Philosophy of Science, 69 (September 2002) pp. S305-S315. 0031-8248/2002/69supp-0026
Copyright 2002 by The Philosophy of Science Association. All rights reserved.
Linnaean Ranks: Vestiges of a Bygone Era
Marc Ereshefsky
University of Calgary
http://www.journals.uchicago.edu/PHILSCI/journal/issues/v69nS3/693026/693026.
html
We tend to think that there are different types of biological taxa: some taxa are species, others are
genera, while others are families. Linnaeus gave us his ranks in 1731. Biological theory has changed
since Linnaeus's time. Nevertheless, the vast majority of biologists still assign Linnaean ranks to
taxa, even though that practice is at odds with evolutionary theory and even though it causes a
number of practical problems. The Linnaean ranks should be abandoned and alternative methods
for displaying the hierarchical relations of taxa should be adopted.
Send requests for reprints to the author, Department of Philosophy, University of Calgary, Calgary,
Alberta T2N 1N4, Canada; ereshefs@ucalgary.ca.
1. Introduction.
Something potentially revolutionary is brewing in biological taxonomy. For
hundreds of years biologists have used the Linnaean system for representing the
organic world's diversity. Organisms are sorted into species, species into genera,
genera into families, and so on. The Linnaean system also tells us how to name
biological taxa. Recently, a handful of biologists and philosophers have called for
the replacement of the Linnaean system (de Quieroz and Gauthier 1992, 1994;
Ereshefsky 1994, 2001). They argue that the Linnaean system is theoretically
outdated: it was designed with creationism and essentialism in mind, whereas now
we live in a Darwinian age. Moreover, the continued use of the Linnaean system
causes a number of practical problems.
The question of whether we should continue using the Linnaean system is a
pressing one for biology and those fields that rely on biological classification.
Nothing less than how we represent the organic world is at stake. The Linnaean
system tells us what types of biological taxa exist and how we should name those
taxa. Consequently, the terms and concepts of the Linnaean system frame all
theoretical questions concerning groups of organisms above the level of the local
population. If the Linnaean system is replaced, much of how we talk about the
organic world will be different. Consider the effect on environmental policy.
Environmental legislation and debates over the conservation of biodiversity are
framed by the terms of the Linnaean system. Species and other types of Linnaean
taxa, we are told, should be preserved. If the Linnaean ranks no longer have a
theoretical basis, then the concepts used in environmental policy need to be reevaluated.
Critics of the Linnaean system cite a number of problems with that system,
everything from whether species should be given binomial names to the
assumption that the organic world can be adequately captured by hierarchical
classification. This paper will highlight just one problematic aspect of the Linnaean
system, the Linnaean ranks. In what follows I will suggest that the Linnaean ranks
should be junked and that an alternative system for representing the relations
among taxa should be adopted. Two general reasons will be offered for getting rid
of the Linnaean ranks. One is theoretical: the Linnaean ranks have little meaning in
contemporary biology. The other is pragmatic: the continued use of the Linnaean
ranks causes a number of practical problems. In the penultimate section of this
paper I will speculate on the prospects of replacing the Linnaean system with an
alternative system of classification.
2. Theoretical Problems.
The Linnaean ranks are often divided into two types: the higher Linnaean ranks,
such as genus, family, and class; and the rank of species. The theoretical problems
of the Linnaean ranks divide along similar lines. Let us start with the higher
Linnaean ranks.
2.1. Higher Taxa.
From Linnaeus to Hennig, biologists have attempted to provide criteria for
determining the ranks of higher taxa. In doing so, they have tried to describe a
feature that makes all taxa of a particular rank taxa of that rank. For example, they
have looked for a feature that makes all genera genera rather than species or
tribes. As we shall see, the search for an adequate ranking criterion for higher taxa
has come up empty-handed. Two prominent proposals were offered in the twentieth
century, one by the founders of evolutionary taxonomy, Ernst Mayr and G. G.
Simpson, and another by the founder of cladism, Willi Hennig.
