[Paleopsych] Robert O'Hara: Population Thinking and Tree Thinking in Systematics
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Population Thinking and Tree Thinking in Systematics
http://rjohara.net/cv/1997Scripta.html
This is a web version of a previously published paper that was first
presented at the Royal Swedish Academy of Sciences in Stockholm as
part of a symposium honoring the 25th anniversary of the journal
Zoologica Scripta. A version of this paper in portable document
format, suitable for printing, is [10]also available. In the web
version the text is reproduced in its entirety but no figures are
included. Editorial insertions in the web version are enclosed in
[INS: {braces} :INS] . Citations to this paper should refer to the
original printed version:
O'Hara, Robert J. 1997.
Population thinking and tree thinking in systematics. Zoologica
Scripta, 26(4): 323-329.
_________________________________________________________________
Population Thinking and Tree Thinking in Systematics
Robert J. O'Hara
Cornelia Strong College and Department of Biology
University of North Carolina at Greensboro
Greensboro, North Carolina 27402 U.S.A.
Abstract
Two new modes of thinking have spread through systematics in the
twentieth century. Both have deep historical roots, but they have been
widely accepted only during this century. Population thinking overtook
the field in the early part of the century, culminating in the full
development of population systematics in the 1930s and 1940s, and the
subsequent growth of the entire field of population biology.
Population thinking rejects the idea that each species has a natural
type (as the earlier essentialist view had assumed), and instead sees
every species as a varying population of interbreeding individuals.
Tree thinking has spread through the field since the 1960s with the
development of phylogenetic systematics. Tree thinking recognizes that
species are not independent replicates within a class (as earlier
group thinkers had tended to see them), but are instead interconnected
parts of an evolutionary tree. It lays emphasis on the explanation of
evolutionary events in the context of a tree, rather than on the
states exhibited by collections of species, and it sees evolutionary
history as a story of divergence rather than a story of development.
Just as population thinking gave rise to the new field of population
biology, so tree thinking is giving rise to the new field of
phylogenetic biology.
Introduction
The history of systematics in the twentieth century can be broadly
divided into two periods. The first is the period of population
systematics, which began at the turn of the century and flourished
especially through the years of the Modern Synthesis of the 1930s and
1940s and beyond (Mayr & Provine 1980). The second period is the
period of phylogenetic systematics which began during the 1960s and
which continues to flourish today (de Queiroz 1997).
During the period of population systematics much of the work of the
systematics community was directed toward studies of geographical
variation, speciation, and microevolutionary processes, and a great
many practical and theoretical advances were made in all of these
areas. The theory of allopatric speciation was comprehensively
developed, especially for vertebrates; large series of specimens for
the study of geographical variation were assembled in museums; the
application of statistical techniques became widespread; and studies
of cytological and biochemical variation began to be added to
traditional studies of gross anatomical variation.
The period of phylogenetic systematics, beginning in the 1960s, has
seen a shift in emphasis toward larger questions of evolutionary
history and the structure of the evolutionary tree, and, just as in
the earlier period, this newer phylogenetic era has seen and continues
to see many advances in systematic theory and practice. The
development of all the tools and concepts of cladistic analysis has
been the most important advance of this period; the distinction
between ancestral and derived character states; the application of
computational techniques for reconstructing trees; the increasing
availability of data on molecular anatomy to supplement the data of
gross anatomy; and more recently the application of phylogenetic
information to problems in many other biological fields from ecology
to physiology to embryology to behavior.
The distinction between the two periods of population systematics and
phylogenetic systematics is not sharp, of course, and there continues
to be much fine work done today in population systematics, just as
there were important contributions to the study of phylogeny before
the 1960s. But the general distinction between these two periods is
real, and it captures a variety of important practical, theoretical,
and disciplinary developments in the history of twentieth-century
systematics.
