the limits of cladism- by david l. hull systematic zoology, vol. 28, no. 4 (dec., 1979), pp. 416-440
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8/10/2019 The Limits of Cladism- by David L. Hull Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440.
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Society of Systematic iologists
The Limits of CladismAuthor(s): David L. HullSource: Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440Published by: Taylor & Francis, Ltd.for the Society of Systematic BiologistsStable URL: http://www.jstor.org/stable/2412558.
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8/10/2019 The Limits of Cladism- by David L. Hull Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440.
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THE LIMITS OF CLADISM
DAVID L. HULL
Abstract
Hull, David L. (Department of Philosophy,University f Wisconsin, Milwaukee, Wisconsin
53201) 1979. The Limits of Cladism. Syst.
Zool. 28:416-440.-The goal
of cladistic
systematics
is to discern sister-grouprelations (cladistic relations) by
the
methods
of
cladistic analysis
and to represent
them
explicitly
and
unambiguously
in
cladograms
and
cladistic classifica-
tions. Cladists have selected
cladistic relations to
represent
for wo
reasons: cladistic relations
can be
discerned with reasonable
certainty y
the methods
of
cladistic
analysis
and
theycan
be represented
with
relative
ease
in
cladograms
and classifications. Cladists
argue
thatfea-
tures
of
phylogeny
other than cladistic relations cannot
be
discerned
with
sufficient ertainty
to warrant ttempting o represent
them
in
either
cladograms
or
classifications
and
could
not
be
represented
if
they
could.
I
argue
that
a
better alternative
is
to work
toward
improving
methods of cladistic analysis (or else
to supplement
them with
other methods) so thatsuch
features ofphylogeny
can be discerned
and to devise methods of
representation capable
of
representing them
in
both
cladograms
and classifications. However, cladograms and classi-
fications cannot represent everything
bout
phylogeny.
It is
better
to
represent
one or two
aspects of phylogeny explicitly and unambiguously than nothing at all. [Cladism; classifica-
tion; evolution; species.]
Our classifications
will come
to
be,
as far
s
they
can be
so made, genealogies;
and will then truly
give what
may
be
called the plan
of creation.The
rules for
classifying
will no doubt
become sim-
pler when
we
have a definite object
in view
(Darwin,
1859:486).
Charles
Naudin's
simile of tree and classifica-
tion
is like mine (and others), but
he cannot,
I
think,have reflectedmuch on the subject, oth-
erwise
he would
see that genealogy by
itself
does not give
classification (Darwin,
1899,
2:42).
One
possible
goal
forbiological
clas-
sification
s
to represent
hylogeny,
o
construct
lassifications
o
that
certain
features
f
phylogeny
an be
read off
n-
equivocally.
However,
Darwin was quite
correctwhen he noted thatgenealogy byitself does not give classification.Differ-
ent
taxonomists
may
select
different s-
pects
of
phylogeny
to
represent.
One
mightchoose
order of branching,
nother
degree
of divergence,
another amount
of
diversity,
nd
so on. Whether or not
the
rules
for classifying
become simpler,
as
Darwin
hoped they
would,
depends on
the features
of phylogeny
which
the tax-
onomist
chooses
to
represent
nd the par-
-ticular
et
of
principles
which
he
formu-
lates to represent them. A classification
which
attempts
o
represent
only
one
as
pect of
phylogeny
s
likely
to be
simpler
than one
which attempts
o
represent
two
simultaneously.
Given any particular
goal,
the simplicity
of the rules
capable
of accomplishing
this goal depends
both
on the
inherent
limitations
of
the
mode
of representation
nd the ingenuity
of
the
taxonomist.For example, the traditional
Linnaean
hierarchy
might
lend itself
to
representing
certain
features
of phylog-
eny
more readily
than
others,
but nothing
precludes
a
taxonomist
from
ntroducing
additional
representational
devices to
re-
move these
limitations.
The end
result
s,
of course,
more
complex
rules for
classi-
fying.
Systematists
are thus
forced
to
strike
balance
between
how much
they
wish
to
represent
and
how complicated
they are willing to make their principles
of classification
and
resulting
classifica-
tions.
In recent
years a
school
of taxonomy
has arisen
whose members
have serious-
ly
taken up
the challenge
of
attempting
to
represent
explicitly
and
unambiguous-
ly
something about
phylogeny.
The
school
is
cladism,
the
aspect
of phyloge-
ny which
the cladists
have chosen
to rep-
resent
is order
of
branching.
The cladists
have given two reasons forchoosing this
416
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8/10/2019 The Limits of Cladism- by David L. Hull Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440.
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1979
LIMITS OF
CLADISM
417
particular
eature
f
evolutionary
evel-
opment
o
represent. irst,
he
Linnaean
hierarchy, s a list of indentednames,
lends itselfmorenaturally o
expressing
discontinuous
han continuous
henom-
ena. Whether r not evolution s gradual
or saltative, hylogeny
s
largely matter
of plitting
nd
divergence.
A
hierachy
f
discrete axa names s
well-calculated o
represent uccessive plitting.t is not s
well-calculated
o
represent arying
e-
grees of divergence. econd, the
cladists
argue that
rderof
branching
an
be
as-
certained
with ufficient
ertaintyo war-
rant he nclusion f such information
n
a
classification
while
other aspects of
phylogeny annot e. The methodwhich
cladistshave
devised
to
discover rder
f
branching
s
cladistic
nalysis.
Cladists
argue that, y and large, ll
a
systematist
as
to
go
on is
the
traits f
the
specimens before him, whether
these
specimens represent
extant
or extinct
forms.
y studying
is
specimens, e
can
discover ested ets
of haracters.
nique
derived
characters istinguish mono-
phyletic roupfrom ts nearest
elatives;
sharedderivedcharacters ombine hese
monophyletic roups ntomore nclusive
monophyleticroups.
n
this
way
the
sys-
tematist
an ascertainnested sets ofsis-
ter-groups. his method, o cladists ar-
gue,
cannotbe used
to discern variety
of
other
elations-speciation
without he
appearance
of at
least one unique
de-
rived character, reticulate evolution,
multiple speciations, nd the
ancestor-
descendant relation-nor
can
any
other
method.
Several questions rise at
this
uncture.
If
the
Linnaean hierarchy annotrepre-
sent
certainfeatures
fevolutionary
e-
velopmentverywell,
is
not that
limi-
tation f
he Linnaean
hierarchy?nstead
of
refusing
o
represent
what
particular
system
f
representation
as
difficulty
n
representing, better lternative
might
be
to
abandon
or
improve
hat
ystem
f
representation.
his is
one avenue
which
the cladists hemselves ave taken Grif-
fiths, 976; Hennig,1966; L0vtrup, 973;
Nelson, 1972a,
1973c;
Patterson nd Ro-
sen,
1977;
Wiley,1979).
Do
the evolu-
tionary
henomena
which
cladistic
nal-
ysis
cannot
discern
actuallyexist?
