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Author: jjh00@eng.amdahl.com (Joel Hanes)
Title: What, if anything, is "Darwinism"?
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In _One_Long_Argument_, Ernst Mayr (evolutionary
biologist, and originator of the Biological Species Concept)
summarizes Darwin's theories, and traces the history of their
acceptance by the world scientific community.
In the Preface , he begins:
`A modern evolutionist turns to Darwin's work again and
again. This is not surprising, since the roots of all
our evolutionary thinking go back to Darwin. Our current
controversies very often have as their starting point some
vagueness in Darwin's writings or a question Darwin was
unable to answer owing to the insufficient biological
knowledge available in his time. But one returns to
Darwin's original writings for more than historical
reasons. Darwin frequently understood things far more
clearly than both his supporters and his opponents,
including those of the present day.'
In Chapter Four, "Ideological Opposition to Darwin's Five Theories",
Mayr summarizes "Darwin's Theory", or "Darwinism", thus:
`In both scholarly and popular literature one frequently
finds references to "Darwin's theory of evolution", as
though it were a unitary entity. In reality, Darwin's
"theory" of evolution was a whole bundle of theories,
and it is impossible to discuss Darwin's evolutionary
thought constructively if one does not distinguish its
various components.
... The term "Darwinism", ... has numerous meanings
depending on who has used the term and at what period.
A better understanding of the meaning of this term is
only one reason to call attention to the composite
nature of Darwin's evolutionary thought.
... One particulary cogent reason why Darwinism cannot
be a single monolithic theory is that organic evolution
consists of two essentially independent processes, as we
have seen: transformation in time, and diversification
in ecological and geographical space. The two processes
require a minimum of two entirely independent and very
different theories.
... I consider it necessary to dissect Darwin's conceptual
framework of evolution into a number of major theories that
formed the basis of his evolutionary thinking. For the
sake of convenience, I have partitioned Darwin's
evolutionary paradigm into five theories, but of course
others might prefer a different division. The selected
theories are by no means all of Darwin's evolutionary
theories; others were, for instance, sexual selection,
pangenesis, effect of use and disuse, and character
divergence. However when later authors referred to Darwin's
theory thay invariably had a combination of some of the
following five theories in mind:
(1) _Evolution_as_such_. This is the theory that the world
is not constant or recently created nor perpetually
cycling, but rather is steadily changing, and that
organisms are transformed in time.
(2) _Common_descent_. This is the theory that every group
of organisms descended from a common ancestor, and
that all groups of organisms, including animals, plants,
and microorganisms, ultimately go back to a single origin
of life on earth.
(3) _Multiplication_of_species_. This theory explains the
origin of the enormous organic diversity. It postulates
that species multiply, either by splitting into daughter
species or by "budding", that is, by the establishment
of geographically isloated founder populations that
evolve into new species.
(4) _Gradualism_. According to this theory, evolutionary
change takes place through the gradual change of
populations and not by the sudden (saltational)
production of new individuals that represent a new
type.
(5) _Natural_selection_. According to this theory,
evolutionary change comes about throught the abundant
production of genetic variation in every generation.
The relatively few individuals who survive, owing
to a particularly well-adapted combination of
inheritable characters, give rise to the next
generation.'
--------------------------------------------------
Let's look at some of the implications of Mayr's analysis.
At first blush, (4) _Gradualism_ seems like it might conflict
with Gould & Eldredge's "punctuated equilibrium" theory;
but on closer examination, not so.
Here [thanks to Robert Low] are two relevant quotes from
_On_The_Origin_Of_Species_:
` ... it is probable that the periods, during which each
[species] underwent modification, though many and long
as measured by years, have been short in comparison with
the periods during which each remained in an unchanged
condition.' (from the final 6th edition, 1872)
`Varieties are often at first local...rendering the discovery
of intermediate links less likely. Local varieties will not
spread into other and distant regions until they are considerably
modified and improved; and when they do spread, if discovered in
a geological formation, they will appear as if suddenly created
there, and will simply be classed as new species.'
Darwin did not claim that evolutionary change is slow and
continuous -- only that it does *not* proceed by "jumps"
_in_a_single_generation_ (what Mayr calls "saltational" change).
That is, despite the distortions of some anti-evolutionists,
Darwin explictly did not think that evolution proceeds by the
production of "hopeful monsters" -- Darwin himself
never proposed that a fully-dinosaur parent
gave birth to fully-bird progeny. Rather, the change took
place in a series of intermediate, perhaps nearly insensible,
steps in successive generations. Note that change over a
thousand generations of any species appears as "sudden" or
"abrupt" change in the fossil record, because a thousand generations
is such an infinitesimally small fraction of Earth's history.