At the broadest level, beyond the development of particular techniques
or concepts, each of these two periods may be characterized by the
introduction and spread of new ways of thinking about systematic and
evolutionary problems, ways of thinking that correspond in scope to
the scientific "themata" described by Holton (1973) for the physical
sciences. Distinctive of the period of population systematics was the
spread of what is commonly called "population thinking" (Mayr, 1959,
1975), and distinctive of the period of phylogenetic systematics has
been the spread of what may be called "tree thinking" (O'Hara, 1988).
My aim here is to outline the components of tree thinking, as a way of
undertanding some of the larger changes that have taken place since
the 1960s. Before we consider tree thinking, however, let us look at
the idea of population thinking by way of comparison.
Population thinking
The term "population thinking" was coined by Ernst Mayr in 1959. In
coining the term Mayr did not claim to be describing something new;
rather he intended to capture with the term a way of thinking that had
swept through systematics and evolutionary biology generally in the
first half of the twentieth century. (Mayr in fact traces the idea of
population thinking back to the early 1800s, but I think it is fair to
say that its hold within systematics did not become widespread until
early in the twentieth century.)
To understand the idea of population thinking it is necessary to
contrast it with the mode of thought it replaced, which Mayr calls
typology or essentialism. In simple terms, an essentialist sees
individual variation within a species as error. An essentialist would
in no way deny the existence of individual variation; it obviously
does exist. But for an essentialist every species has a natural form,
a true type, and individual variation within that species represents
accidental deviation from that true type caused by external
environmental influences. In the absence of external influences that
cause individuals to deviate from their true type all individuals of a
species would be forever the same, because each species' type remains
fixed through time.
The French naturalist Buffon expressed the essentialist view well in
his Histoire Naturelle in 1753 (Sloan 1987: 121):
There is, in nature, a general prototype in each species upon which
each individual is modeled, but which seems, in realizing itself,
to be altered or perfected by circumstances. So that, relative to
certain characteristics, there is an unusual variation in the
appearance in the succession of individuals, and at the same time a
constancy in the species as a whole which appears remarkable. The
first animal, the first horse, for example, has been the external
model and the internal mold upon which all horses which have ever
been born, all those which now exist, and all which will arise,
have been formed. But this model, which we know only by its copies,
has been able to be altered or perfected in the communication and
multiplication of its form. The original impression subsists in its
entirety in each individual, but although there might be millions
of them, none of these individuals is similar in entirety to any
other, nor, by implication, to the impressing model.
Elliott Sober (1980, 1994) has provided a very thorough examination of
the idea of essentialism as it applies to species, drawing on what he
calls the "natural state model" of Aristotle, and I recommend his work
to all who are interested in this subject. Sober's discussion can be
fruitfully compared with those of Toulmin (1961) and Kuhn (1977) on
the conceptual framework of early chemistry and physics.
In contrast to the essentialist, the population thinker rejects
entirely the idea that species have "types" or "natural states."
Individual variation within a species is not deviation from a natural
state under the influence of external forces, a natural state to which
the species will return if the forces are removed. Rather, the range
of individual variation within a species is the result of ongoing
processes of mutation and recombination, the production of phenotypes
in the available environments, and then the selection of those
phenotypes from generation to generation. Nothing remains invariant
across time because new individuals are not produced from some
permanent "internal mold," but instead are produced directly from
their parents, and they incorporate new heritable variations in each
generation. This allows species to "depart indefinitely" (Wallace
1858) from their ancestors, and in so doing it dissolves the idea of
an enduring species type altogether.
In passing it is worthwhile to note that even though population
thinking has by now thoroughly permeated systematics and evolutionary
biology generally, there are other biological fields, most notably
medicine, where it has made little headway. Medical notions of health
and disease have strong essentialist overtones, and as medicine has
come to focus more on the genetic traits of individuals (as opposed to
external agents of infection) there is a tendency on the part of
medical practicioners to pathologize normal variation in human
populations, and in so doing to resurrect the idea of a "natural type"
for Homo sapiens, an idea long ago rejected by evolutionary biology.
