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Ärende: Re: Direction of Evolutio
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ZOOLOGY 304 

Concepts and Questions in Evolutionary Biology.

Is evolution progressive?

It is widely believed (or presumed) that evolution is progressive.  In
response to the question, "Is evolution progressive?", students in ZOOL
304 gave these answers:


Yes
Yes, complexity and optimization to niche (although perfection cannot be
attained).
Yes.
Yes, better ability to survive.
Only if conditions are changing.
Yes, traits increase (physical traits and genetic traits).
Yes, better adapted (but no species is better than another, just
different).
Yes.
Yes, more suitable to it's own environment.
Yes, in some areas.
Yes, it continues without stopping.
Yes, it's a continuing process.
Yes.
Yes, ability to survive increases.


However, it is also frequently declared (especially by evolutionary
biologists) that evolution is NOT progressive. 
"Does [the genetical theory of natural selection] necessitate a belief
in progress?  Many biologists have stated that it does, and many more
have tacitly assumed this position.  I would maintain, however, that
there is nothing in the basic structure of the theory of natural
selection that would suggest the idea of any kind of cumulative
progress."
         George Williams, Adaptation and Natural Selection,
1966, p. 34.  (See Chapter 2 in this book for a more substantial
discussion of "progress" in evolution.)

What's going on?  Could this really be an issue of some significance?
The "no progress" stance is in part a response to some widespread
misconceptions about evolution.  Many people believe that evolution
has a goal, some target that the entire process has been approaching
ever since life first began.  

In part, this is a hold-over from preDarwinian explanations for the
history of life that interpreted the Great Chain of Being (Aristotle's
"scala naturae" or "ladder of nature", with all organisms arranged in
rank order from plants to plant-like animals to worms, various
invertebrates, fish, reptiles, mammals, men, and angels) as reflecting a
striving for perfection, with men (or angels) as the goal.  In this
view, living invertebrates are in some real sense "inferior" animals
compared to vertebrates, living reptiles are "inferior" to mammals, and
living non-human vertebrates inferior to people.  This view remains
"obvious" to many people, but makes no sense from a modern evolutionary
perspective that recognizes that all extant species are successful
descendents of lineages that have remained unbroken since life began.
 

More subtly, the "progressive" view holds that extinct taxa must
represent inferior versions of their respective taxa.  In this view,
evolution is a process which replaces poorly-adapted species with
better-adapted species.  But this is not how evolution works.
 Within a population, adaptive evolution favors a trait-variant which
yields greater reproductive success than another existing variant, under
the particular circumstances which prevail at the moment of
selection.  The result has been some impressive adaptations, but the
process itself has never been working "toward" any overall, long term
improvement. 

And in part, the view that evolution is "progressive" is simply a matter
of definition.  Of course, when history is read backward, all past
events represent "progress" toward the future.  But this really is not
the question.  Past scientific attempts to identify "progress" in
evolution have involved efforts to identify some characteristic of life
which shows some consistent, directional increase or advance in a
measureable value.  But, as discussed further below, no such
characteristic has been found.  

This issue matters, because a presumption of progress can interfere with
clear understanding of just how undirected, how local and immediate and
unconcerned with future consequences, the evolutionary process actually
is.

One meaning for "progress" is simple continuation of a process, without
halting.  In this sense (as noted by two students), evolution surely
does continue without stopping, probability inevitably so.  But this
is a very weak definition for "progress".  The qualified answer, "only
when conditions change", is similarly weak.  An iceberg floats at the
surface of the sea.  When the sea level changes, the iceberg rises or
falls to match that change.  Thus the iceberg always moves "toward"
the level of the sea.  But this is not "progress" in the common sense
of the word.  

Much more commonly, "progress" is defined and understood not just as
"change which maintains some condition" but as "advancement toward a
higher, better, or more advanced stage".  Progress in this sense
seemed to be meant by at least seven students.  But if evolution is
progressive in the sense of advancement or improvement, what quality is
it that advances or improves? 

Isn't there an obvious sense in which evolution MUST be progressive?
 Doesn't natural selection assure that species are always becoming
better adapted, so that degree of adaptedness must be increasing over
time?  Doesn't the fossil record document continuing advancement
toward improved design and complexity?  Doesn't the process of
adaptative radiation (continuing speciation with adaptation) guarantee
that the ecological world will be ever more precisely subdivided into
niches occupied by ever-increasing numbers of species?

In short, no.  No one has yet demonstrated any measureable parameter
that shows a consistent, reliable increase over time as evolution
proceeds.  This is an important point.  Belief that evolution is
always necessarily "improving" something can interfere with clear
appreciation of the actual mechanism of evolution, which is simply the
replacement of one heritable variant by another because, in specific
conditions which include the presence of both variants, one does better
than the other.