According to Mayr (1969, 223ff.) and Simpson (1961, 222ff.), the rank of a higher
taxon is determined by a number of factors. One is the phenotypic diversity found
within a taxon. Another is the uniqueness of a taxon's adaptive zone. A third is the
breadth of a taxon's adaptive zone. Generally, the higher a taxon scores on these
factors, the more inclusive the taxon. The members of an order, for example, are
more phenotypically varied than the members of a genus. Similarly, the difference
between the adaptive zones of two families in a tribe will be greater than the
difference between the adaptive zones of two genera in a family.
As any student of biological taxonomy knows, Mayr and Simpson's criteria for
ranking higher taxa have been subject to much criticism. The arguments launched
against them are standard ones used to criticise the taxonomic school evolutionary
taxonomy. Hennig (1966, 156) argues that there is no nonarbitrary method for
measuring phenotypic diversity. He playfully asks "whether the morphological
divergence between an earthworm and a lion is more or less than between a snail
and a chimpanzee"? Measures of phenotypic diversity, he concludes, are based on
"sheer prejudice." Similar objections have been launched against the concept of
`adaptive zone.' Evolutionary taxonomists raise the taxon Aves to the rank of class,
because birds, they argue, live in a significantly different adaptive zone than
reptiles. Cladists respond that the concept of `adaptive zone' is ambiguous and is
applied inconsistently across phyla (Wiley 1981, 254). In brief, the criticism
launched against Mayr and Simpson's criteria is that the concepts of phenotypic
diversity and adaptive zone are too malleable to serve as measures of a taxon's
rank.
Hennig ([1965] 1994, 272) offers an alternative way of defining the higher
Linnaean ranks. He suggests that the taxa of a higher Linnaean rank should
originate during a single time period. Here Hennig draws an analogy with geology.
Just as geological strata fall into different geological categories depending on their
time of origin, biological taxa should be assigned Linnaean ranks according to their
time of origin. Classes, for example, could be defined as all and only those taxa that
originate during the Late Cretaceous. Orders would be taxa that originated during a
more recent time period.
Unfortunately, Hennig's suggestion for defining the higher ranks is problematic
as well. Ideally, cladists would like the taxa of the same higher rank to have a
similar degree of branching. However, taxa originating in the same period often
have different genealogical structures (Eldredge and Cracraft 1980, 222). For
example, some taxa originating during the Late Cretaceous are quite successful
and contain a number of orders and genera. But other taxa originating during the
same period are monotypic and contain only a single basal taxon. According to
Hennig's criterion, some classes are monotypic while other classes contain an array
of subtaxa. From a phylogenetic perspective, Hennig's criterion places different
types of taxa under a single rank. This result has caused cladists, including later
Hennig (1969), to abandon the idea of correlating the rank of a taxon with time of
origin.
Stepping back, we have looked at two methods for ranking higher taxa: one
offered by evolutionary taxonomists, another by the founder of cladism. Neither
method is successful. The current status of the higher Linnaean ranks is the
following. Higher taxa of the same rank are not comparable in any absolute sense.
Families, for instance, can vary in their age, their degree of inclusiveness, their
internal diversity, and the breadth of their adaptive zone. At best, calling a taxon a
`family' indicates that within a particular classification, that taxon is more inclusive
than a genus and less inclusive than a class. Beyond that, calling a taxon a `family'
or an `order' is an empty designation.
2.2. Species.
Thus far, I have argued that the higher Linnaean ranks lack theoretical meaning.
What about the other type of Linnaean rank, the rank of species? Is there a property
distinctive of all species taxa? Once again, I will argue no. The taxa we call
`species' are non-comparable as well. Consequently, and this is a very
controversial claim, the rank of species lacks a theoretical foundation.