As
I
understand
volution and
evolutionary
theory,
wo
of
these
phenomena
learly
occur (ancestorsgivingrise to descen-
dants and
reticulate
evolution),
one
is
quite
ikely
multiple
peciation),
nd the
fourth
s
doubtful
speciation
without
e-
viation,
although
the
unique
derived
character
may
be
all but
undetectable).
s
not
the
inability f cladistic
analysisto
discern
the
preceding
phenomena,
as-
suming
they
take
place,
a
limitation
f
the
methods f
cladistic
nalysis?
nstead
of
declining
o
deal with such
phenom-
ena because the methods of cladistic
analysis cannot
discern
them,
perhaps a
better
trategy
ight e
to
attempt
o
m-
prove
upon
the
methods f
ladistic
nal-
ysis
or
to
supplement hem
with
other
methods.
The
crucial
question with
re-
spect to
cladism,
as
I
see
it,
is
what is
cladistic
nalysis?
What re
its
imits? s
cladistic
nalysis
one
way
of
scertaining
phylogeny,
he
only
way of
ascertaining
phylogeny,
he
only
way
of
obtaining
knowledge fanyphenomena?
One fact
bout
cladism
which
compli-
cates
attempts o
answer
the
preceding
questions
s that
ladism,
ike all
scien-
tific
movements,
s
neither
mmutable
nor
monolithic. t
any one
time,
ladists
can be
found
isagreeing
with
ach other
about
particular
rinciples
and
conclu-
sions.
In
addition,
f
one
traces
the
de-
velopment
f
cladism
hrough
ime,
rom
its
inception
in
the
works
of
Hennig
(1950), through ts introductiono En-
glish-speaking
systematists
Hennig,
1965,
1966;
Brundin,
1966;
Nelson,
1971a,
1971b,
1972a,
1972b,
1973a,
1973b,
1973c,
1974)
to
the
latest
publi-
cations
of
present-day
ladists,
signifi-
cant
changescan be
discerned.
he
most
significant
hange whichhas
taken
place
in
cladism
is
a
transition
rom
pecies
being
primary
o
characters
being
pri-
mary.
or
Hennig
1966,
1975)
and
Brain-
din (1972a), the basic unitsof cladistic
analysis
are
species, characterized
y
at
least
one
unique
derived
haracter.
ow,
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418
SYSTEMATIC
ZOOLOGY
VOL.
28
for a growing
number
of cladists, any
monophyletic roup whichcan be char-
acterizedby appropriate
raits
an func-
tion in cladistic analysis,
regardlessof
whether
hat
group
s
more nclusive or
less inclusive than traditionally-defined
species. Cladistic classifications
o not
represent heorder fbranching
f ister-
groups,but the order of emergence of
unique derived characters,
whether or
not the development f
these characters
happens to coincide with speciation
events.
t
is
not
the
emergenceof
new
species which
s
primary
ut
the
emer-
gence
of new traits
Tattersall
nd
El-
dredge, 1977:207; Rosen,
1978:176).
In
general, ladists seem to be moving o-
ward the
position
hat he
particulars
f
evolutionary evelopment
re not rele-
vant o cladism.
t
does notmatter heth-
er
speciation
s
sympatric
r
allopatric,
saltativeor gradual,Darwinian or La-
marckian,ust
so
long
as
it
occurs
nd
is
predominantlyivergentCracraft,
974;
Bonde, 1975; Rosen, 1978;
andforthcom-
ing works by Nelson and
Platnick,El-
dredge
and
Cracraft,
nd
others).
The writingsof most cladists have
been
concernedwith
developing
meth-
ods
of
ascertaining
he
cladistic elations
between
biological axa,
ut
Platnick
nd
Cameron
1977)
have shown thatthese
same techniques
can
be
used
to
discern
the cladistic relations xhibitedby
lan-
guages
and
texts see
also
Kruskal, yen,
and
Black, 1971; Haigh,
1971; and Nita,
1971).
n
general
ladistic nalysis an be
used to
discover
the cladistic relations
between any entitieswhichchange by
means of modification
hrough escent
(Platnick, 979).
As
general
s thisnotion
of
cladisticanalysis s,
it still
retains
necessary emporal imension.
ransfor-
mation series must be established
for
characters,
ot
ust
an
abstract
temporal
transformation
eries like the cardinal
numbers r the
periodic
table, but
a se-
ries of ctualtransformations
n
time.For
example,
the vast
majority
f
matter
n
theuniversehappens tobe hydrogen.n
isolated pockets of the universe,more
complex
molecules have
developed,
but
theyhave not developed by
progressing
up
the periodic table, from
ydrogen o
helium, ithium,
eryllium, tc.
From the
beginning,Gareth Nelson
(1973c) seems to have been
developing
twonotions f cladistic nalysis imulta-
neously, one limited to
historically e-
veloped patterns
cladismwith a small
'c'),
the
other more
generalnotion p-
plicable to all
patternsCladism with a
big C'). His
method f
component nal-
ysis
is
a generalcalculus for
discerning
and
representing
atterns
f
ll sorts. or
example,
n a
recent
discussion,Nelson
(1979:28) states that
cladogram
s
an
atemporal
oncept
...
a
synapomorphy
scheme. It remains o be seen whether
all
of
the terms f cladistic
nalysisfrom
sister
group to
synapomorphy can
be
defined o that
none of them neces-
sarily resupposes
temporal imension.
Although think
hat the principlesof
cladistic
nalysis
an
be
extended
o any
system
which
genuinelyevolves (i.e.,
changes through
ime
by means of
mod-
ification hrough
descent), I
will limit
myself n this
paper to phylogeny nd
biological evolution. also will not ad-
dress
myself
o
Nelson's
more general
notion.
Attemptingo discussthe princi-
ples of cladistic
nalysis s
they pply to
phylogeny s a
sufficiently
ifficultask
without
ttempting
o
present nterpre-
tations f
these
principleswhich re also
consistent
with
Nelson's more
general
program.
METHODOLOGICAL
PRINCIPLES
AND
OBJECTIONS
In
spite
of
certain bservations
which
cladists
have made
periodically
boutthe
evolutionary rocess,
the
principles
of
cladistic
analysis
do not concern
evolu-
tion
but the
goals
of
biological
classifi-
cation
nd the means
by
which
they
an
be realized.
These
principles
re
primar-
ily
methodological.
s
I
see
it,
the
goal
of cladism
s to
represent
ladistic
rela-
tions
in
both
cladograms
nd classifica-
tions s explicitlynd unambiguouslys
possible. Thus,
cladistic
methodology
has two
parts:
ules for
iscerning
ladis-
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8/10/2019 The Limits of Cladism- by David L. Hull Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440.
5/26
1979
LIMITS OF CLADISM
419
tic
relations nd
rules for
representing
them
in cladograms nd
classifications.
For example,
Nelson raises
two objec-
tions o the
ancestor-descendant
elation:
first,
ancestral
pecies cannot
be iden-
tified s such in thefossilrecord Nel-
son,
1973b:311),
and
second,
they
are
also
inexpressible
in
classifications
(Nelson,
1972a:227).
One sourceof
the confusion
whichhas
accompanied the
controversyver clad-
ism is the
interpretationf
their meth-
odological
principles
s
empirical
eliefs
aboutevolution. he claim that
dichoto-
mous
speciationneveroccurs
s an em-
pirical
claim to be
tested by
empirical
means.The claim thatwe can neverdis-
tinguish
between dichotomous
pecia-
tion events and
more complex
sorts of
speciation is a
methodological claim
about
the limitsof our
methodsof
sci-
entific
nvestigation.
inally,
the
claim
thatmultiple
peciation,
f
t
occurs and
if it can be
discerned,
annotbe
repre-
sented
unambiguously
in
cladograms
and/or
lassifications
s
a comment
bout
the
limitationsf
these modes of repre-
sentation. nce theprinciples fcladism
are
recognizedforwhat
they re,
meth-
odologicalprinciples,
he
ogic
of
he cla-
distic positionon
a
variety
f ssues
be-
comes
much
clearer.