What would it mean to become progressively better at surviving or
reproducing, to become progressively better adapted?  Well, if
adaptation implies an increase in fitness, and if fitness is measured as
reproductive success, then an ongoing progressive process of adaptative
evolution should yield increasing reproductive success over
time.  And this would imply that descendent species should reproduce
more successfully than their ancestors.  Thus a presumption of
progress leads to an unreasonable conclusion of ever-increasing
reproductive rates over time. But evolution is just a process whereby
one variant is replaced by another.  Perhaps surprisingly, there
really is no expectation that any quality will be better after the
replacement than before, only different.  Natural selection works on
relative fitness, not on some "absolute" measure of adaptedness which
might show ever-increasing improvement.  The net result is typically
not improvement, just change.  There is no reason to believe that
progressive increase in reproductive success has occurred in ANY
lineage, certainly not in most lineages.  

What matters in evolution is variation, the relative differences in
reproductive success that occur among members of a population.  By
definition, selection favors those variants which out-reproduce their
fellows.  But in what sense can that yield a consistently
progressive increase in some measurable quality?  

For example, imagine an environment which supports one hundred
individuals.  Mutations appear which make the individuals which carry
the mutant alleles stronger and more aggressive.  At first, the mutant
individuals drive others away from resources, so within a few
generations all individuals are similarly strong and aggressive.  The
environment still only supports one hundred individuals (maybe fewer,
because they are all expending more energy on growing strong and
fighting).  No one is better off than before.  In fact, fewer
resources may now be available for reproduction because of the costs of
conflict. 

 Adaptation has changed the basis for reproductive success.  But
where is the progress?  Overall, what has increased, or "improved"?
 

In one famous example (cited by over 200 websites, with a couple links
in this paragraph), adaptive evolution drives each of three lizard
genotypes in an endless cycle of predominance, with each one replacing
the preceding one in turn, mimicking the game of "rock, paper, scissors"
(more, more).  This and the preceding imagined example are not
peculiar or unusual.  They represent the basic mechanism by which
evolution proceeds.  One heritable variant replaces another because
under it was more successful under certain particular conditions which
included the presence of the other variants .  This does not imply any
long-term advancement in a progressive sense.  

What about improved function?  An impression of increasing complexity,
associated with advanced engineering design, is easily obtained by
picking an exceptionally complex organism (human beings are an obvious
choice) and following its lineage backwards in time toward ancestral
simplicity.  But the impression of "progress" is determined by the
choice of organism.  

Hummingbirds and ospreys are certainly better at flying than are
herrings or sharks.  But fish, even ancient placoderms and ancestral
agnathans, are far better at being fish than are most birds (penguins
excepted?).  In an evolutionary sense, the only measure of quality
engineering design which has any significance is, "good enough; adequate
to continue reproduction in competition with other variant designs".
 As one student noted, "no species is better than another, just
different".  This applies to species in ancestor-descendent
relationships just as much as to coeval species.

In many lineages (most famously parasite species, which may be more
numerous than host species), evolution has proceeded with reduction
toward simplicity from an ancestral state.  (One might even argue that
the predominant trend in evolution has been toward greater parasitism.)
 And of course, in many other lineages (especially bacteria, which
outnumber eukaryotes by several orders of magnitude), there appears to
have been no significant increase in complexity whatsoever.

Regarding overall species diversity, it is true that there have been
times when diversity has increased.  But these seem to have been
momentary (geologically speaking) fluctuations associated with arbitrary
incidents of mass extinction.  On average, speciation seems to be
largely balanced by extinction.  There is no evidence, and no
theoretical expectation, that diversity in the Paleozoic and Mesozoic
eras was any less than that in the Cenozoic.

Ever since Aristotle, people have had an inclination to rank living
things in a single dimension of "lower to higher" or "primitive to
advanced".  Such rankings have a name, "the Great Chain of Being" or
"the Ladder of Life".  But such rankings have no basis in evolutionary
biology.  All living organisms occupy equivalent positions on the tips
of the latest twigs in phylogeny.  The "lowliest" worm or microbe is
just as "advanced", just as successful at adaptation and reproduction
throughout its lineage, as is the 'highest" primate or social insect.
 "Progress" was an essential feature feature of some pre-Darwinian
evolutionary theories, notably Lamarck's believe in evolution driven by
inward striving toward improvement.  But modern evolutionary theory
supports no clear expectation of progress, at least not in any dimension
that has yet been explored.

Not convinced?  That's okay.  What matters is clear thinking about
the evolutionary process.  The question of progress in evolution
remains open.  Prevailing opinion among professional biologists is
that no progressive trend will be found.  But an intuitive suspicion
persists, that something might be increasing over time in many or even
most lineages.  Possibly something like efficiency of genomic
information utilization, or genomically-based facility at sustaining the
evolutionary process itself.

So, if you hold a position that evolution is indeed progress, you must
be aware that this is a challenging position to maintain.
 Intellectual integrity obliges you to justify any opinion by
considering its implications.  In this case, that means identifying
just what measurement would use to demonstrate a progressive advancement
over time.

304 home page 
Comments and questions: dgking@siu.edu 
Department of Zoology e-mail: zoology@zoology.siu.edu 
Comments and questions related to web server: webmaster@science.siu.edu 
SIUC / College of Science / Zoology / Faculty / David King / ZOOL 304  

URL: http://www.science.siu.edu/zoology/king/304/progress.htm 
Last updated:  21 January 2003 / dgk 

"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well without it."
 Stephen Hawking
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