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1. The Modern Synthesis and its Competing views of
the explanatory structure of evolutionary theory
Philippe Huneman
IHPST (CNRS/ Unniversite Paris I Sorbonne)

2. Context: extending or expamding
the Modern Synthesis
• Perspectives from:
Evodevo
Molecular biology
Ecology
Gilbert Opitz and Raff (1994), Gould (2002), Pigliucci (2007),
Muller and Pigliucci eds. (2011), Gilbert (2009), etc.

3. Outline
• Chracterizing MS from viewpoint of the
critiques. ( switch in explanatory structure)
• MS as a synthesis? Competing views 0f
explananda
Theme 1. Evolution and genetics ( Fisher, Wright,
1930s)
Theme 2. Adaptation and diversity ( Mayr,
Dobzhansky, Simpson, Wright, 1950s)

4. A view from now:
"Dead parts of the Modern Synthesis"
• It is important to be clear about common, though not necessarily universal, assumptions of mid-20th
Century biology that have been discarded. A partial listing would include at least the following:
• •The genome is always a well-organized library of genes.
• Genes usually have single functions that have been specifically honed by powerful natural selection.
• Species are finely adjusted to their ecological circum-stances due to efficient adaptive adjustment of
biochemi-cal functions.
• The durable units of evolution are species, and within them the organisms, organs, cells, and
molecules, whic are characteristic of the species.
• Given the adaptive nature of each organism and cell their machinery can be modeled using principles
of efficient design.
• "The new biology: beyond the Modern Synthesis" Rose and oakley biology direct
2007

5. • Dead parts of the Modern Synthesis
• •The genome is always a well-organized library of genes.
• Genes usually have single functions that have been specifically honed by powerful natural
selection.
• Species are finely adjusted to their ecological circum-stances due to efficient adaptive
adjustment of biochemi-cal functions.
• The durable units of evolution are species, and within them the organisms, organs, cells, and
molecules, which are characteristic of the species.
• Given the adaptive nature of each organism and cell their machinery can be modeled using
principles of efficient design.
"Natural selection , acting on the heritable variation provided by
mutation and recombination of a Mendelian genetic
constitution, is the main agency of biological evolution." .
(Huxley to Mayr, 1951)

6. Core of the MS?
• Population genetics is the science of the process of evolution
by natural selection. (1)
Evolution is change in gene frequency
Variation = mutation/ recombination. Inheritance is genetic
• Natural selection is the main cause of the change of traits,
evolution, adaptation, etc. (2)
• Developement and inheritance are distinct; only the latter
matters for evolution (3)
Macroevolution has same processes as microevolution

7. How to characterize the
current challenges?

8. Define evolution
• Textbook definition: evolution is a change in
genotypic frenquencies in a population (see 1)
• Contrast with (2005):
• Evodevo definition 2005 (Sean Caroll) Evolution is
transformation of developmental pathways
• Should the extended synthesis to be
understood as decoupling evolution from
genetics ?
Genes?

9. Genes and form
(1,3)
• Modern synthesis was about genes in
evolution. Form was left out. (Ghiselin 1981,
Love 2003 etc.). New advances in Evodevo
are about form (Pigliucci 2007)
• Evo devo, morphology, etc. Form is back
(commonalities in structure, eg Tetrapod limb)
• -> need for an " extended synthesis"
Genes?

10. Form and adaptation
(2)
• perspectives: structuralists ( evodevo) vs
adaptationnist ( MS) (Amundson 2005):
MS is adaptationist (i.e. externalist) and Evo
devo is structuralist.
• But in this viewpoint, chances of a genuine
new synthesis are low.
Adaptation?

11. Dilemma
• Pigliucci, Muller (2011), Gilbert Opitz & Raff
(1996), etc. Build a new ( extended)
synthesis
• Amundson: synthesis not very plausible
• How to decide?