Tree Thinking
If the spread of population thinking characterized the period of
population systematics, then the spread of what we may call "tree
thinking" (O'Hara 1988; Maddison & Maddison 1989; de Queiroz 1992;
Doyle & Donoghue 1993; Wake 1994) has characterised the period of
phylogenetic systematics. Tree thinking is in no sense a successor to
population thinking, which is just as important today as it has ever
been. Tree thinking is simply the phylogenetic counterpart to
population thinking, and like population thinking it has brought a
more completely evolutionary perspective to systematics (de Queiroz
1988, 1992, 1997; O'Hara 1988, 1992, 1996).
What constitutes tree thinking, and more especially what constitutes
the absence of tree thinking? If population thinking is contrasted
with essentialism, then with what should we contrast tree thinking?
Tree thinking may be contrasted with two other ways of thinking about
systematics and large-scale evolutionary phenomena. The first of these
I call "group thinking," and the second I call "developmental
thinking." Let us consider each in turn, and consider how tree
thinking differs from them.
Group thinking has been a long-standing way of thinking in
systematics, and group thinking equates "systematics" with
"classification." Just as we can classify many kinds of
objects--landforms, books, minerals, stars--so in the same way can we
classify species, says the group thinker. Group thinking in
systematics (and classificatory thinking in general) treats each
member of a particular group as an independent replicate, and this is
key. Each neutron star, for example, is an instance of the class of
neutron stars, an independant replicate that can teach us something
about the nature of neutron stars as a class. Each drumlin is an
independent replicate of the landform group "drumlins" and can give us
insight into the common causes of drumlins--the common processes
responsible for the formation of all drumlins. The goal is to abstract
from the replicate instances a general picture that will describe all
members of the class and account for their existence.
Group thinking of this kind--seeing members of a group as replicate
instances--is quite appropriate for many kinds of scientific inquiry,
such as the study of stars or landforms, but it breaks down when we
try to apply it to species. It breaks down for the fundamental reason
that species are not independent replicates: they are parts of a
connected tree of ancestry and descent, and they inherit most of their
attributes in a way that stars and landforms, for example, do not.
Tree thinking, in contrast to group thinking, considers species in a
phylogenetic context, not as independent replicates but as parts of a
single phylogenetic tree. If we seek to understand common causes
acting in evolution then the replicates we need to examine are not
species, but the evolutionary events that are of interest in a
particular study, and this can only be done by plotting those events
on a tree. If we are interested in why ten species in a larger group
exhibit a particular trait (say a trait that is correlated with the
occupation of a certain environment) then we must first ask, in the
context of a tree, whether this situation represents ten independent
originations of the trait, or eight with two subsequent speciations,
or five, or three, or perhaps only one independent origination event
with the ten separate species all retaining the trait through
inheritance. These questions can only be answered in the context of a
tree.
The focus on explaining evolutionary events rather than the states of
supposedly replicate species, and on determing where the events occur
on a phylogeny, is central to tree thinking. This new phylogenetic
orientation has in recent years opened the door to a whole range of
important studies of adaptation, ecology, physiology, and other areas
that long been approached from ahistorical, synchronic perspectives
(Fink 1982; Lauder 1982; Felsenstein 1985; Huey 1987; Coddington 1988;
Ronquist & Nylin 1990; Wanntorp et al. 1990; Brooks & McLennan 1991;
Harvey & Pagel 1991; Vane-Wright et al. 1991; Stiassny 1992).
Although tree thinking as I have described it is an aspect of
systematic biology, the idea of tree thinking isn't necessarily tied
to living things--all it requires is descent and inheritance. A
fascinating inorganic example of tree thinking can be found in a
recent paper on the motion of asteroids (Milani & Farinella 1994), an
example which makes use of many of the same ideas I have just
outlined. In examining the orbits of asteriods it is often possible to
identify groups of asteroids that have motion characteristics in
common. One might be tempted to assume that there is something about
the composition of this group of asteroids or about their location
that causes this common "phenotype" (if you will) to exist. But Milani
and Farinella have shown that these asteroids do not share certain
characteristics of motion because of some common set of external
forces acting on them; they share common patterns of motion because
they literally inherited that motion from an ancestral asteroid of
which they once were parts and which subsequently broke up into the
pieces we now see. The asteroids in this group are not independent
replicates that constitute a class, but rather are parts of a tree of
inheritance and their common characteristics can be explained by
reference to their shared history.