Scientists re interested
n
truth,
ot
Absolute
Truth,
but truth
nevertheless.
Although
hilosophershave
yet to pro-
duce
a
totally
nproblematic
nalysis
of
science as
the
pursuit
of truth
see, for
example,Laudan,
1977),
think
hat
ci-
entists re correct n the focus of their
attention.
cientists also
present
argu-
mentsto buttress
heir mpirical
laims
and
try
o make their
rguments
s
good
as
possible.
Unfortunately,
cientists
re
rarely ble to
present
heir rguments
n
the
unproblematic
orms hus
far ana-
lyzed
successfully by
mathematicians
and
logicians.
The
reasoning
which
goes
into the
formulationnd
testing f sci-
entific
heories
s a
good
deal more
om-
plex and informal hanany explicitra-
tional
reconstructionas
yet
been
able to
capture.
cientists
re
interested
n
both
truthnd
cogencyof argument,
ut they
are a
good deal more
nterested
n
truth
than
n
cogency fargument.f
a line of
reasoningwhich
ed to a
particular on-
clusion turns out
to be
somewhat ess
thanperfect,treallydoes notmatter ll
that
much, ust so long
as the
conclusion
depicts
reality
with
greater
idelity
han
previous
ttempts.
Scientists ave
limited
patience when
it
comes to discussing
rguments. hat
patience s even more
imitedwhen the
argumentsre
methodological. s
strange
as
it
may sound
coming
from
philoso-
pher,
am
highly ceptical of
methodo-
logical principles
nd
prescriptions,
s-
peciallywhenthey represented n the
midst
fscientific
isputes.They tendto
be
suspiciously
elf-serving,esigned
to
put one's opponentsat a
disadvantage
while
shoring p
one's
own
position.
n-
variably the advocates of a
particular
methodology an know
what
they
need
to
knowwhile their
pponents an never
hope
to know what
theyneed
to know.
For example,
hepheneticists
laim that
we can
analyze
traits
nto
unit
haracters,
butwe can neverhope to establishgen-
uine,
evolutionary
ransformationeries
among
haracters. one too
surprisingly,
the
pheneticists
eed toknow
he former
but
not the
latter.
Only
the
cladists and
evolutionists
eed
to know
the unknow-
able.
Similarly,
he cladists
laim
that
we
can establish
transformation
eries with
sufficient
ertainty
o warrant he role
which
they
play
in
cladistic
ystematics
but that
ancestor-descendant
elations
are unknowable.As luck would have it,
cladistsneed to
knowthe former ut not
the
atter. nly the
evolutionists eed to
knowthe
unknowable.
As
cynical as the
precedingremarks
may
ound,
think
heyhave some valid-
ity.
t is
no
accident that ll
four f the
phenomenawhich
he cladists laim can-
not be
known,
annotbe
discerned by
the traditional
eans
ofcladistic
nalysis
and
cause
problems
for he
explicit
nd
unambiguous representation f sister-
group relations
n
both
cladograms nd
classifications.
t is
also
no
accidentthat
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6/26
420
SYSTEMATIC
ZOOLOGY VOL. 28
one of the
phenomena
which cladists
claim cannot
be
known
(the
ancestor-de-
scendant relation) plays
a central
role
in
the research program of their chief
ri-
vals-the evolutionists. I do not mean
to
imply that the preceding remarks apply
uniquely to the
cladists. They apply to
scientists
n
general.
If
I
were
investigat-
ing the pheneticists or the
evolutionists,
comparable
observations would
apply
as
readily to them. These are the sorts of
games
which scientists
(not
to mention
philosophers)
play with each other.
In
general,
I think
t
is
very bad strat-
egy
for
proponents
of a
particularscien-
tific research program to stake their fu-
ture on epistemological considerations,
especially
on
our
inability
to
know some-
thing. Phenomena
which
scientists
in
one
age
claim
can
never
be
known often
become common
knowledge
at a
later
date.
The
history
of science
is
littered
with
the bodies
of
scientists who
staked
the success of their movements on
what
we can never know.
I
agree
with
Einstein
(1949:684),
who,
in
response to philo-
sophical criticismsof his
work, stated:
Science without
epistemology
is-insofar as it
is
thinkable
at
all-primitive
and
muddled.
How-
ever,
no
sooner has the
epistemologist,
who
is
seeking
a clear
system, fought
his
way
through
to
such
a
system,
han
he
is
inclined to
interpret
the
thought-content
f science
in
the sense
of his
system
and
to
reject
whatever does not fit
nto
his
system.
The
scientist,however,
cannot afford
to
carry
his
striving
for
epistemological system-
atic that far.
I
hardly want
to argue against
meth-
odological rigor n science, but I also do
not want to see scientificprogress
sacri-
ficed to
it.
Invariably methodological rig-
or is a
retrospective
exercise, carried on
long after
ll
the
Nobel prizes have been
won.
From the
point of view of current
methodologies, scientists will, as
Ein-
stein
(1949:684) noted, appear to
the
systematic epistemologist as a type
of
unscrupulous opportunist. Instead
of
cladists
insisting
that
certain aspects of
phylogeny can never be known and
could
not be
represented cladistically
if
they could be known,
wiser strategy
would be
to
attempt
o devise methods f
analysiscapable
of discerning hese fea-
tures of evolutionary
evelopment
nd
representational
echniquessufficiento
representhem. n thispaper, have at-
tempted o
produce an internal riticism
of
cladism;
that
s,
I have
accepted
the
goals
of cladism
and
have
set
myself
he
task of decidingwhich
methodological
principles
re central
o the
undertaking,
which
peripheral,
nd which could
be
modified
r abandoned without oss
and
possibly
with
ome
gain.
CLADOGRAMS,
PHYLOGENETIC TREES,
AND EVOLUTIONARY SCENARIOS
A curious featureof scientific
evel-
opment
s
the
frequency
with
which a
new movement
s named
by its oppo-
nents. ocial
Darwinists o morewanted
to be called
Social
Darwinists han
clad-
ists have
wanted to be called cladists.
Twentyyears
ago, JulianHuxley 1958)
coined
the terms
clade
and
grade
to
distinguish
etweengroups
f
organisms
with common enetic rigin
nd
groups
distinguished y differentevels of or-
ganization.
Later Mayr (1965:81)
and
Camin and
Sokal
(1965:312)
introduced
the term
cladogram o refer o
a
dia-
gram depicting
the
branching
of the
phylogenetic
reewithout espect
o rates
of
divergence Mayr,
1978:85). Finally,
Mayr 1969:70)
invented he term clad-
ism as a substitute or phylogenetic
ys-
tematics, he name preferred
y Hennig
and
his
followers.
Gradually
he cladists
themselves ave cometo use theterm o
refer o
themselves, lbeit
grudgingly.
Whether
he
Hennigian
school
of
sys-
tematics
s
called
phylogenetic
r cla-
distic
s
not
very mportant,
ut the
pre-
cise nature
f
cladograms
s
they
unction
in
cladistic
nalysis
s.