Confusion over the distinctive property of species goes back to Darwin and
earlier. In a letter to Joseph Hooker, Darwin writes:
It is really laughable to see what different ideas are prominent in various
naturalists' minds, when they speak of "species"; in some, resemblance is
everything and descent of little weight in some, resemblance seems to go
for nothing, and Creation the reigning idea in some, sterility an unfailing
test, with others it is not worth a farthing. It all comes, I believe, from trying
to define the indefinable. (December 24, 1856; in Darwin 1887, Vol. 2, 88)
So according to Darwin, there is no common and distinctive property among those taxa we
call `species.'
Today, we still have a wealth of species concepts. Many of those concepts fall
into two groups: interbreeding concepts (Mayr 1970; Patterson 1985) and
phylogenetic concepts (Mishler and Brandon 1987; Ridley 1989). Roughly,
proponents of interbreeding concepts believe that species are groups of organisms
that successfully interbreed and produce fertile offspring. Proponents of
phylogenetic concepts believe that species are basal monophyletic taxa. The
problem that Darwin saw with the species category remains today. The
interbreeding and phylogenetic approaches to species highlight different types of
taxa.
Consider the processes that maintain the existence of such taxa. Interbreeding
species are bound by gene flow. Some phylogenetic species are bound by
interbreeding, but many are not. Those phylogenetic species not bound by
interbreeding are held together by other processes, such as selection and genetic
homeostasis. Examples of such phylogenetic species are monophyletic basal taxa
consisting of asexual organisms and monophyletic basal taxa consisting of isolated
groups of sexual organisms (Ereshefsky 1999, 291 292).
Alternatively, consider the patterns of interbreeding and phylogenetic species.
Phylogenetic species must be monophyletic. Some interbreeding species are
monophyletic, but, again, many are not. Ancestral species are a good example
(Ereshefsky 2001, 136 137). Suppose a handful of organisms forms a peripheral
isolate that gives rise to a new species. The original species, the ancestral species,
continues to survive as a successful interbreeding species. However, the ancestral
species does not contain all and only the offspring of a common ancestor. The
ancestral species is not monophyletic and fails to be a phylogenetic species, even
though it is a good interbreeding species.
The conclusion to draw from this discussion is that the species category is a
heterogeneous class. Some taxa we call `species' are paraphyletic and form
cohesive gene pools, others are monophyletic and consist of populations that do
not exchange genetic material. The heterogeneity found within the species
category is not limited to a few isolated cases, but is widespread (Ereshefsky 2001,
130ff.). Many phylogenetic species fail to be interbreeding species, and many
interbreeding species fail to phylogenetic species. The species category lacks a
significant unifying feature and is in the same boat as the higher Linnaean ranks.
The Linnaean ranks are problematic all the way down.
3. Practical Problems.
I would like to switch gears now and discuss the practical problems that come
from using the Linnaean ranks. Many of these problems stem from the theoretical
difficulties just outlined. Consider the suggestion that the higher Linnaean ranks
lack significant meaning. As we have seen, the higher Linnaean ranks have no
interclassificatory meaning; they are ontologically empty designations.
Nevertheless, many biologists believe that taxa assigned the same higher Linnaean
rank are similar regardless of their classification. A common but incorrect
assumption is that the Linnaean ranks represent real levels of genealogical
inclusiveness in nature. Unfortunately such assumptions encourage debate over
the ranks of taxa, even though such disagreements lack an objective basis for
resolution. Accordingly, Hennig ([1969] 1981, xviii) suggests that the continued use
of the Linnaean ranks is a source of many "unfruitful debates" in biological
taxonomy. So one practical problem with the Linnaean ranks is that valuable
research time is wasted assigning ranks to higher taxa when no such ranks exist in
nature.