There is another aspect of group thinking that tree thinking is
supplanting, and that is the traditional inclination to regard taxa of
equal rank within certain large groups as equivalent and comparable in
some sense. (This is a higher level version of the
species-as-replicates perspective.) An example concerns the
traditional orders of birds, the largest of which is the Passeriformes
which by itself contains about half of all bird species, with the
other 30 or so traditional orders containing all the rest. The
ornithologist Robert Raikow wrote a paper called "Why are there so
many kinds of passerine birds?" (1986) in which he argued in part that
this question is misplaced because it assumes that the various
"orders" of birds are in some way comparable groups when in fact they
are not. And further, even if we frame a more precise comparison
between the Passeriformes and their sister clade, and ask why each of
these two groups differs in species richness, here the validity of the
question will depend upon the internal structure of the passeriform
tree (Fig. 1). Framing these questions in the context of a tree is
essential if progress is to be made, a point that some of Raikow's
commentators did not appear to fully grasp (Raikow 1988).
Let us now turn from group thinking as contrasted with tree thinking,
to what may be called "developmental thinking" and contrast this also
with tree thinking.
By "developmental thinking" I mean thinking that sees evolutionary
history as a story of individual development or unfolding--a story of
"evolution" in the original sense of the word. There is a
long-standing tradition in evolutionary writing of describing the
course of evolution as a developmental course running from monad to
man. This tradition pre-dates evolution certainly; the evolutionary
version is really a temporalization of the ancient idea of the Chain
of Being (Lovejoy 1936).
Evolutionary histories of the developmental type don't narrate a
tree--a branching history--they select one important endpoint (usually
us) and then trace up from the root through the tree to that endpoint,
employing a variety of narrative and nomenclatural devices that
minimize the branching aspect of evolution. In other papers (O'Hara
1988, 1992, 1993) I have discussed in detail the narrative and
graphical devices that have traditionally been used to minimize the
branching aspect of evolutionary history and to thereby create a
linear, developmental aspect.
Tree thinking, in contrast to this sort of developmental thinking,
emphasizes the divergent character of evolutionary history and the
complexity and irregularity of the evolutionary tree. I'm afraid to
say, however, that while many contemporary systematists no longer
think of evolution as a developmental story and no longer draw
diagrams that show humans as the pinnacle of life, most of the general
public and most of our students still do. A survey of beginning
biology students' understanding of evolutionary history almost
invariably produces images of the developmental type with a long main
line reaching to vertebrates, mammals, or humans (Fig. 2). One of the
main objectives of the systematics community for the next decade
should be the preparation of educational materials for beginning
students to teach them to become tree thinkers (O'Hara 1994). Just as
beginning students in geography need to be taught how to read maps, so
beginning students in biology should be taught how to read trees and
to understand what trees communicate (Figs. 3, 4). One effective
method of jarring students out of the traditional pattern of
developmental thinking is to show them trees that are purposely drawn
from a different evolutionary perspective (Fig. 5), although few such
trees are now available.