In
fact,
ncertain-
ty ver
what
t s
that ladograms
re
sup-
posed
to
depict
and how
they
are
sup-
posed
to depict
it
has been
the
chief
source
of confusion
n
the
controversy
overcladism.Like all terms,hemeaning
of
cladogram
has
changed
through
he
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LIMITS OF
CLADISM
421
years.
t
no longermeansto cladistswhat
Mayr,
Camin
and
Sokal
proposed. The
meanings
of cladism and
cladistic
analysis
have
changed
accordingly.
n
an
attempt o
reduce the
confusion verthe meaning of cladogram, cladists
have introduced he
distinction
etween
cladograms,
hylogeneticrees,
nd evo-
lutionarycenarios.'
Once again,
he par-
ticular erms
sed tomark hese
distinc-
tions are
not important;he
distinctions
themselves
re.
It
is
also truethat the
cladists
have devised
these distinctions
for
their
own
purposes. Others
might
want o
draw
ther
istinctions
rto draw
these distinctions
differently.
n
anycase, all three ppellationsrefer o rep-
resentations,ot o the
phenomena
eing
represented. Cladogram nd
tree re-
fer
o two
sorts
f
diagrams, hile scen-
ario refersto a
historical narrative
couched
n
ordinary iological
anguage.
Phylogenetic rees
re designedto de-
pict
phylogenetic evelopment,
ndicat-
ing which
taxa are extinct, hich
extant,
which
gave
rise to
which,degrees
of
di-
vergence, nd so on. As
diagrams
hey
do
not include discussions ofthe methods
used to construct
hem,
he natural
ro-
cesses which
produced
the
phenomena
theydepict,
nd
a
variety
f
other onsid-
erations
f
equal importance.
tree
s a
diagram,
ot an
entire
axonomicmono-
graph.
All
sorts f
conventions ave been
devised to
represent hylogeny
n a dia-
grammatic
orm; or xample,
ines
usu-
ally represent
ineages,
forks
epresent
speciation
vents,
he
slant nd
length
f
a line reflect herapiditywithwhichdi-
vergence ookplace, and
the termination
of line
indicates xtinction.
ther
ech-
niques of
representation ave
also been
used on
occasion; for xample,
dots
in-
dicating uestionable
connections,
ines
of
varying
hicknesses
eflecting
elative
1
The
distinction
between
cladograms,trees,
and
scenarios
discussed
in
Tattersall
nd
Eldredge
(1977)
was takenfrom manuscriptbyGarethNelson. See
Mayr
(1978)
for early
definitionsof
such
terms as
i'cladogram
.
phenogram, and
phylogram.
numbers
of
organisms,
and so
on. How-
ever,
as a
diagram
in
two-dimensional
space, a
tree can
include
only
so much
information
efore
a point of
diminishing
returns ets in. Eventually attempts o in-
clude additional
sorts of
information
e-
sult in
the
loss
of
nformation. f
trees are
supposed to
be
systems of
information
storage
and
retrieval, the
information
must be
retrievable.
Scenarios, as
cladists
use the
term,
re
not
diagrams.
They
are
historical
narra-
tives
which
attemptto
describe not
only
which
groups
gave
rise
to
which
(the
sort
of
information
contained
in
trees)
but
also the ecological changes and evolu-
tionary
forces
which
actually
produced
the
adaptations
which
characterize the
organisms
discussed.
Romer's
(1955:57)
story of the role which
the
drying up
of
ponds and streams
played
in
the
transi-
tion
from the
crossopterygians
to early
amphibians is
by now
a
classic
example
of an
evolutionary
scenario.
Because
scenarios
are
presented
in
ordinary
an-
guage-supplemented
with a host
of
technical biological terms-the phenom-ena which scenarios can
describe are
lim-
ited
only by
the
limitations
of
language.
Hennig's early
cladograms
(Hennig,
1950:103;
1966:59, 71)
give every
ap-
pearance
of
being
highly stylized
trees.
The
circles
arrayed along
the top of
the
diagram
apparently
represent extant
species,
while
those
at
the
nodes
repre-
sent
extinct, ommon
ancestors
(see
Fig.
la).
At
one
stage
in
the
development
of
the cladogram, Nelson (1972b, 1973b) ar-
gued that the
circles at
the
nodes
did
not
represent
real common
ancestors
but
hypothetical
ancestors.
The
term is
problematic.
All
taxa that
ever
existed
are
equally
real.
Only
our
ability
to
discern
them varies.
We
can
usually discern
ex-
tant
species with
greater
certainty
than
extinct
forms,
but the
postulation of
any
taxon,
whether
extant or
extinct,
nvolves
highly
complex
inferences and
requires
evidence
which is
frequently quite dif-ficult to obtain.
(Recall
the
objections
which
pheneticists
raised to
the
biologi-
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422
SYSTEMATIC
ZOOLOGY
VOL. 28
A
B C
FIG.
1.-The
evolution of the cladogram. (a) A
cladogram
in
which the darkened circles
at the ter-
mini represent
extant
species,
while those at the
nodes represent
ommon
ancestors. b) A cladogram
in which
the darkened circles at the
termini rep-
resent species,
both extantand extinct,
while the
circles at the nodes represent hypothetical
com-
mon ancestors,
i.e., morphotypes. c)
A cladogram
in which the darkened
circles at the termini rep-
resent species, both extant
and extinct,while the
nodes represent
speciation events
and/or
he emer-
gence of unique derived
characters.
cal
species
concept
even when
it is
ap-
plied to extant
forms.)
f
extinctforms
re
hypothetical
because
of their inferen-
tial basis, then so
are extant
forms.The
difference
between
extant
and
extinct
species
is
not between observation
and
inference, but between
inferences of
varying degrees of
certainty.
Another
nterpretation f hypothetical
ancestor
is that he
circles
which
appear
at the nodes of cladograms are not sup-
posed
to
be taxa
at
all
but
morpho-
types,
rational constructs characterized
by
the
defining
traitsof the taxa
listed at
the termini
of the
cladogram
and
no
oth-
ers.
As Platnick
1977c:356)
observes,
the
nodes
of cladograms
representonly
in-
ferred
species (or,
more accurately, only
minimum sets of synapomorphic
charac-
ters).
In this sense, hypothetical
ances-
tors are
hypothetical,
but they are not
ancestors. They are not even taxa (see
Fig. lb).
In
present-day
cladograms,
all
taxa,
whether
extinct or extant, appear
along
the top
of the
cladogram.
Clado-
grams
would
be much
less
misleading
if
they
included nothing
at the nodes (see
Fig. ic).
If
the nodes
in
a cladogram
do
not rep-
resent real taxa
(ancestral
or
otherwise),
what do the
lines and
forks
n
cladograms
represent?
Two answers
have been
given
to this question, which may or may not
'be reducible
to the
same
answer.
A fork
in
a cladogram can
representeither
a
spe-
ciation vent
the splittingfone
species
intotwo),
or the emergence f an
evolu-
tionary
ovelty a unique derived
char-
acter),
or both.