12. claim: The Ms/ ext synthesis
alternative is about explanatory
structure
• Amundson 2000:
• EvoDevo, alternative view:
"importance of homology over
adaptation"
• MS: importance of adaptation over
homology

13. MS explanatory structure
Natural
selection
Adaptation
Adaptive radiation
Diversity Analogies -> Unity
Common descent -
homology
« Unity of type »
Quite faithful to
Darwin's Origin, chap.
6

14. Alternative view
Natural selection Plasticity/NC Developmental constraints
Adaptation
Diversity Unity
Adaptive radiation

15. Is it really the MS view?
• Textbook def: evolution is a change in allelic
frequency in population
But see already in the MS:
• “Evolution is not a change in gene frequencies, as is claimed so
often, but the maintenance (or improvement) of adaptedness
and the origin of diversity. Changes in gene frequency are a result
of such evolution, not its cause.” Mayr 1997

16. Theme 1. Genetics and
evolution
• Roughly said: "genetics underlies evolutionary processes"
is a common claim in MS
• For example:
• Mayr: "mutation, recombination selection and isolation
are the four cornerstones of evolution" ("genetic change
and environment", 1955)
• Dobzhansky: "Evolution is a change in the genetic
composition of populations. The study of mechanisms
of evolution falls within the province of population
genetics". (GOS 1951 8)

17. • Genetics of the transmission of hereditary materials, and genetics of
development, are concerned with individuals as units. The former seeks to find
out the rules which govern the formation of gene constellations in individual
zygotes, so that the probable distribution of genes in the offspring may be
predicted from a knowledge of the genotypes of the parents. The latter studies
the mechanisms of gene action in ontogeny. A third subdivision of genetics has
as its province the processes which take place in groups of individuals, in
populations, and is therefore called genetics of populations. (Dobzhansky,
GOS, 1951, 10)
• Evolution is a process of change or movement. Description of any movement
may be divided into two parts: statics, which treats of the forces producing a
motion and the equilibrium of these forces, and dynamics, which deals with the
motion itself and the action of forces producing it ( Dobzhansky, GOS, 1951, 15)
• Mutation, recombination// selection and migration // "The origin
and functioning of the reproductive isolating mechanisms"

18. • But the relation between genetics and evolution can be conceived of
in two ways, as Fisher indicates
• The status of genetics within evolution is not always the same comparing
HALDANE' S sub-ject, "Can evolution be explained in terms of present known
genetical causes ?" with the heading under which I chose to speak, "The
evolutionary "modification of genetic phenomena." My own address might
equally well have been entitled, "Can genetical phenomena be explained in
terms of known evolutionary causes ?" The one approach, as you perceive, is
analytic and deductive. Genetic studies are regarded as revealing the
mechanism connecting cause and effect, from a knowledge of which the
workings of the machine can be deduced and the course of evolutionary
change inferred. The other approach is inductive and statistical; genetics
supplies the facts as to living things as they now are, facts which, like the
living things in which they occur, have an evolutionary history and may be
capable of an evolu-tionary explanation, facts which are not immutable laws of
the workings of things but which might have been different had evolutionary
history taken a different course.
(THE EVOLUTIONARY MODIFICATION OF GENETIC PHEnomena 1932)

19. • Application: The problem of the evolution of dominance
• Out of 221 different mutations available for classification I
found that 208 were described as recessive; that is to say,
that in these cases the cross-bred or heterozygous fly
appeared to be com-pletely normal, and the mutation was
entirely concealed. In not one case did the cross- bred fly
resemble the pure- bred mutant. But in the 13 remaining
cases it was intermediate, showing a defect of the same
kind as the mutant, but of less intensity. The distribution
of these mutations in respect of dominance or
recessiveness was therefore extremely one-sided.
• The whole body of observations may be summarised in the
two rules : that the wild- type gene is completely dominant
to all other genes of the series, whilst absent.

