Download Origin

History of a concept in complexity studies
The origins of natural design
The origins of particularly
complex and perfect organs
(analogy with artifacts)
“There cannot be design without a
designer; contrivance, without a
contriver; order, without choice”
Young Darwin impressed by Paley’s
descriptions of “adaptations”
Natural Theology, 1802
Charles Darwin’s
Transmutation Notebooks
1836 - 1839
SOME RADICAL DOUBTS… (1837-1838)
Imperfection: “When one sees nipple on man’s breast, one does
not say some use. So with useless wings under elytra of beetles,
born from beetles with wings and modified. If simple creation, surely
would have been born without them” (Not. B, Sept. 1837)
Evolution is not superiority: “It is absurd to talk of one animal
being higher than another. We consider those, where the cerebral
structure, or intellectual faculties, most developed, as highest. A
bee doubtless would when the instincts were” (Not. B, Sept. 1837)
Adaptive contingency of characters: “Chance and
unfavourable conditions to parent may become favourable to
offspring” (Notebook E, 19 Oct. 1838)
“Every species is due to adaptation AND hereditary structure”
(Not. B, 1837)
The tree (or coral) of life
(branching evolution) as
description
Natural selection
(differential survival and
reproduction) as
explanation
Notebook B, July 1837
ADAPTATION WITHOUT FINALISM
(Notebook E, Dec. 1, 1838)
“No adaptation to an end, but adaptation to
varying circumstances… owing to external
contingencies and relations with other species, not
owing to mandate of God”
(commenting William Whewell’s “History of the Inductive Sciences” and his
idea of a providential adaptation)
“Structure is due to external agency and
circumstances, without final cause, either in
present, or past generation”
(Notebook E, Dec. 14 1838)
A pessimistic passage in Notebook C, July 1838…
“We never may be able to trace the steps by
which the organization of the eye, passed
from simpler stage to more perfect,
preserving its relations. The wonderful
power of adaptation given to organization.
This really perhaps greatest difficulty to
whole theory” (CD)
DIFFICULTY: is there any contradiction between continuity of
change, by natural selection, and the need of a persistent (even
“infinitesimal”) functional advantage?
CD is quoting Henri Milne-Edwards (“Histoire
naturelle de crustacés”, 1834, paper 1838):
“Il est aussi digne de remarque que l’instrument
affecté à cet usage insolite n’est pas un organe
nouveau introduit ad hoc dans la structure des
Crustacés à brachies infèrieures, mais un
appendice qui existe dans tous les animaux de
cette catégorie, et qui est seulement en partie
détourné de sa destination ordinaire et
lègérement modifié dans sa conformation pour
devenir apte à remplir ses fonctions nouvelles”
(in, Notebook E, Oct. 19, 1838)
The “five percent of a wing” Problem
How can evolution ever make a wing in
Darwin’s gradualist and functionalist way
if the five percent of a wing cannot
possibly provide any benefit for flight?
George Mivart’s objection to Darwin
(1871): the incompetency of natural
selection to account for the incipient
stages of useful and complex structures
Risky predictions…
“To suppose that the eye with all its inimitable
contrivances for adjusting the focus to different
distances, for admitting different amounts of
light, and for the correction of spherical and
chromatic aberration, could have been formed
by natural selection, seems, I freely confess,
absurd in the highest degree”
(CD, Origin, Sixth Edition, 1872, p.)
Mivart’s objection: answer 1
GRADUAL IMPLEMENTATION
“When it was first said that the sun stood still and the world
turned round, the common sense of mankind declared the
doctrine false; but the old saying of Vox populi, vox Dei, as
every philosopher knows, cannot be trusted in science.