If
the
emergence
of a
unique derived character
ccurs
always
and onlyat speciation, hen the twoan-
swers lways oincide.
f
not,not. imilar
observations
old for he term cladistic
relation.
t
can refer
o order f splitting
of taxa,
or to order fappearance
of evo-
lutionaryovelties, r
both.The decision
hinges
on how operational
ne wishes
to
make cladistic nalysis.Since
specia-
tion events
re inferred y
means
of
the
appearance
of evolutionary
novelties,
one
inferential
tep
can be eliminated
y
talking nly bouttheemergence fevo-
lutionarynovelties
and not speciation
events.
CERTAINTY
AND LEGITIMATE DOUBT
One reason
for he
cladists' ntroducing
the
distinction between
cladograms,
trees, and
scenarios
was
to clarify
he
technical
ense
in
which heywere
using
the term
cladogram.
Too manypeople
were misinterpretingladograms
s bi-
zarre, ighly tylized rees.A secondrea-
son was
to establish
he
ogical
and
epis-
temological
priority
f
cladograms
ver
trees nd
of reesover
cenarios.
As clad-
ists define
hese terms, n inclusion
re-
lation
xists etweenthem.
cenarios
n-
clide
all
the information
ontained
in
trees,
and more besides. Trees
include
all
the
nformationontained
ladograms,
and more
besides.
Thus, any knowledge
required
for
cladogram
s
required
for
a tree, nd anyknowledge equiredfor
tree
is
required
for
scenario,
but not
vice versa.
Thus,
cladograms
re more
certain
han
rees,
nd treesmore ertain
than
scenarios.
As harmless
s the dis-
tinction
etween cladograms, rees,
nd
scenariosmay
seem on
the
surface,
t
re-
sults
n
cladistic
nalysisbeing
in
some
sense
basic
to
all
evolutionary
tudies.
The relations
et outabove do
obtain
between
cladograms,
rees,
and scenar-
ios-as the cladists definethese terms.
That is one reason why non-cladists
might refer
therdefinitions. ut even
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1979
LIMITS OF
CLADISM
423
if
one were to accept the cladists'
defi-
nitions, ertain onclusionswhich
clad-
istshave claimedfollow rom he
preced-
ing ine ofreasoning o not.For
example,
in claiming hat trees should always
be
based on cladograms nd that cenarios
should follow rom rees,
Tattersall nd
Eldredge (1977:205)
are confusing ogi-
cal and
epistemological
rder
with tem-
poral order.The
conclusion
to
an argu-
ment
follows ogically
rom
ts
premises.
That does not mean
that
people
must
think f the premisesfirst nd only
then
thinkof the conclusion.Sense
data are
epistemologically
rior
o
all
knowledge,
but that
does
not mean
that
scientists
should beginevery cientifictudywith
an
extensive
nvestigation
f their own
sense data.
f scientists ad actually ro-
ceeded
in
this
fashion,
we would
still
be
awaiting Ptolemaic
astronomy, ot to
mention
elativityheory.
he
inferences
which
take
place
in
the
actual
course of
scientificnvestigations
re
very ntricate
and
frequently
feed
back
upon each oth-
er, as Tattersall nd Eldredge 1977:205)
note.
n
retrospect,
here s considerable
pointtounravelinghese nferencesnd
setting hem ut
n
some ogically
oher-
ent
fashion,
ut the insistence
hat
sci-
en-tists
must
always begin
at
the
begin-
ning and proceed step by step
according
to some sort f ogical or epistemological
order would
stop
science
dead
in its
tracks.2
Cladists
also
make
much
of
the
differ-
ences
in
certainty
etween
cladograms,
trees,
and
scenarios.
n
actual practice,
scientistswanderfromne level ofanal-
ysis to
another
n
the course of their
n-
vestigations.
As a result knowledge ac-
2The pheneticists
presented similar arguments
for the
epistemological
priority
f
phenetic
classi-
fications.
According to
the
pheneticists, the only
place at which systematists
an legitimately begin
is
phenetic characters nd
the construction f
pure-
ly phenetic
classifications. Once such
a phenetic
classification
is
erected,
then a systematistcould
put various interpretations on his classifications
and
possibly produce
one or more special pur-
pose
classifications; see e.g., Sneath
and Sokal
(1973).
quisition
s a
process of
both
reciprocal
illumination
Hennig,
1950;
Ross,
1974)
and
reciprocal
lundering.
etting
ne
element
right
elps
improve
ur
under-
standing fthe
other
lements, ut
errors
feed throughhe systemust as readily.
Although ladists
might
well
admit
that
knowledge
acquisition
in
general
is
a
process
of
reciprocal
llumination,
hey
also
tend
to be
so
sceptical
oftrees
and
scenariosthat
hey
see all
the
illumina-
tion
going in
a
single
direction-from
cladogramsto
trees and
scenarios. At
times
ladists eem
to
argue
hatnot
only
should
evolutionary
tudies
begin with
cladogramsbut
also
they should
end
there as well. Cladistic relations are
knowable;
verything
lse
about
phylog-
eny
s
unknowable.
For
example,
Platnick
1977d:439) ar-
gues
that
he
onlyway
that
hylogenetic
trees an
be tested
s
by the
same
means
used
to
test
cladograms,
by
evaluation
in
the
lightof
newly
discovered
harac-
ters.
Any
hree axa
can
be
ordered
nto
22
different
rees.
omeof
hese
rees an
be
rejected
by
discovering
ppropriate
characters,.e., the appropriate utapo-
morphies
and
synapomorphies.
Other
trees
can
be
rejected
only by
claiming
that
no
autapomorphies
xistfor
he rel-
evant
taxa.
Since
the most hat
system-
atist
an
legitimately
laim
s that
he has
yet
to
detect uch
autapomorphies,
e is
not
ustified n
rejecting
hese
trees.Plat-
nick
1977d:440) concludes
that
phylo-
genetic
rees
re
not estable
y character
distributionsnd
thus hat
cientific
hy-
logenyreconstructions not possible at
the evel
of
phylogenetic
rees nd must
be
restrictedo
the evel of
cladograms.
But
the
same
sort
f
rgumentan be
pre-
sented
against
he
cladists.
They
do
not
need
to
know
ancestor-descendantela-
tions,
ut
they
do
need
to establish
rans-
formation
eries nd
to
determine he
po-
larity
f
these
series.
They do
need to
individuate
haracters nd
decide which
are
autapomorphic,
ynapomorphic,
tc.
For instance, synapomorphies an be
used to
test
cladograms
f
they re
syn-
apomorphies,
ut
no one
has
yetto
sug-
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424 SYSTEMATIC ZOOLOGY VOL. 28
gest any infallible means
for
deciding
whether
r
not a trait s
actually syn-
apomorphy.
he same can
be said
for
such relations s
thepolarity
f transfor-
mation
eries. In fact,
Tattersall
nd El-
dredge 1977:206)state, In practice t s
hard, even
impossible, to
marshal a
strong,ogical
argument or
given
po-
larity
formany
characters
n a given
group. The
sortof
argument
rom eg-
ative
evidence
which the
cladists
use
againstothers t
the evel of taxa
can be
used
againstthem
at the
level of
traits.
Of
course,
their
pponents
lso
must n-
dividuate nd
categorize raits.
hus, the
cladists'
position s
refuted t
only
one
level of analysis,while the positionof
their
opponents s
refuted t
two.
Put
more
directly, uch
arguments efute o
positions
whatsoever.