20. • Thesis: "the recessiveness
of mutations is itself a
consequence of a
prolonged evolutionary
process, by which each
species reacts to the
unfavourable mutations
with which it is persistently
peppered."
• The evolution of dominance is
an adaptive Mechanisms
which buffers the species
against generally deleterious
mutations
• "That the vast majority of mutations
should be deleterious is a perfectly
natural consequence from the view
that the organism is maintained in a
highly adapted condition by natural
selection, for a highly adapted
condition can mean nothing else
than one which is more easily injured
than improved by a change in its
organisation"
Evolution of dominance
Adaptive character of organisms
Deleterious ( in general) character of mutations

21. • The (empirically noticed) generally deleterious
aspect of mutations implies that mutations can
not be directed
• "Just where does the theory of natural
selection place the creative causes which
shape evolutionary change? In the actual
life of living things; in their contacts and
conflicts with their environments,"
Fisher, eeddington Lecture 1950.
• ->> it is not mutations which cause
evolution (argument against any
lamarckism)

22. • What adaptation is: a reversal of probabilities.
Natural selection makes highly probable the
very improbable. Exemple of the eye.
("Retrospect of the criticisms", Evo as a process, 1954)
• Idea of "design" as a highly improbable
structure (Hume, Kant, etc.). The statistical
understanding of selection explains design.
A genetical theory of adaptation: Fisher (vs
Wright & Dobzhansky)

24. "the
intricacy
of
design"

25. • "Natural selection is not evolution" (GTNS
1931) : a genetical theory of adaptatiom
• FTNS: increase in population mean fitness
due to selection equals genetic additive
variance. (Price, 1972, Edwards 1994)
• The FTNS analytically ties the part of
evolution caused by selection, with the
increase in fitness, hence adaptation
A genetical theory of adaptation: Fisher (vs
Wright & Dobzhansky)

27. A genetical theory of adaptation:
Fisher (vs Wright & Dobzhansky)
• FTNS: deterioration
of environnement (
eg epistasis,
pleiotropy, etc,
change of
reproductive value
of alleles regarding
the genetic
background) can
swamp the
adaptive-selective
part of evolution
• But the theory of dominance
shows that selection acts upon
the level of the genic structure
of populations

28. The bearer of adaptation is individual: never a species, but the individual
organism or its genes ( "immediate progeny")
• The injury observed on close inbreeding is thus exposed in an entirely
new light. It is not perpetual self- fertilisation, but the first few
generations, and especially the first generation, that is dangerous. The
inbred lines show no perceptible further deterioration after eight or ten
generations. Moreover, it is not the racial potentialities that are injured,
but only the individual expression of them. It is not the species, but the
individual, which suffers. The various devices which exist in nature to
ensure exogamy are not for the benefit of the species, for which, so
to speak, Natural Selection cares nothing, but to ensure the well-
being of the immediate progeny ; to guard them against the
recessive parents. (THE BEARING OF GENETICS ON THEORIES OF
EVOLUTION. 1932)
Compare with Dobzhansky: mendelian population is the unit of
adaptation
A genetical theory of adaptation: Fisher (vs
Wright & Dobzhansky

29. A genetical theory of adaptation:
Fisher vs Dobzhansky
• The injury observed on close inbreeding
is thus exposed in an entirely new light. It
is not perpetual self- fertilisation, but the
first few generations, and especially the
first generation, that is dangerous. The
inbred lines show no perceptible further
deterioration after eight or ten
generations. Moreover, it is not the racial
potentialities that are injured, but only the
individual expression of them. It is not the
species, but the individual, which suffers.
The various devices which exist in nature
to ensure exogamy are not for the
benefit of the species, for which, so to
speak, Natural Selection cares nothing,
but to ensure the well-being of the
immediate progeny ; to guard them
against the recessive parents. (THE
BEARING OF GENETICS ON THEORIES
OF EVOLUTION. 1932)

30. • Wright (1932): adaptation is not the only process of
evolution (random drift and SBT theory)
(hence Evolution is not only natural selection. And compare to
Fisher.)
• But they deal with a similar adaptive problem ("the intricacy
of design")
• Wright: the cohesive coadaptive charact of genotypes
makes evolution unlikely to maximise adaptation directly via
selection
• Idea of adaptive landscape: populations cans be stacked
on a local fitness peak ( local maximum of adapted design)
A genetical theory of adaptation: Fisher vs
Wright

33. • Shifting balance theory: episodes of random drift
allow switching from one peak to a higher one.
• Wright's SBT answers through non adaptive
processes to the question of the consevation of
"intricacy" through evolution
(ie variation (mutation, recombination) tends to
dismantle adaptiveness of organisms but still evolution
occurs)
• Fisher's view of the evolution of genetic make-
up (dominance recessivity etc.) answered it by
an adaptive process altering the nature of
alleles.
A genetical theory of adaptation: Fisher vs
Wright