Reason tells me, that if numerous gradations from a
simple and imperfect eye to one complex and perfect
can be shown to exist, each grade being useful to its
possessor, as is certainly the case; if further, the eye ever
varies and the variations be inherited, as is likewise
certainly the case; and if such variations should be useful
to any animal under changing conditions of life, then
the difficulty of believing that a perfect and complex eye
could be formed by natural selection, though insuperable by
our imagination, should not be considered as subversive of
the theory” (p. 144)
Mivart’s objection: answer 2
FUNCTIONAL SHIFT
“Natural selection might specialise, if any advantage were
thus gained, the whole or part of an organ, which had
previously performed two functions, for one function alone,
and thus by insensible steps greatly change its nature. …
Again, two distinct organs, or the same organ under two very
different forms, may simultaneously perform in the same
individual the same function, and this is an extremely
important means of transition. In all such cases one of the
two organs might readily be modified and perfected so as to
perform all the work, being aided during the progress of
modification by the other organ; and then this other organ
might be modified for some other and quite distinct purpose,
or be wholly obliterated” (p. 147)
Mivart’s objection: answer 1+2
PRE-ADAPTATION
“The illustration of the swimbladder in fishes is a good
one, because it shows us clearly the highly important fact
that an organ originally constructed for one purpose,
namely, flotation, may be converted into one for a
widely different purpose, namely, respiration. The
swimbladder has, also, been worked in as an accessory to the
auditory organs of certain fishes. All physiologists admit that
the swim bladder is homologous, or "ideally similar" in
position and structure with the lungs of the higher vertebrate
animals: hence there is no reason to doubt that the swim
bladder has actually been converted into lungs, or an organ
used exclusively for respiration” (p. 148)
Flexibility
Redundancy
Structure/Function
PRE-ADAPTATION (Ernst Mayr): continuity in differential
reproductive success, not in the same function
- CURRENT USE not always => HISTORICAL ORIGIN
- SUB-OPTIMALITY of adaptation (CONSTRAINTS)
“Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than, the other inhabitants of the same
country with which it comes into competition. And we see that this is
the standard of perfection attained under nature. Natural selection
will not produce absolute perfection, nor do we always meet, as
far as we can judge, with this high standard under nature” (p. 163)
“I am convinced that Natural Selection has been the most important,
but not the exclusive, means of modification” (CD, Origin, p. 4)
XX Century vulgata:
ADAPTATIONISM (strong)
- A) power of natural selection as an optimizing
agent, at the genetic level
- B) breaking an organism into unitary "traits" and
proposing an adaptive story for each one
considered separately
- C) Trade-offs among competing selective
demands exert the only brake upon perfection
(non-optimality is thereby rendered as a result of
adaptation as well)
A “PANGLOSSIAN” PARADIGM?
(Gould-Lewontin, 1979)
Criticisms:
- 1) SELECTION ACTING ON CONSTRAINTS
- 2) DISTINCTION BETWEEN CURRENT UTILITY AND
REASONS FOR ORIGIN
- 3) INFALSIFICABILITY OF ADAPTIVE STORIES
- 4) failure to consider adequately such competing themes
as random fixation of alleles, production of non-adaptive
structures by developmental correlation with selected features
(allometry, pleiotropy, material compensation, mechanically
forced correlation) and current utility as an epiphenomenon of
nonadaptive structures.
- Coming back to Darwin…
Daniel Dennett’s “Reverse Engineering”
(Darwin’s dangerous idea)
EXPLANATORY ADAPTATIONISM
- Not only adaptations in nature, but adaptations
explain evolution
- Adaptive problems in the past
evolved to solve them
adaptations
- Evolutionary problem-solving based on the
universal “darwinian algorithm”
Hypothesis of optimality
“QUIRKY FUNCTIONAL SHIFT”
Exaptation: a missing term in the science of form
(S.J. Gould, E. Vrba, 1982)
APTATION: any features now contributing to fitness
- AD-APTATION: a feature directly crafted for a
current utility by natural selection
- EX-APTATION: a feature coopted for a current
utility following an origin for a different function,
or for not function at all.
“Exaptation”: character useful (aptus) as a consequence of (ex) its form
1941 - 2002
“To paraphrase Mr. Huxley in a famous context, I am
prepared to go to the stake for exaptation; for this
new term stands in important contrast with
adaptation, defining a distinction at the heart of
evolutionary theory, and also plugging an
embarassing hole in our previous lexicon for basic
processes in the history of life”
(S.J. Gould, “The Structure of Evolutionary Theory”, 2002, p. 1234)
THE END OF ADAPTATIONISM - A NEW TAXONOMY
OF FITNESS (Gould-Vrba 1982; Gould, 2002)
Process
Character
Natural Selection
shapes the character
for a current use
Adaptation
Darwinian
Function
Exaptation 1
(by cooptation)
preadaptation
Darwinian
Effect
Exaptation 2
(by nonaptation)
Structural
Effect
A character, previously shaped by
natural selection for a particular
function (an adaptation), is
coopted for a new use
A character whose origin cannot
be ascribed to the direct action
of Natural Selection (a nonaptation), is coopted for a
current use
Usage
Ichthyosaurus
TAXONOMY OF THE EXAPTIVE POOL
as structural basis of “evolvability” in life’s history
(S.J. Gould, 2002)
A – Inherent potentials (unexploited)
B – Available things (“spandrels”)
B1) As architectural consequences (structural and
non-adaptive origin)
B2) As historical unemployments (historical, non-adaptive
origin) (ex. vestigia)
B3) As invisible introductions (historical, non-adaptive
origin) (ex. neutral drifts, founder effects)
The Spandrels of
San Marco
A character whose origin
cannot be ascribed to the
direct action of Natural
Selection (a non-aptation),
could be coopted for a
current use
Imperfections in human physiology
Due to:
- Previous adaptations
- Selective trade-offs
- Exaptations
- Spandrels and vestigia
The Five Percent of a Wing Problem - 2011
Archaeopteryx lithographica
Solnhofen, 1861
(Richard Owen)
ADAPTIVE STORIES?