The
cladists
have weakened
the force
of
their
rguments
y
presenting
hem
n
a
needlessly dichotomous
ashion.The
only
thing
hat
hey
need to
know
o pro-
duce
cladograms
re cladistic
relations,
and
they re
knowable.Others
who wish
to produce
trees
need to
know much
more; e.g., ancestor-descendantrela-
tions,
the
existence of
multiple specia-
tion and
reticulate
volution, tc. These
phenomena re
totally
nknowable. he
cladists
need not
present
such an ex-
treme
and
suspiciously
elf-serving)o-
sition. Estimations
f
cladistic
relations
are
inferences
nd
as such
carrywith
them he
possibility ferror.
owever, t
is certainly
ruethat
everything
hich
the cladists
need to know
the
evolution-
istsalso need toknow,whiletheevolu-
tionists
need to
know more besides.
These additional
phenomena
lso
carry
with
them
certain
egree
of
uncertain-
ty.
Hence,
evolutionary lassifications
are
bound to be
moreuncertain
han
la-
distic
classifications. he
question re-
mains
whether
ll
evolutionary
henom-
ena
other han
cladisticrelations
re so
uncertain hat
o
attempt
houldbe made
to
include
informationbout
them n a
classification.n thesucceeding ections
of this
paper,
I
will
investigate
arious
phenomenawhich
cladists
rgue cannot
be
known or
at
least
cannot
be known
with
ufficient
ertainty)
o
see
why hey
are so
difficult
o discern. s it
that
hey
cannotbe
known, r
that
hey
annotbe
knownby
means of
cladistic
nalysis?
s
therenowaytoacquireknowledge f he
world
other than
by
cladistic
analysis?
The
pointof
his
ection s,
however, hat
the
difference
n
our
ability o ascertain
cladistic
elations nd
other
volutionary
phenomena s
one of
degree,not
kind.
Differences
r1
degree of
certainty re
neither
eryneat
nor
esthetically.
leas-
ing,but
theyare
the
most
thatcladists
can
justifiably
laim. What
they
ose
in
elegance
and
simplicity,
hey gain
in
plausibility.
Certain
ladists,
n
more
recent
publi-
cations,
eem
to be
moving
n
precisely
this
direction. For
example,
Eldredge
(1979:169)
states hat,
ontraryo
hisear-
lier
opinions:
I
no
longer
oppose the
construction
f
phyloge-
netic trees
outright,or
for
that
matter,
scenar-
ios (which
are,
after
ll,
the
most
fun),
but
mere-
ly point
out
that,
in
moving
through the
more
complex
levels,
we
inevitably
become further e-
moved from he original data base in adding as-
sumptions
and ad
hoc
(and
largely
untestable)
hypotheses.
As
long as we
understand
precisely
what
we are
doing
at each
step
in
the
analysis,
which includes
having
an
adequate
grasp
of the
probability
that
we are
wrong
and
of what the
assumptions are what we have
added
along
the
way,
there no
longer
seems to me
any
reason
for
anyone to
tell anyone
else what
notto
do.
Although hoseworkers
ngaged
n
the
production f
trees and
scenarios
might
differwith Eldredge on the situationbeingas extreme s he makes touttobe,
they oo
are aware
ofthe
difficulties.
or
example,
Lucchese
(1978:716)
concludes
a
paper on the
evolution
f sex
chromo-
somes
with
he
followingemarks:
...
for
n
individual
to
profess
n
insight
nto
the
biological
changes
that
have
occurred
through
time
remains
an
act of faith
of
considerable mag-
nitude.
[Yet,
within]
the
limitationsust
set forth,
the
purpose
of
this
article
has been
to
spin a rel-
ativelyreasonable evolutionarytale in the hope
of
expanding
the
current
perception of a
highly
significant
xample
of genetic
regulation
in eu-
karyotes.
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11/26
1979
LIMITS
OF CLADISM
425
THE
PRINCIPLE
OF
DICHOTOMY
Few
principles
attributed
to cladistic
taxonomists
have caused
more
conster-
nation and
confusion
than the claim
that
all cladogramsand classificationsmust be
strictly
dichotomous.
When cladism
was
first
ntroduced
to English-speaking
sys-
tematists,
cladists
and anti-cladists
alike
agreed
that the principle
of
dichotomy
was
essential
to
the
philosophy
of
Hen-
nig and
Brundin
(Nelson,
1971a:373;
see
also
Darlington,
1970:3;
Mayr,
1974:
100;
Bonde,
1975:302;
Platnick,
1977d:438).
Although
Cracraft
1974:79)
agrees
that phylogenetic
classification
sensu Hennig and Brundin would appear
to require
the
assumption
of dichotomous
branching,
cladistic
classification
in
a
more
general
sense
does
not necessitate
dichotomous
branching,
and the exact
pattern
f branching
s determined
by
the
manner
in
which
shared
derived
charac-
ter-states
cluster
taxonomic
units. The
question
remains
why
the principle
of
dichotomy
has seemed
so central
to most
cladists
and continues
to seem
so to
some.
Although
no
cladist
has ever
main-
tained
that the
principle
of
dichotomy
s
an empirical
claim
about the
speciation
process,
few
cladists
have
been
able
to
resist the
temptation
to justify
t
by ref-
erence to empirical
considerations.
For
example,
Hennig (1966:210,
211) begins
by stating
hatdichotomy
s
primarily
no
more than
a methodological
principle
and then goes
on
to
add,
A
priori
it is
very improbable that a stem species ac-
tually
disintegrates
nto several
daughter
species
at once.
Cracraft
1974:79)
in-
terrupts
the
discussion quoted
above
with the
remark
that the
principle
of
di-
chotomy
can
be
justified
on
theoretical
grounds
not
associated
with classifica-
tion.
Bonde
(1975:302)
agrees.
Although
dichotomy
s
a
methodological
principle,
in nature
speciations
are probably
near-
ly
always
dichotomous.
Whether multiple speciation seems
possible
or
impossible
depends
on
the
unit
of
time one
selects to
define
simul-
taneous.
As
Hennig
(1966:211)
notes,
the
issue
of dichotomous
speciation
can
be trivialized
by defining
simulta-
neous
too
narrowly.
If
it
is defined
in
terms
of split
seconds,
then
multiple
spe-
ciation is impossible. Conversely, if si-
multaneous
is
defined
too
broadly,
everything
an be
made to occur
at
the
same
time.
The question
of
dichoto-
mous speciation
can
be
made significant
only
by
the specification
of a unit of
time
which
makes
sense
in
the context of
the
evolutionary
process.
In the absence
of
such
a unit,
the notion
of
simultaneity
can
be
expanded
or contracted
t will
and
the
issue
decided
by
fiat
see
Cracraft,
1974:74).
Whether
multiple speciation
as an em-
pirical
phenomenon
seems plausible
or
implausible
depends
on
the
model
(or
models)
of
speciation
which one
holds.
If
speciation
is viewed
as the
gradual
split-
ting
and
divergence
of
large
segments
of
a species,
as Hennig (1966:210)
main-
tains,
then
multiple
speciation
looks
less
likely
than
if it is
viewed
as
a
process
in
which
small,
peripheral
populations
be-
come isolated and develop into separate
species,
as
Wiley (1978:22)
supposes.
