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[ Pobierz całość w formacie PDF ]Philosophy of Science Association
Evolution, Population Thinking, and Essentialism
Author(s): Elliott Sober
Source: Philosophy of Science, Vol. 47, No. 3 (Sep., 1980), pp. 350-383
Published by: The University of Chicago Press on behalf of the Philosophy of Science
Association
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EVOLUTION,
POPULATION
THINKING,
AND
ESSENTIALISM*
ELLIOTT
SOBERt
University
of
Wisconsin-Madison
Ernst
Mayr
has
argued
that
Darwinian
theory
discredited essentialist modes
of
thought
and
replaced
them with what he has called
"population thinking".
In this
paper,
I characterize essentialism as
embodying
a certain
conception
of how variation in nature is to be
explained,
and show how this
conception
was undermined
by evolutionary theory.
The Darwinian doctrine of evolution-
ary gradualism
makes
it
impossible
to
say exactly
where one
species
ends
and another
begins;
such
line-drawing problems
are often taken to
be the
decisive reason for
thinking
that essentialism is untenable.
However,
according
to the view of essentialism I
suggest,
this familiar
objection
is not fatal
to essentialism. It is rather the essentialist's use of what I call the natural
state model for
explaining
variation which clashes with
evolutionary theory.
This model
implemented
the essentialist's
requirement
that
properties
of
populations
be defined in terms of
properties
of member
organisms. Requiring
such constituent
definitions
is reductionistic in
spirit; additionally, evolutionary
theory
shows that such definitions are not
available, and, moreover,
that
they
are not needed to
legitimize population-level concepts. Population thinking
involves
the thesis that
population concepts may
be
legitimized by showing
their connections with each
other,
even when
they
are not reducible to
concepts
applying
at lower levels of
organization.
In the
paper,
I
develop
these
points
by describing
Aristotle's ideas on the
origins
of
biological variation; they
are a classic formulation of the natural state model. I also describe how
the
development
of statistical ideas in the 19th
century
involved an
abandoning
of the natural state model.
1. Introduction.
Philosophers
have tended to discuss essentialism as
if it were a
global
doctrine-a
philosophy
which,
for some uniform
reason,
is to be
adopted by
all the
sciences,
or
by
none of them.
Popper (1972)
has taken a
negative global
view because he sees
essentialism as a
major
obstacle to scientific
rationality.
And
Quine
(1953b), (1960),
for a combination of semantical and
epistemological
reasons,
likewise wishes to banish essentialism
from
the
whole of
scientific discourse. More
recently, however,
Putnam
(1975)
and
Kripke (1972)
have advocated essentialist doctrines and have claimed
that it is the task of each science to
investigate
the essential
properties
of its constitutive natural
kinds.
*Received
April 1979;
revised December
1979.
tSuggestions
made
by
William Coleman, James Crow, Joan
Kung,
David Hull,
Geoffrey Joseph,
Steven
Kimbrough,
Richard
Lewontin,
Ernst
Mayr,
Terrence
Penner,
William
Provine,
Robert
Stauffer,
Dennis
Stampe
and Victor Hilts
helped
me consider-
ably
in
writing
this
paper.
Philosophy of
Science,
47
(1980)
pp.
350-383.
Copyright
? 1980
by
the
Philosophy
of Science Association.
350
EVOLUTION,
POPULATION
THINKING,
AND ESSENTIALISM
351
In contrast to these
global viewpoints
is a tradition which sees
the
theory
of evolution as
having
some
special
relevance to essentialist
doctrines within
biology.
Hull
(1965)
and
Mayr (1959)
are
perhaps
the two best known
exponents
of this
attitude; they
are local anti-es-
sentialists. For
Mayr,
Darwin's
hypothesis
of evolution
by
natural
selection was not
simply
a new
theory,
but a new kind
of theory-one
which discredited essentialist modes of
thought
within
biology
and
replaced
them
with what
Mayr
has called
"population
thinking".
Mayr
describes essentialism as
holding
that
...
[t]here
are a limited number of
fixed, unchangeable
"ideas"
underlying
the observed
variability [in nature],
with the eidos
(idea) being
the
only thing
that is fixed and
real,
while the observed
variability
has no more
reality
than the shadows of an
object
on a cave wall . . .
[In contrast],
the
populationist
stresses the
uniqueness
of
everything
in the
organic
world. . . . All
organisms
and
organic phenomena
are
composed
of
unique
features and
can be described
collectively only
in statistical terms.
Individuals,
or
any
kind of
organic
entities,
form
populations
of
which we
can determine the arithmetic mean
and the statistics of variation.
Averages
are
merely
statistical
abstractions, only
the individuals
of which the
population
are
composed
have
reality.
The ultimate
conclusions of the
population
thinker and of the
typologist
are
precisely
the
opposite.
For the
typologist
the
type (eidos)
is real
and the variation an
illusion,
while for the
populationist,
the
type
(average)
is an abstraction and
only
the variation is real. No two
ways
of
looking
at nature could be more different.
(Mayr
1959,
pp. 28-9).
A
contemporary biologist reading
this
might
well conclude that essen-
tialists had no
scientifically respectable way
of
understanding
the
existence of variation in nature. In the absence of
this,
typologists
managed
to
ignore
the fact of
variability by inventing
some
altogether
mysterious
and unverifiable
subject
matter for themselves. The notion
of
types
and the kind of
anti-empiricism
that seems to
accompany
it,
appear
to bear
only
the most distant connection with modern
conceptions
of evidence and
argument.