Protowings?
Transitional stages?
Dinosaurs with plumage?
1996-2006:
AMNH of NY
A) Arboreal Theory
(for
gliding in tree-dwelling
ancestors)
B) Cursorial Theory
(from running terrestrial
dinosaurs)
C) Wing-assisted
incline running in avian
ancestors (Dial, Randall, Dial,
BioScience, 56, n. 5, May 2006)
Velociraptor mongoliensis with Mononykus olecranus
Anyway: Exaptation
(type 1) of avian flight
“A firm step from water to land”
Tiktaalik roseae –
Ellesmere Island,
Nunavut, Arctic Canada
(Shubin, Daeschler,
Jenkins, Nature, 440,
2006)
(Nature, 440, April 2006)
Tetrapods
359 M
Single intermediate
fossil? Missing link?
What is the right
pattern for
vertebrate
transition from
water to land?
365 M
385 M
Fishes
Classic picture Devonian–Carboniferous: LINEAR ANAGENESIS?
Cladogram of the pectoral fins of taxa on
the tetrapod stem
Life in shallow
water
- Multiple adaptive solutions (different combinations of
“retained” and “modern” characters)
- Exaptation fins-limbs
- Not always the present is the key for the past
(Henry Gee)
EVO-DEVO: HOX-MUTATIONS IN PHYLOGENY
- Same Hox genes for the
entire animal kingdom
- Nat. Selection and Dev.
Constraints
- Traits without adaptation
(structural effects)
Evolution and bricolage:
functional shifts and
cooptations
Extensive application of exaptation today
- Palaeontology
- Evo-Devo
- Genetics
(primary/secondary adaptations)
(selection on developmental constraints)
(molecular cooptations; symbioses)
- Multilevel
another)
selection
(adaptations at one level could be exaptations at
- Niche construction (the active role of organisms shaping the frame of
selective pressures; triggers for new niches)
- Evolution of unselfish behaviours
(selective triggers
exaptations; or exaptations and then selective reinforcement)
-
and
then
Human evolution (evolution of mind; cultural evolution)
- Evolutionary psychology (where the strong adaptationist approach is
more and more failing)
-
Theoretical biology and complexity (“adjacent possible”)
What exaptation does not mean:
1) A CONFUTATION OF THE AGENCY OF NATURAL
SELECTION IN EVOLUTION, but: a) trade-offs between
functions and structures; b) non-adaptive structures.
2)
A BREAK IN THE CONTINUITY OF EVOLUTIONARY
PROCESSES (it could allow major and rapid novelties, but
without statements about “saltations” in evolution)
3) THE “AD HOC” HYPOTHESIS WHEN ALL THE OTHER ONES
FAILED (exaptation needs observative and experimental
supporting data)
4) A UNIVERSAL EVOLUTIONARY PROCESS (but: a prominent
“pattern” in a pluralistic framework of evolutionary processes)
AN EXTENDED NEO-DARWINIAN
SYNTHESIS
Understanding the phenotype
(phenotypic plasticity,
macroevolution, origins of form)
New views on
inheritance
(epigenetic
inherit., niche
inherit.)
Advances of NeoDarwinism (population
genetics, drift, speciation)
Evo-Devo
(innovation,
modularity,
evolvability)
Selection and adaptation
reformed (neutralism,
multilevel selection, niche
construction, exaptation)
The evolution of the structure of Neo-Darwinian Research Program