But, as
the cladists
have reminded
us
often
enough,
dichotomy
is
a
methodo-
logical
principle.
If
speciation
were
al-
ways
dichotomous,
this fact
about
evor
lution
would
certainly
lend support
to
dichotomy
s
a methodological
principle,
but
good
reasons might
exist
for
produc-
ing exclusively
dichotomous
cladograms
and
classifications
even
if
speciation
is
not always dichotomous.
Cladists
have
presented
two sorts
of
justification
for
their preference
for the
principle
of
dichotomy:
our inability
to
distinguish
dichotomous
speciation
from
more
complex
sorts
of speciation
and our
inability
to
represent
unequivocally
more
complex
sorts
of speciation
in
cladograms
and classifications.
If
one
limits
oneself
ust to
traditionally-defined
traits
e.g.,
presence
of
various sorts
of
appendages, dentitions,etc.) and if one
assumes
that these
traits have
been
ap-
propriately
individuated
and identified,
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12/26
426
SYSTEMATIC ZOOLOGY
VOL.
28
then he argument hichthe
cladistsput
forth
s
reasonably traightforward.
f a
cladist
tarts ith
nytwo pecies
chosen
at random nd compares third
pecies
to them, wo ofthese species
will be cla-
disticallymore closely relatedto each
other
than
either
is
to the remaining
species.
If
the cladist continues o add
species to his study ne at a time,he will
produce a consistently dichotomous
cladogram
ntil
ne oftwothings ccurs:
either he happens upon
a
genuine
in-
stance of multiple peciation
or else he
is unable to resolve this complex situa-
tion nto he ppropriateequence
of uc-
cessive dichotomies.
ecause dichotomy
is the simplesthypothesis,t should al-
ways
be
preferred,
vidence
permitting.
But as the cladists see it, giventhe sort
of
data available to the
systematist
nd
the resolvingpower of his methods of
analysis, e
will
always
end
up
wherehe
begins-with
doubt
or
with
dichotomy.
Never is
a
systematistustified
n
con-
cluding
that
speciation
vent
s
genu-
inely trichotomous ecause he is never
justified
n
dismissing hepossibilityhat
an apparent richotomys really pair of
unresolveddichotomies.
If
the preceding
s a fair
haracteriza-
tion of the cladists'argument,
t has
two
weaknesses.
First,
he cladists re
selec-
tive
in
acknowledging ossible
sources
of error.After
ll,
a
systematist
annot
guarantee
hat trait
which
he considers
to be unique and derived actually is.
There
is
always the possibility
hathe
is
dividing singlegroup
nto woor ump-
ing two groups nto one. Ifa trichotomy
represents ither genuine richotomyr
two unresolveddichotomies, hen a
di-
chotomy
could
just
as
well
represent
either
genuinedichotomy,
lumped
ri-
chotomy,
r a
single ineage
dividedmis-
takenly nto
two. When
a
systematist
claims that
particular peciation
vent
is trichotomous,e maybe mistaken, ut
he
may
lso
be
mistaken
n
claiming
hat
it s dichotomous. uring heearly tages
of nvestigation,oubtsaboutthe actual
cladistic elationswhichcharacterize he
groups under study are legitimate.
Many
trichotomies re very likely to
be resolv-
able into
dichotomies
after
urther tudy,
many of the groups treated as
single may
have to be split
into two or
more groups,
and vice versa. However, as the groups
are studied
more exhaustively, the legit-
imacy of continued doubt decreases.
If
after xhaustive study,three
species con-
tinue to appear
to be related
trichoto-
mously, the claim
that they mightactual-
ly be related by a pair of dichotomies
begins to ring rather
hollow. Descartes
has shown where that sort of
mindless
scepticism
leads.
If
a
systematist
an be-
come
reasonably certain
that
he
has
cor-
rectly discerned a genuine dichotomy,
see no
reason why
he
cannot also
attain
that same
level of certainty
n the iden-
tification of
trichotomies-even
using
just the
traditional methods of cladistic
analysis.
But
systematists re not
limited
just to the study
of ordinary taxonomic
traits.There
is
no reason fornot
using the
methods of
cladistic analysis on other
sorts of evidence, e.g.,
the
data
of histor-
ical biogeography.
As Platnick and Nel-
son (1978: 10) remark:
Trichotomous
cladograms are of no signifi-
cance
as tests
or
as initial
hypotheses)
unless
the
cladograms
for
all available
test
groups
are
tri-
chotomous,
in
which case we
may
suspect
that
an event disconnected
an area
into
three smaller
areas
simultaneously. Testing
this
hypothesis by
biotic relationships
seems
impossible (because
we have no
way
of
distinguishing
those clado-
grams reflecting
ctual trichotomiesfrom those
reflecting
only our
failure
to
find the relevant
synapomorphies
that would
resolve
a dichoto-
mous cladogram),but it should be subject to in-
dependent testing by
data
from
historical
geolo-
gy,
which
can
either be
in
accordance
with such
a
synchronoustripartite
isconnection of
the
to-
tal area or not.
If
multiple
speciation can occur
in na-
ture and
if n
certain circumstances
t can
be
discerned,
then cladists would be
wise to devise
methods of
representing
multiple speciation
in
their cladograms
and
classifications.One theme
which the
cladists repeatedlyvoice is thatanything
represented
in
a
cladogram
and
classifi-
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1979
LIMITS OF CLADISM
427
cation should be
represented xplicitly
andunambiguously.fcladistswere will-
ing to arguethat richotomouspeciation
never took place, then trichotomiesn
cladograms
would not be
ambiguous.
They would always represent pair of
unresolveddichotomies.
s
things tand
now, they are ambiguous,representing
either
genuine richotomyr
else a
pair
of unresolveddichotomies.
Whether
r
not cladists
think
hat nstances
f
mul-
tiple speciation can be discerned, the
adoptionof such
an
ambiguousmode of
representation eems counter-produc-
tive. A better lternative ould be to re-
serve
trichotomies or representing
ri-
chotomouspeciation vents hewaythat
dichotomies are used to representdi-
chotomous
peciationevents,epistemo-
logical doubts o one side, and to devise
another
way
of
indicating ncomplete
knowledge.For example,
f
systematist
is reasonably ure that two species are
each other's
losest relatives,
e can
in-
dicate
this
by
a
dichotomy.
f
he thinks
--they ight
e but
s
not ure,
he can
con-
nect them
y dots.
Similar emarks
pply
to trichotomiessee Fig. 2). No system f
representation
an
represent verything,
but
if
doubt
s
importantnoughtorep-
resent
in
a
cladogram,
t
is
important
enough
to
represent nequivocally.
Similar
emarks re equally applicable
to classifications. or example, Nelson
(1973c) distinguishes
etween
the
two
sorts
f
representationommonly
sed
in
the Linnaean
hierarchy-subordination
and
sequencing-and
shows
how
com-
binationsof these two conventions an
represent variety f evolutionary he-
nomena including trichotomouspecia-
tion. More
recently,Wiley (1979)
has
suggested using
sedis mutabilis when-
ever the order
of
isting
axa
n
a
classi-
fication
means
nothing.
n
addition o the
difficulties hich he
recognition
f
mul-
tiple speciation
aises
for
ladograms
nd
classifications,
t
also
poses problems
for
theproperdefinition f monophylynd
related terms (Wiley, 1977; Platnick,
1977c).
