,
the notion of a passive trend links up with a traditional set of
questions about the role of chance or randomness in evolution
(Millstein 2000). Indeed, the passive/driven distinction looks a lot
like the distinction between random genetic drift and selection in
population genetics. Drift could be construed as a passive trend in
gene frequencies. Grantham (1999) argues that the notion of a passive
trend points toward a certain sort of explanatory pluralism: a trend
that is best explained as a stochastic phenomenon at the macro-level
could receive a more deterministic explanation at a lower level, and
those explanations need not conflict. Turner (2014) suggests that the
notion of a passive trend poses a challenge to popular interventionist
ideas about causal explanation. Explaining a trend by showing that it
is passive could turn out to be a non-causal explanatory strategy.



One issue that remains under-explored, philosophically, is how the
distinction between passive vs. driven larger-scale trends might
intersect with philosophical disagreements about whether neo-Darwinian
evolutionary theory is best understood as a causal theory (a view
given its canonical formulation by Sober 1984) or a statistical theory
(Matthen and Ariew 2002; Walsh, Lewens, and Ariew 2002). There may be
some question about whether it is best to understand the
passive/driven distinction as a distinction between trends generated
by different causal mechanisms, vs. a distinction between different
sorts of statistical phenomena.


## 8. The Zero-Force Evolutionary Law



Many paleobiologists have assumed that driven trends must be driven by
natural selection. For example, if body size increase is driven,
surely that means that larger body size has conferred some survival or
reproductive advantage (Hone & Benton 2005). McShea (2005) and
McShea and Brandon (2010) have forcefully challenged that assumption.
McShea (2005) argues that increase in structural complexity is a
driven trend, but one that need not be driven by selection. That is
because increasing structural complexity might just be the zero-force
condition for evolving systems, an idea that McShea and Brandon (2010)
develop in great detail. McShea and Brandon's zero-force
evolutionary law--or *ZFEL*--has elicited critical
responses from other philosophers (Barrett, et al. 2012). Whereas
traditional evolutionary thinkers, from Darwin on down, assumed that
natural selection would have to explain complexity increase, McShea
and Brandon see complexity increase as the default expectation, and
they invoke selection to explain deviations from that default
expectation--such as cases of eye loss in cave-dwelling species.
They treat eye loss in cave dwellers as a case where natural selection
favors a reduction in structural complexity. Alternatively,
O'Malley, Wideman, & Ruiz-Trillo (2016) consider the
potentially wide-ranging and macroevolutionary role of simplification
in conferring "adaptive" advantage.



McShea and Brandon also argue that the *ZFEL* has the virtue of
giving a unified account of both micro- and macro-evolutionary
phenomena. They invoke the *ZFEL* to explain a wide variety of
different patterns, ranging from complexity increase in the history of
animal life, as represented by the fossil record, to increasing
genomic heterogeneity over time. Whatever one thinks about their claim
that the *ZFEL* is "biology's first law",
their project raises a question worth exploring: is it reasonable to
think that explanations of macroevolutionary patterns should take
fundamentally the same form as explanations of patterns at vastly
different spatiotemporal scales? How much does scale matter?



The *ZFEL* also raises some fundamental philosophical questions
about the structure of evolutionary theory. How, for example, should
we determine what counts as the default expectation, or zero-force
condition, for an evolutionary system (Gouvea 2015)? Is there
any way to distinguish empirically between theories that disagree
about the default expectation? Is there even a fact of the matter
about what the default expectation should be? These questions about
default expectations for evolutionary systems may link up with larger
questions about the function of null hypotheses in scientific
reasoning. These questions about default assumptions in
macroevolutionary theory also prompt comparison with questions about,
for instance, neutral theory in molecular evolution (Kimura 1983) and
in community ecology (Hubbell 2006). For relevant philosophical
discussion, see e.g., Dietrich (1994); Bausman and Halina (2018).


## 9. Major Evolutionary Transitions



If one way to think about the larger picture of the history of life on
Earth is to focus on evolutionary trends (size increase, complexity
increase, etc.), another approach is to focus on major transitional
events in the history of life (Maynard Smith & Szathmary
1995; Szathmary & Maynard Smith 1995). The point of
departure for most work on major transitions has been Szathmary
and Maynard Smith's (1995: 228) list of game-changing
alterations in evolutionary history:


* Replicating molecules to populations of molecules in
compartments
* Unlinked replicators to chromosomes
* RNA as gene and enzyme to DNA and protein (genetic code)
* Prokaryotes to eukaryotes
* Asexual clones to sexual populations
* Protists to animals, plants, and fungi (cell differentiation)
* Solitary individuals to colonies (non-reproductive castes)
* Primate societies to human societies (language)



One of Maynard Smith and Szathmary's insights is that the
very mechanisms of evolution--the way evolution works--have
changed over the course of evolutionary history (Calcott &
Sterelny 2011).