A similar practical problem occurs when we assign taxa the rank of species. All
species taxa, it is often assumed, occupy a unique and common role in nature. As
we have seen, however, the species category may be nothing more than a set of
non-comparable taxa. If that is the case, then we have laboured under a false
assumption. This mistake has practical implications. If the species category lacks a
significant unifying feature, then time and energy is wasted when we argue over the
correct definition of the species category. Similarly, time and energy are wasted
when we argue over whether a taxon should be called a `subspecies,' a `species,'
or a `genus.' It is perhaps worth mentioning that the reality of particular taxa, such
as Homo or Homo sapiens, is not being called into question. Nor is the
classificatory hypothesis that Homo sapiens is part of Homo. What is being called
into question is whether the designations `species' and `genus' add anything to
those facts. If they do not, then the continued use of the Linnaean ranks wastes
valuable research time.
Theoretical problems with the Linnaean ranks lead to further practical problems.
These have to do with nomenclature. The current Linnaean system contains
various rules of nomenclature. A centrepiece of those rules is the idea that the rank
of a taxon be incorporated in its name. Species, for example, are given binomial
names, such as Fundulus heteroclita. Higher taxa are assigned uninomial names,
such as Homo. In addition, the names of many higher taxa are given rank-specific
endings. The rank of the family Hominidae, for instance, is indicated by the suffix -
idea. The rank of the tribe Hominini is indicated by the suffix - ini.
Suppose that the Linnaean ranks do indeed lack meaning. If that is the case,
then we should drop the requirement that the rank of a taxon be indicated in its
name. That requirement enshrines misleading and theoretically useless information
in the name of a taxon. It requires that a name tells us the rank of a taxon even
though no such rank exists in nature. If the higher Linnaean ranks have no basis in
nature, then biologists should not be required to assign rank-specific endings to the
names of taxa. Similarly, if there is no significant species/higher taxa divide in
nature, then biologists should not be required to highlight that divide in the names of
taxa.
One might reply that surely there is no harm in incorporating information about
Linnaean ranks in taxon names even though no such ranks exist in nature. After all,
we are just talking about names, and what harm can names do? One harm is that
the continued use of Linnaean names furthers the misleading impression that the
ranks of the Linnaean hierarchy correspond to levels in nature.
Perhaps a greater harm is that the use of Linnaean names places biologists in
the awkward position of having to determine the rank of a taxon even though no
Linnaean ranks exist. Suppose a biologist discovers a new taxon. She believes that
it has a high degree of inclusiveness, so she must attach an appropriate suffix to its
name. To determine which suffix to use, she must first determine that taxon's
Linnaean rank. If the Linnaean ranks lack objective existence, then she is being
asked to find a non-existent distinction before she can even name the taxon.
Theoretical problems with the Linnaean ranks ramify into practical problems: nonexistence distinctions must be found, and then those distinctions must be codified in
the names of taxa.
Stepping back from these problems, there is a general point to be made. The
Linnaean rules place an undue burden upon biologists. Time spent deciding the
correct ranks of taxa would be saved if we stopped assigning Linnaean ranks.
Arguments over the correct definition of the species category would be avoided if
we opted out of the Linnaean system. And, taxon names would be more easily
assigned if we dropped the rule that a taxon's name reflects its rank.
We have only scratched the surface of the practical problems that face the
Linnaean system. There are many more, and many of these occur regardless of
whether one is sceptical of the Linnaean ranks. One such problem occurs when
classifications are revised, when, for example, a taxon thought to be a family is
reclassified as a genus. Taxonomic revision is the norm and not the exception in
biological taxonomy. Unfortunately, the Linnaean rules of nomenclature make such
revisions doubly hard. Biologists must not only reclassify taxa during taxonomic
revision, they must also alter the names of those taxa. The Linnaean rules of
nomenclature are themselves a source of instability in classification. Drop the
Linnaean requirement that a taxon's name indicates a taxon's rank and one source
of instability in biological taxonomy will disappear.
The Linnaean rules lead to further problems, for example, when biologists
disagree on the rank of a taxon. In such situations the Linnaean rules require that
biologists assign two different names to what they agree is the very same taxon.
The list of practical problems facing the Linnaean system goes on (see Ereshefsky
2001, 221ff. for an overview). Instead of introducing more problems, I would like to
ask some programmatic questions about the future of the Linnaean system.