Systematics and palaetiology
When William Whewell, the nineteenth-century polymath, compiled his
comprehensive survey of all the sciences (Whewell 1847), he placed
systematic zoology and systematic botany along with mineralogy in the
category "classificatory sciences." Elsewhere in his survey, however,
Whewell created a new class of sciences which he called by the awkward
name "palaetiological sciences"--the sciences of history and
historical causation. Into this new category Whewell put such
seemingly disparate fields as geology and comparative philology,
fields he saw as united by their common aim of historical
reconstruction (O'Hara 1996). Charles Lyell's geological work, which
was new at the time, helped to shape Whewell's characterization of the
palaetiological sciences. When Charles Darwin began to work seriously
on the species question he didn't take as his model the approaches of
the classificatory sciences; he took as his model the palaetiological
science of Lyell. Indeed, the Origin of Species is almost a casebook
of the palaetiological principles that Whewell had outlined. Darwin in
effect took systematic biology out of the classificatory sciences and
placed it squarely among the palaetiological sciences, and in so doing
he set for us a range of historical problems the full implications of
which are still being discovered today (de Queiroz 1988; O'Hara 1988,
1992, 1993; de Queiroz & Gauthier 1992, 1994; Williams 1992).
"As buds give rise by growth to fresh buds," wrote Darwin in one of
his more literary passages (1859: 130), "and these, if vigorous,
branch out and overtop on all sides many a feebler branch, so by
generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and
covers the surface with its ever branching and beautiful
ramifications." The tree of life has proven to be a subtle construct,
more subtle perhaps than Darwin suspected. But the spread of tree
thinking throughout systematics in the last thirty years, and its more
recent spread from systematics to other fields, has brought a new
clarity to our understanding of the tree of life, an idea that is
fundamental to all of evolutionary biology.
Acknowledgements
I am grateful to Per Sundberg, Marit Christiansen, and Fredrik Pleijel
of Zoologica Scripta, as well as to the Norwegian Academy of Science
and Letters and the Royal Swedish Academy of Sciences for their kind
invitation to participate in this anniversary symposium. Margareta
Wilberg handled all the arrangements expertly, and Per Sundberg
extended especially kind hospitality during my stay in Sweden. Jeremy
Ahouse directed my attention to Scott's tree of butterfly evolution.
Gregory Mayer, Kevin de Queiroz, and Fredrik Pleijel offered valuable
comments on the manuscript.
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[INS: {Figure Captions} :INS]
Fig. 1. -- Two sister taxa differing in species richness (A). One
might be inclined to assume that the speciose taxon possesses a "key
innovation" that has caused it to speciate at a greater rate than its
sister taxon. Such an assumption may or may not be warranted depending
upon the internal structure of the speciose clade. If the internal
structure is as shown in (B) then it is unlikely that clade B
possesses any special innovation, although its sub-clade B' may.
Fig. 2. -- An evolutionary tree drawn by an undergraduate biology
student at the University of Wisconsin--Madison. At the beginning of a
course each student was asked to "sketch an evolutionary tree of life,
and label as many branches as you can. Don't worry if your tree is not
perfect or if you can't remember technical terminology; this is not a
graded exercise, and you should not even put your name on the page."
Most trees the students produced have as their longest branches the
ones leading humans or to mammals or vertebrates generally.
Fig. 3. -- A phylogeny of three taxa shown in four different graphical
styles (A-D), from O'Hara (1994: 14). All four of these diagrams
convey exactly the same information about the three taxa.
Non-specialists and beginning biology students need to be taught to
read modern evolutionary trees just as beginning students of geography
need to be taught to read maps.
Fig. 4. -- A phylogeny of eight taxa (A), and two simplified versions
of that phylogeny (B-C), from O'Hara (1994: 14). If students and
non-specialists are to become tree thinkers they must learn to
recognize how trees can be differentially simplified (or
differentially resolved) to show the details of particular branches.
Fig. 5. -- "The evolutionary tree of animals, especially those along
the line that evolved into butterflies," from Scott (1986: 87).
Vertebrates appear on the lower left. Trees such as this can jar
students and non-specialists into thinking about the assumptions
behind traditional human-centered trees such as the one shown in Fig.
2. Numbers on this tree represent millions of years.
References
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12. http://rjohara.net/cv/1992BP.html
13. http://rjohara.net/cv/1993SB.html
14. http://rjohara.net/cv/1994AZ.html
15. http://rjohara.net/cv/1996Milan.html
16. http://rjohara.net/
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