One philosophical concern about the theory of major transitions is
that the original list was something of a mixed bag (McShea &
Simpson 2011). For example, some of the transitions, like the
evolution of eukaryotes and the evolution of multicellularity, seem to
involve jumps in part/whole complexity. But others, like the evolution
of sexual reproduction, do not. So it is not clear at all that these
are transitions of the same kind. Indeed, Maynard Smith and
Szathmary suggest that the transitions have three features: (1)
changes in the way that information is stored and transmitted; (2) new
division of labor; and (3) individuals that could once reproduce
independently can no longer do so after the transition. Criterion (3),
in particular, links the issue of major transitions to the larger
debate about the nature of biological individuality. One potential
problem, though, is that not all the transitions on the original list
satisfy all three criteria. For example, the shift from primate
societies to human societies clearly involves (1) and (2), but not
(3). To complicate things further, Szathmary (2015) revisits
the original list of transitions and proposes some changes.



Related to this worry that the major transitions do not comprise a
unified type of evolutionary event, some theorists (e.g.,
O'Malley 2014) argue that other significant events should be
count as major transitions. O'Malley & Powell (2016) think
that the biological oxygenation of Earth is a major transition, and
Currie (forthcoming) makes the case for treating mass extinctions as major
transitions.



A second philosophical concern is that Maynard Smith and
Szathmary's original idea looks a bit teleological, and
in a way that likely raises the hackles of some scientists and
philosophers (e.g., Dawkins 1986; Dennett 1995). The transitions in
Szathmary and Maynard Smith's (1995) list all seem like
necessary steps on the way to the evolution of humans, and cynically,
one might wonder if their relation to the later appearance of humans
is what makes the transitions "major". Relatedly, one
might wonder if the idea of major transitions comes a little too close
to reviving pre-evolutionary thinking about the great chain of being.
This also connects with traditional questions about evolutionary
progress (Ruse 1997). While some of the concepts surveyed in this
entry--especially passive trends and historical
contingency--might seem like threats to the belief in
evolutionary progress, the theory of major transitions coheres well
with progressivist views about evolution.



In spite of these potential concerns, however, the major transitions
really are historical events that pose distinct explanatory
challenges. Arguably, our picture of macroevolution will not be
entirely complete without tackling some of those challenges.



It is noteworthy that neither Maynard Smith nor Szathmary are
paleontologists, and so there may be a bit of a disconnect between
their original proposal and the way that many paleontologists have
come to think about major transitions. Several of the transitions from
the original list must have occurred way back in the deep past; the
fossil record is not likely to give us much insight into the origin of
eukaryotes, for instance. Molecular tools may be able to shed some
light on macroevolutionary queries about the deep past, however
(O'Malley 2016). And paleontologists sometimes take a much
broader (and looser) view of major transitions, such that any major
evolutionary novelty would count as a major transition. For instance,
the evolution of placentation in mammals, or the evolution of feathers
in archosaurs, might count as major transitions on this broader view.
Currie (forthcoming) draws a distinction between theory-driven
approaches to major transitions and phenomena-driven approaches.
Paleontologists tend to fall in the latter camp, since they usually
begin by thinking about major morphological or phenotypic changes that
show up in the fossil record.


## 10. The (Dis)unity of Macroevolutionary Theory



One significant and remaining challenge in the philosophy of
macroevolution is to work out how the various ideas about
macroevolution surveyed above may or may not fit together. What
exactly is the relationship between punctuated equilibria and species
selection? Or between PE and the major transitions? Or between species
selection and historical contingency? Or between contingency and the
major transitions? Do (some, all of) these ideas weave together to
form a unified picture of evolution at large scales? Or do they
represent something more like a heterogeneous grab bag of scientific
ideas that are more or less useful in different contexts? A further
question is how the relative (dis)unity of macroevolutionary theory
might bear on the longstanding question of how macroevolution is
related to microevolution.



Finally, the investigation of macroevolution challenges the purported
distinction between the experimental and historical sciences (Sober
1993; Cleland 2001). Investigation in this domain is historical and
often emphasizes the deep past, to be sure, but it also involves
theoretical and prospective modeling, making predictions about what
the models will or will not find, making predictions about what
paleontologists will or will not find, running bottle and other kinds
of experiments, simulations, and more. The scientific study of
macroevolution is a philosophically rich and challenging field with
many deposits to mine.