(5) _Natural_selection_, doesn't account for some of the kinds
of variation that we see in species -- particularly non-adaptive
traits -- but you'll notice that Darwin didn't claim that
natural selection explained all traits, merely the adaptive
ones.
After Darwin, some biologists distorted the theory of natural
selection into the doctrine of "strict adaptionism", in which
every feature of every organism was held to be produced by
natural selection, (and thus some explanation of why the
feature is adaptive was required.) But Darwin didn't say
that _all_ selection is natural (adaptive) selection -- only
that natural selection is the source of _some_ change, and
can explain why adaptive change occurs. Modern biologists
have proposed other mechanisms for change -- neutral selection,
genetic drift, the "founder effect", etc., and Darwin himself
thought that sexual selection could be important. None of these
contradict the idea of natural selection -- they augment it,
as additional mechanisms for genetic change over time.
Here [thanks to Ken Smith] is a quote from the final chapter
of the sixth edition of _On_The_Origin_of_Species_:
`But as my conclusions have lately been much misrepresented, and
it has been stated that I attribute the modification of species
exclusively to natural selection, I may be permitted to remark
that in the first edition of this work, and subsequently, I placed
in a most conspicuous position -- namely, at the close of the
Introduction -- the following words: "I am convinced that
natural selection has been the main but not the exclusive means
of modification."
This has been of no avail.
Great is the power of steady misrepresentation; but the history of
science shows that fortunately this power does not long endure.'
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Mayr recaps the history of Darwinist theories, and addresses
the claims that Darwinism has been disproved or superseded in
Chapter Ten: "New Frontiers in Evolutionary Biology".
`Just as in the decade after the rediscovery of Mendel's rules,
since about 1970 the claim has been made increasingly often that
"Darwinism is dead."
...
Opponents of the [modern evolutionary] synthesis consistently
confound three schools of Darwinism:
(1) neo-Darwinism, a tem coined by Romanes in 1896 to designate
"Darwinism without an inheritance of acquired characters";
(2) early population genetics, a strongly reductionist school
that defined evolution as the modification of gene frequencies
by natural selection; and
(3) the holistic branch of the [modern evolutionary] synthesis,
which continued the traditions of Darwin and the naturalists
while accepting the findings of genetics.
...
Darwinism is not a simple theory that is either true or false
but is rather a highly complex research program that is being
continuously modified and improved. This was true before the
[modern evolutionary] synthesis, and it continues to be true
after the synthesis. Table 2 lists many of the significant
stages in the modification of Darwinism that one might recognize.
Yet recognizing such seemingly discontinuous periods is in many
respects an artificial enterprise. ... each of these periods
was heterogeneous to some extent, owing to the diversity in the
thinking of different evolutionists. Most critics who have
attempted to refute the evolutionary synthesis have failed to
recognize this diversity of views and thus have succeeded in
refuting only the reductionist fringe of the Darwinism camp.
...
Table 2 Significant stages in the modification of Darwinism
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Date Stage Modification
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1883;1886 | Weismann's | End of soft inheritance;
| neo-Darwinism | diploidy and genetic
| | recombination recognized
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1900 | Mendelism | Genetic constancy accepted
| | and blending inheritance
| | rejected
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1918-1933 | Fisherism | Evolution considered to be a
| | matter of gene frequencies and
| | the force of even small
| | selection pressures
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1936-1947 | Evolutionary | Population thinking emphasized;
| synthesis | interest in the evolution of
| | diversity, geographic
| | speciation, variable
| | evolutionary rates
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1947-1972 | Post-Synthesis | Individual increasingly seen
| | as target of selection; a more
| | holistic approach; increased
| | recognition of chance and
| | constraints
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1954-1972 | Punctuated | Importance of speciational
| equilibria | evolution
- - - - - -+- - - - - - - - -+- - - - - - - - - - - - - - - -
1969-1980 | Rediscovery of | Importance of reproductive
| importance of | success for selection
| sexual |
| selection |
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References:
_One_Long_Argument_
Charles Darwin and the Genesis of Modern Evolutionary Thought
Ernst Mayr, Harvard University Press, Cambridge Massachusetts 1991
ISBN 0-674-63905-7
QH371.M336 1991
575 - dc20
_On_The_Origin_of_Species_ by Means of Natural Selection
or the Preservation of Favored Races in the Struggle for Life
Charles Darwin, First Edition 1859
Sixth Edition 1872
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