J.
Paleont. 62(3), 1988, pp.
319-329
Copyright ©1998. The Paleontological Society
0022-3360/88/0062-0319$03.00
PRESIDENTIAL
ADDRESS TRENDS
AS CHANGES IN VARIANCE: A NEW SLANT ON
PROGRESS AND DIRECTIONALITY IN EVOLUTION STEPHEN
JAY GOULD
Museum of Comparative Zoology, Harvard University,
Cambridge, Massachusetts 02138
ABSTRACT-Trends are the primary phenomenon of macroevolution,
but they have often been misinterpreted because an old and deep conceptual error
has induced us to misread, as anagenesis in abstracted entities, a pattern that
actually records changes in variance by increase or decrease in diversity or disparity
among species within clades. These patterns are actually produced by the "entity
making and breaking machine" of differential species success, but we misread
history as anagenesis because we focus on extreme values (though they may only
represent tails in variance of a system), or on measures of central tendency that
shift passively as species birth and death work in their differential way. I discuss
several examples in two classes: "increase trends mediated by growth in variance,
and "decrease trends" produced by diminution of diversity or disparity.
I present two examples in extenso based on original data: threefold occurrence
of Cope's rule in planktonic foraminifers as a consequence of increasing species
diversity from small founding lineages (increase trend), and disappearance of.
400 hitting in baseball as a decrease trend recording symmetrical decline of variance
with increasing excellence of play, not the gradual extinction of a valued "thing."
A proper appreciation of trends as changes in variance flows from and into the
two most important revisionary themes in modern evolutionary theory: 1) constraint
and structure as an antidote to overreliance upon adaptation (questions about
why founding lineages tend to be small, and why size ranges are constrained, lie
primarily in structural, not adaptational, domains); 2) hierarchy (increase and
decrease trends are powered by differential species sorting not by extrapolated
anagenesis of competition among organisms within populations)
INTRODUCTION
G
EOLOGISTS, with good humor, have tried to enliven the Latin mottoes that still
emblazon the professional logos of academe. The International Geological Congress, in pleasant alliteration, proclaims mente et malleo--with
mind and hammer. Our Society uses a different version of the same idea, in the
even more iconoclastic form of an unabashed, but sublime, joke: frango ut patefaciam--I
break in order to reveal. The statement, no doubt, was meant literally
as a reference to Estwings and bedding planes. But I suggest that we might also
find some inspiration in a metaphorical meaning. The greatest impediments to scientific
progress are often conceptual locks, not factual lacks. The missing pieces of
a puzzle may well deprive us of an adequate solution, but a far more serious bar
to understanding lies in mental frameworks that limit conceivable solutions to
a restricted, and false, subset of larger possibilities. When we think that we
proceed with absolute and comprehensive objectivity, we are even more likely to
be lost, for then we unconsciously cloak our own disabling biases and sally forth
down a primrose path masquerading as the straight and narrow road to final truth.
Gunnar Myrdal, the Swedish historian and sociologist who wrote the
greatest book on American racism (An American Dilemma. 1944), expressed
this fundamental quandry of intellectual life in writing: There
must be ... countless errors ... that no living man can yet detect, because of
the fog within which our type of Western culture envelops us. Cultural influences
have set up the assumptions about the mind, the body, and the universe with which
we begin; pose the questions we ask; influence the facts we seek; determine the
interpretation we give these facts; and direct our reaction to these interpretations
and conclusions. We must break conceptual bonds, not only rocks,
if we wish to understand the history of life. I propose, in this address,
that we reexamine the conceptual basis of evolutionary trends--the key phenomenon
behind a century of claims (or denials) that life's history provides a unique
and essential input to biological theory. I suggest that the most important contemporary
revision of evolutionary theory--the insight that selection (and other forces)
acts simultaneously at several levels of a genealogical hierarchy, with effects
propagating to levels above and below--might also reorient our interpretation
of evolutionary trends. The traditional view depicts trends as
a product of anagenetic change within lineages. What else could a trend be? You
have to get from here to there through steps in between, don't you?1
This conventional reading reflects two central beliefs that are rarely questioned
or even stated: first, that trends are powered by entities either rolling or pushed
in certain directions; second, that the motor of pushing shall be found in ordinary
Darwinian forces of selection and competition. Thus, an environmental change from
woodlands to grasslands, and a subsequent shift of optimal eating strategies from
browsing to grazing powered the most famous of all evolutionary sequences-the
linear museum parade of Hyracotherium to Equus. And the adaptive benefits
of mentality marked the ascent from Australopithecus through primitive
species of Homo to our exalted selves. (Professionals, of course, recognize
the bushiness of equid and hominid trees, but still view the lone survivors as
end-products of a coherent anagenetic sequence within the bush.) Similar
anagenetic interpretations apply to changes in habitat as well as form:____________________
1 The annual symposium of the Paleontological
Society directly proceeded the luncheon at which I delivered this Presidential
Address on Tuesday, October 27, 1987. Devoted to evolutionary trends,
this symposium beautifully illustrated my claim about conceptual locks.
All speakers, with the sole exception of Steve Stanley, treated trends
as anagenetic sequences only. Many elegant techniques were presented
for assessing variations in rate within such sequences, but no one even
suggested that what we perceive as trends may be produced by other processes
(proliferation rather than directed motion), or that such anagenetic
accumulations might play little part in the sum total of changing character
gradients within clades.
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