4. The Future of the Linnaean Hierarchy.
It is one thing to highlight the problems of the Linnaean hierarchy, it is quite
another thing to successfully build a case for its replacement. In the remainder of
this paper I will explore the possibility of replacing the Linnaean hierarchy with an
alternative system of classification. A successful case for replacing the Linnaean
hierarchy needs to address three questions. Are there alternatives to the Linnaean
hierarchy? Are those alternatives any better than the old system? Even if there are
better systems, is it practically feasible to abandon the Linnaean hierarchy and
adopt an alternative? The quick answer to these questions is "yes, yes, and yes."
Consider the first question: Are there alternatives to the Linnaean hierarchy? A
number of them have been suggested in the last 40 years. In the 1960's, Hull
(1966) and Hennig (1969) offered alternative systems. More recently, de Queiroz
and Gauthier (1992, 1994) have written a series of papers articulating a
phylogenetic system. And just in the past year, a handful of biologists have
introduced a system of phylogenetic nomenclature (Cantino 2000). I've been
working on a post-Linnaean system as well (Ereshefsky 2001). So, there are lots of
alternatives out there.
Turning to the next question: Are any of these systems better than the Linnaean
hierarchy? A full-scale comparison of the Linnaean hierarchy to its alternatives
cannot be conducted here. Let us just consider the problems of the Linnaean
hierarchy highlighted in the previous section. If an alternative system is to overcome
those problems, it must do two things. First, it must indicate the positions of taxa in
classifications without using ranks. Second, it must divorce the function of naming
taxa from the function of classifying taxa
the name of a taxon should merely be a
name, it should not indicate the rank of a taxon.
All of the alternative systems cited above satisfy these two requirements. Each
indicates the positions of taxa without Linnaean ranks. Instead, they indicate the
position of a taxon within a classification by using either numerials or indentations.
Consider the numerical system proposed by Hull (1966) and Hennig (1969). A
standard Linnaean classification looks like this:
Subclass Reptilomorpha
Infraclass Aves
Infraclass Mammalia
Division Monotremata
Division Theria
Cohort Metaheria
Cohort Eutheria.
A classification of the same taxa using positional numbers would be the following:
2.4. Reptilomorpha
2.4.1. Aves
2.4.2. Mammalia
2.4.2.1 Monotremata
2.4.2.2. Theria
2.4.2.2.1. Metaheria
2.4.2.2.2. Eutheria.
Positional numbers capture various types of information. A taxon's degree of inclusiveness
is indicated by the number of digits in its positional number: the fewer the digits, the more
inclusive the taxon. Positional numbers indicate which taxa are parts of more inclusive taxa.
Eutheria's number shows that Eutheria is a part of the more inclusive taxon Theria, which is
part of Mammalia.
Positional numbers have advantages over Linnaean ranks. Positional numbers
are not part of a taxon's name, so the functions of classifying taxa and naming them
are divorced. If a taxon needs to be reclassified there is no need to change that
taxon's name, only its positional number needs to be altered. Positional numbers
also have the advantage of not corresponding to any absolute ranks. A positional
number merely indicates the placement of a taxon within a particular classification.
Recall that the Linnaean ranks carry with them the misleading assumption that taxa
of the same rank are comparable; they also carry the false assumption that the
Linnaean ranks correspond to real divisions in nature. Positional numbers are free
of such assumptions. They indicate the positions of taxa within classifications
without carrying any suspect theoretical baggage.
Post-Linnaean systems avoid the problems associated with the Linnaean ranks.
Do they also avoid the problems of Linnaean nomenclature? They do, and in a very
simple way: when a taxon is assigned a name in a post-Linnaean system that name
does not indicate the rank of the taxon, it is merely a name. Consequently, the
practical problems of the Linnaean hierarchy highlighted earlier are avoided. A
taxon can be named prior to knowing its classification. Biologists can assign a
single name to a taxon even though they disagree on its classification. Furthermore,
taxon names remain stable during taxonomic revision.