34. A genetical theory of adaptation:
Fisher vs Wright
• Wright looks for the conditions under which evolution can take
place, as a switch from peaks (and they are restrictive)
• Analogously the FTNS allows to determine conditions under
which evolution is adaptive (ie: the change directly caused by
selection overcomes the deterioration of the environnement)

35. Theme 2 : adaptation
and diversity

36. • Mayr: critique of the
"beanbag genetics"
• From Wright, idea of
coadapted genotypic
complexes (the
selective value of a
gene depends upon the
genetic background)
•
Mayr1955
The idea of adaptedness as "intricacy of
design"

37. • Some of the conceivable
combinations, indeed a vast majority
of them, are discordant and unfit for
survival in any environment. Others
are suitable for occupation of certain
habitats and ecological niches.
Related gene combinations are, on
the whole, similar in adaptive value.
The field of gene combinations may,
then, be, visualized most simply in a
form of a topographic map, in which
"contours" symbolize the adaptive
values of various combinations.
( GOP 7)
•
Dobzhansky GOP
ch 9 261
Reproductive isolation,
hence the concept of a
species, explains the
preservation of adapted
genotypes

38. Adaptation and diversity: Dobzhansky /
Simpson vs.Wright
Natural
selection
Adaptation
Adaptive radiation
Diversity Analogies -> Unity
Common descent -
homology
« Unity of type »
• Dobzhansky:
adaptative landscape
explains diversity
•

39. Dobzhansky GOP 8

40. • Between Wright and Simpson / Dobzhanski
(1950s) the adaptive landscape changes
meaning and comes to express species
diversity !

41. Dobzhansky GOP 8
• Adaptation yields diversity via radiation
and the SBT

42. Probleme 1: conflict with darwinian idea
of diversity, based on homology

43. Divergences are due to
differences in ancestry
( distinct homologies).
Adaption is noise on the
patterns of diversity

44. • Probleme 2: "The
hierarchical nature of
the biological
classification reflects
the objectively
ascertainable
discontinuity of adaptive
niches."
• This link between unity,
diversity and adaptation
is problematic because
the explanatory theory
(la SBT by Wright) is not
truly adaptationist !
• -> irony: Fisher (1950)
uses results by
dobzhansky on
Drosophila melanogaster
to criticise Wright's SBT
Adaptation and diversity:
Dobzhansky / Simpson
vs.Wright

45. • namely that it is not only small
isolated populations, but also large
populations, that experience
fluctuations in gene ratio. If this is
the case, whatever other results
isolation into small communities
may have, any effects which flow
from fluctuating variability in the
gene ratios will not be confined to
such subdivided species, but will
be experienced also by species
having continuous populations.
This fact, fatal to "The Sewall
Wright Effect," appeared in our
own researches from the
discovery that the year-to-year
changes in the gene ratio in a wild
population were considerably
greater than could be reasonably
ascribed to random sampling, in a
population of the size in question.
We presumed that random
sampling fluctua-tions must always
be present, but that other causes
must be actingtoo, with an intensity,
which, even in a population of no
more than 1000, seems to be
greater than the effects of random
sampling. But it is only the random
sampling fluctuation which is
accentuated by the small size of an
isolated population ; other causes,
like selective survival varying from
year to year, will influence large
populations equally. Indeed we
pointed to other researches,
notably those of Dobzhansky,
demonstrating such fluctuations in
large populations.
Fisher & Ford, Heredity, 4: 117-119,
(1950).

46. Conclusions

47. • The entanglement between theory of adaptation and
evolutionary aspect of genetics is proper to Fisher. His theory
of adaptation is not so pervasive in MS. But the problem of
'buffering and evolving intricacy of design' is a common one.
• The difference between Fisher, Wright and Dobzhansky is not
only an empirical question of the degree or amount of
adaptation in evolytion (the RV on the Wright / Fisher debate).
• The explanatory structure (adaptation- diversity- selection) is
not constant across the MS
• The question whether one can extend or overcome the MS is
not so well formulated when MS is understood as a historical
set of views, concepts, theories etc.