But this reaction raises a
question
about the
precise
relation of evolution to essentialism. How
could the
specifics
of a
particular
scientific
theory
have mattered
much
here,
since the main obstacle
presented by
essentialist
thinking
was
just
to
get people
to be scientific about nature
by paying
attention
to the evidence? The
problem
was to
bring people
down to earth
by rubbing
their noses in the
diversity
of nature. Viewed in this
352
ELLIOTT SOBER
way, Mayr's position
does not look much like a form of local
anti-essentialism.
Other
perplexities
arise when a
contemporary biologist
tries to
understand
Mayr's
idea of
population thinking
as
applying
to his or
her own
activity.
If
"only
the individuals of which the
population
are
composed
have
reality,"
it would
appear
that much of
population
biology
has its head
in
the clouds. The Lotke-Volterra
equations,
for
example,
describe the interactions of
predator
and
preypopulations.
Presumably, population thinking, properly
so
called,
must allow that
there is
something
real over and above individual
organisms. Population
thinking
countenances
organisms
and
populations; typological thinking
grants
that both
organisms
and
types
exist. Neither embodies a resolute
and
ontologically
austere focus on individual
organisms
alone. That
way
lies
nominalism,
which
Mayr (1969)
himself
rejects.
Another issue that arises from
Mayr's conception
of
typological
and
population
thinking
is that of how we are to understand his
distinction between
"reality"
and "abstraction." One natural
way
of
taking
this distinction is
simply
to understand
reality
as
meaning
existence. But
presumably
no
population
thinker will
deny
that there
are such
things
as
averages.
If there are
groups
of
individuals,
then
there are numerous
properties
that those
groups possess.
The
average
fecundity
within a
population
is no more a
property
which we invent
by
"mere abstraction" than is the
fecundity
of individual
organisms.
Individual and
group properties
are
equally
"out there" to be discov-
ered. And
similarly,
it is unclear how one could
suggest
that
typologists
held that
variability
is
unreal; surely
the historical record shows that
typologists
realized that differences between individuals exist.
How,
then,
are we to understand the difference between essentialism and
population thinking
in terms of what each holds to be "real" about
biological reality?
Answering
these
questions
about the difference between essentialist
and
population
modes of
thought
will
be
the main
purpose
of this
paper.
How did essentialists
propose
to account for
variability
in
nature? How did
evolutionary theory
undermine the
explanatory
strategy
that
they pursued?
In what
way
does
post-Darwinian
biology
embody
a novel
conception
of
variability?
How has
population thinking
transformed our
conception
of what is real? The form of local
anti-essentialism which I will
propound
in what follows will be
congenial
to
many
of
Mayr's
views. In one
sense, then,
our task
will be to
explicate
and
explain Mayr's insight
that the shift from essentialist
to
populationist
modes of
thinking
constituted a shift in the
concept
of
biological reality.
However,
I will
try
to show
why
essentialism
was a
manifestly scientific working hypothesis. Typologists
did not
EVOLUTION,
POPULATION
THINKING,
AND ESSENTIALISM
353
close their
eyes
to variation but rather tried to
explain
it in a
particular
way.
And the failure of their
explanatory strategy depends
on details
of
evolutionary
theory
in
ways
which have not been much
recognized.1
The
approach
to these
questions
will be somewhat historical.
Essentialism about
species
is
today
a dead
issue,
not because there
is no conceivable
way
to defend
it,
but because the
way
in which
it
was defended
by biologists
was
thoroughly
discredited. At first
glance,
rejecting
a
metaphysics
or a scientific research
program
because
one of its formulations is mistaken
may appear
to be fallacious.
But more careful attention vindicates
this
pattern
of evaluation. It
is
pie-in-the-sky metaphysics
and science to hold on to some
guiding
principle simply
because it is
possible
that there
might
be some
substantive formulation and
development
of it.
Thus, Newtonianism,
guided by
the maxim that
physical phenomena
can
be
accounted for
in terms of matter in
motion,
would have been
rejected
were it not
for the success of
particular
Newtonian
explanations.
One evaluates
regulative principles by
the
way
in which
they regulate
the actual
theories of scientists. At the same
time,
I will
try
in what follows
to
identify precisely
what it is in essentialism and in
evolutionary
theory
that makes the former a victim of the latter. It is an
open
question
to what
degree
the source of this
incompatibility
struck
working
biologists
as central.
As I will
argue
at the end of this
section,
one
diagnosis
of the situation which seems to have been
historically
important
is much less decisive than has been
supposed.
The essentialist's method of
explaining variability,
I will
argue,
was
coherently
formulated in
Aristotle,
and was
applied by
Aristotle
in both
his
biology
and in his
physics.
17th and 18th
century biologists,
whether
they argued
for evolution or
against
it,
made use of Aristotle's
Natural State Model. And to this
day,
the model has not been
refuted
in mechanics. Within
contemporary biology, however,
the model met
with less success. 20th
century population genetics
shows that the
model cannot be
applied
in the
way
that the essentialist
requires.
But the Natural State Model is not
wholly
without a home in
contemporary
biology;
in
fact,
the
way
in which it finds an
application
there
highlights
some salient facts about what
population thinking
amounts to.
An essentialist view of a
given
species
is committed to there
being
some
property
which all and
only
the members of that
species possess.
'Mayr (1963)
has
argued additionally
that essentialist errors continue to be made
in
population biology
in the form of the distortions of
"bean-bag genetics."
The
assumption
that the fitness of
single genes
is
independent
of their
genetic
context
is and has been known to be
mistaken;
but how this
simplifying assumption
is essentialist
in character is obscure to me.
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