There is one last question I would like to consider: Is it practically feasible to
abandon the Linnaean hierarchy and adopt an alternative? Suppose that an
alternative system is more theoretically consistent with evolutionary theory than the
Linnaean hierarchy. Suppose, also, that the adoption of that alternative would make
the job of biologists easier. Are these reasons sufficient for switching? A common
response is "no." Some argue that the Linnaean hierarchy is so deeply entrenched
in biology that there is little hope of replacing it (Jeffrey 1989).
I find this pessimism too self-fulfilling. If at the start we decide that scientific
change is too hard, then there will be no change. If inconvenience were an
overriding concern in science, then scientific progress would grind to a halt. Writing
new textbooks is not easy, nor is telling people that the world is different than
they've previously thought. Nonetheless, those are the results of scientific progress.
Progress in biological taxonomy is no different: it might be inconvenient at first, but
well worth the effort in the end.
Having said that, however, I would like to suggest that the adoption of a postLinnaean system would not be grossly inconvenient. The primary change that a
post-Linnaean system would bring is the elimination of the Linnaean ranks from
biological classification. That modification leaves much of current classification
untouched. Biological classifications are hypotheses of relations among taxa. That
is the primary purpose of classification. In a post-Linnaean taxonomy, hypotheses
concerning the relations among taxa would not be affected. What would change is
the how we represent those relations. Instead of Linnaean ranks, positional
numbers or indentations would indicate the relations among taxa.
Another change would concern the meaning of taxon names. In a post-Linnaean
system, taxon names would no longer convey information concerning the Linnaean
ranks of taxa. Taxon names would be considered merely names, not indicators of
rank. This change would not affect the names of taxa. We would continue calling a
taxon by its current name. We would just need to recognize that the various endings
of taxon names no longer indicate the ranks of taxa.
One might nevertheless protest that the Linnaean hierarchy will not be replaced
because its terms are too entrenched in biology and general society. The term
`species,' for example, is not only found in biological taxonomy, but in many
textbooks, field guides, television nature programs, and environmental laws. One
might argue that even though some biologists and philosophers might want to get
rid of the term `species' for worthy technical reasons, its occurrence is too prevalent
to eliminate. The response to this sort of concern is that the switch to a postLinnaean system does not have to be done in one fell swoop. A post-Linnaean
system could be introduced in a piece-meal fashion. For example, at first we could
abandon the higher Linnaean ranks but keep the species rank in place. Eventually,
the species rank itself would be eliminated, but again that too can be done in steps.
Perhaps the first step would be to eliminate the term `species' from biological
taxonomy. Later the term would be dropped from general biology texts. How this
would be done does not need to be spelled out here. The point is that the switch to
a post-Linnaean system of classification does not have to be all or nothing. It can
come in stages.
From a practical perspective, I do not think that the switch to a post-Linnaean
system is as radical as it sounds. Taxon names would remain constant, so would
classifications. Just the theoretical overlay of the Linnaean hierarchy would be
altered. As we have seen, the Linnaean ranks are a remnant of pre-evolutionary
biology. Moreover, their continued use causes a number of practical problems: time
is wasted assigning taxa meaningless and misleading ranks, and it is further wasted
when taxon names must be altered during taxonomic revision. The practical thing, it
seems, is to drop the Linnaean ranks altogether.
5. Conclusion.
A potential revolution is brewing in biological taxonomy. The very way we
represent the organic world is being challenged. This challenge has broad
implications. From theoretical questions concerning the ontology of nature, to
practical ones concerning how to frame environmental legislation. This is an area
relatively untouched by philosophers. Yet, it is an area that would benefit from the
attention of philosophers. Many central questions in the philosophy of science are
present here. The question of scientific realism arises when we ask whether
Linnaean ranks should be treated realistically or instrumentally. The question of
how theoretical entities should be named arises when we ask whether taxon names
should be defined descriptively or as rigid designators. The debate over the
Linnaean hierarchy is as much philosophical as it is biological.
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