A B 7
c
FIG.
2.-The ambiguitybetween genuine trichot-
omies and unresolved
dichotomies. (a) A clado-
gram which represents
unambiguously a pair of
successive dichotomies;
epistemological doubts, if
any, are not indicated.
(b) A cladogram which is
commonly used to represent a pair of unresolved
dichotomies, a genuine trichotomy, ommon ances-
try nd reticulate evolution.
(c)
A
cladogram which
indicates doubt about the
actual relations between
the species indicated.
Such cladograms might in
time resolve to (a) a pair
of dichotomies or (b) a
genuine trichotomy.
ommon ancestry and reticu-
late evolution must be
represented in some other
but equally unambiguous fashion.
ANCESTOR-DESCENDANT
RELATIONS
Although some disagreement exists
over the
occurrence of
multiple specia-
tion, everyone
agrees thatcertain species
which
once existed no
longer
exist and
that some of these
species gave rise to
Recent species. The
cladists' complaints
about the
ancestor-descendant relation
cannot possibly be interpreted s empir-
ical. They are clearly methodological.
Once again the distinctionmust be made
between difficulties n
discerning
ances-
tor-descendant relations and
difficulties
in representing them.
Farris (1976:272)
acknowledges
this
distinction when
he
presents two reasons fornot treatingfos-
sil
species
as
ancestral
to later species:
First, there
is
no obvious way of using a
classi-
fication
to
represent ancestor-descendant
rela-
tionships.A taxonomic ierarchy ith tswell-
nested axa s naturally
uitedonly o the repre-
sentation
f sister-groupelationships.econd
while
ister-group
elationships ay
more
r ess
readilybe established hroughhedetection f
apomorphous
imilarities, ncestor-descendant
relationships aynot
be so established.
A
second
distinction
of
equal impor-
tance
is between the
recognition
of
ex-
tinct
species
as
species
and
deciding
which
species gave
rise to which.
Those
cladists who are willing to recognize ex-
tinct
species
as
species
while
maintain-
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428
SYSTEMATIC
ZOOLOGY VOL. 28
ing that he
ancestor-descendantelation
is unknowable
re put
in
the
positionof
explaininghow we can
knowthe former
but
notthe atter
Engelmann
nd
Wiley,
1977).
Those cladistswho
arguethat
nei-
ther s knowable re spared hisdilemma
but
are faced
with the
problem
of
what
to do with
fossils.L0vtrup
1973) argues
that
xtinctpecies are so
poorlyknown
that hey hould
be totally xcluded
from
classifications. rowson
1970) sees the
differences
n
our
ability
o
recognize x-
tinct nd extant
orms o be
sufficiently
great
hat hey
houldbe included
n
sep-
arate classifications.
elson
(1972a:230)
reasons
hat ossilgroups
houldbe fitted
intoclassifications hich lso contain e-
cent
groups
but
distinguished
y
some
convention.
Finally, Rosen
(1978:176)
agrees that extant and
extinct forms
should be
distinguishedbut that
they
shouldbe
treated
s
the same sorts
fen-
tities-groups
distinguished y
unique
derived
characters.
n
this
section
will
deal
first
with difficultiesn
discerning
species
and
ancestor-descendant
ela-
tionsand then
withproblemsof
repre-
sentation.
Hennig
(1966, 1975)
and
Brundin
(1972b) maintain
that
the biological
species
concept
s
central
o
phylogenet-
ic systematics.rom
the
beginning,
rit-
ics of
thebiological
pecies concepthave
argued
that t is
not
sufficiently
opera-
tional even
for
xtant
pecies.
Perhaps
it is
possible to
ascertainwhich
organ-
isms actually
have mated with
one
another
o producefertile
ffspring
nd
whichorganisms annotmate witheach
other nd/or
roduce
fertile
ffspring,
ut
it
is impossible
to discoverwhich
could
have done so but did
not.
When the
species
under
investigation
re
extinct,
so
the critics
rgue,
nothing
an be
dis-
covered
directly
bout their
reeding
e-
lations.
t
is
certainly
rue that
the bio-
logical species
concept
is
not very
operational,
ut
as
I
have
argued
else-
where,
no
theoretically ignificant
on-
cept n science s (Hull, 1968, 1970).The
interesting uestion
is whether
iologi-
cal
species
can be
discerned often
enough and with sufficientertainty o
justify ystematists'ttemptingo repre-
sent hem
n
their
lassifications.
n
even
more fundamental uestion is whether
biological species
are
sufficientlympor-
tantto the evolutionary rocess to war-
rant he attention hey re given. But in
any case, the existence nd extent f bi-
ological species are almost always in-
ferred
ndirectly
rom
haracter istribu-
tions. Hence, mistakes will be made.
When these species are extinct,mistakes
are even
more ikely
and
our abilityto
correct hem ven more imited.No one
could possibly laim that
ll
pterodactyls
belonged
to
a
single species,
but
finer
distinctionsrehighly roblematic.
Thus, those cladists who continue to
maintain hatbiological species are the
ultimate nits
n
their nvestigationsre
forced
o admit hat heir
ladogramsnd
classificationsmightbe mistaken.They
mighthave lumped several species into
one
or divided a
single species
into
two
or more
pecies.
For extinct
orms,
hese
mistakes re difficult,
f
not impossible,
to remedy. ladistswho have abandoned
the biological pecies concept re spared
this problem. They are not classifying
species
but are
grouping rganisms
c-
cording
to
the
possession
of
particular
traits.The
only
doubt associated with
such an
exercise
is
whether
he
organ-
isms actuallyhave the trait ttributed o
them,whether r not
t s
actually
trait
and
not several,
and
whether
t
has
the
distribution
ttributedo
t.
The
question
remainswhether
t is
any easier to indi-
viduate raits han t s to ndividuatehe-
oretically ignificant
axa.The two
seem
both
conceptually
nd
inferentiallyery
closely connected.
If
extinct pecies cannot e recognized
as
genuinegroups
f
some
sort, hey
an-
not be recognized
s
sistergroups.
But
assuming
hat
extinct orms
an
be rec-
ognized
as extinct
pecies
or
as
extinct
groupsdelimited y
a
particularrait,
an
ancestor-descendant
elations
between
extinct and extant forms lso be dis-
cerned?Cladists
re
all butunanimous
n
claiming
that
they
cannot.
As
Nelson
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8/10/2019 The Limits of Cladism- by David L. Hull Systematic Zoology, Vol. 28, No. 4 (Dec., 1979), pp. 416-440.
15/26
1979
LIMITS
OF
CLADISM
429
(1973b:311) states, Indeed,
I
have as-
sumed that ancestral species cannot be
identified s such in the fossil record, and
I have pointed out that this assumption
is fundamental to Hennig's phylogenetic
systematics. Two considerations seem
central to the
cladists'
claim
that
ances-
tor-descendantrelations are unknowable.
First, any distribution of characters
which would
imply
that one
taxon
was
ancestral to
another
would equally imply
that one was the sister group of the other,
and no other way
exists for
deciding
ancestor
relations.
Second, given
the
vast
number of species which
must have ex-
isted from he beginning of time and the
relatively few which have left records, it
seems very unlikely
that
very many
ac-
tual common ancestors have been discov-
ered.
I
find the second argument more
top related