Download Neutrality

IHPST
Philbio
20/03/2008
Ecologie et
macroévolution
Julien Delord
CERES-ERTI
Neutral Theories in Ecology and Evolution
• Introduction: Do ecological communities exist (and evolve)?
• 1. Presentation of the Unified Neutral Theory of Biodiversity
(UNTBB)
• 2. The (epistemological) non-neutrality of neutral models
- Neutrality and falsity (in biology)
- Neutrality as a strategy towards generality and unification
• 3. Neutrality and the unification of evolutionary theory
- Neutral theories in genetics and in ecology
- « L-neutrality » and « L+1 neutrality »
- « Grand Unification » or « domain unification »?
- Communities as evolutionnary entities
• Conclusion
- Theory change in ecology
« Are phenomenological communities causal systems? »
(Sterelny 2006)
= Are communities structured, functionally organized with
effects on the fate of populations they are composed of?
(downward causation)
3 criteria:
- Boundedness
- Internal regulation
- Emergent property effects
 For Sterelny, they fail to meet these criteria…
 For most ecologists, communities are structured by niche
competition and niche assembly rules!!!
Dynamiques des communautés végétales
Organicisme
Frederic CLEMENTS (1874-1945)
« Comme un organisme, la formation naît,
croît, mûrit et meurt. En outre chaque
formation climacique [ayant atteint le climax]
est capable de se perpétuer, en reproduisant
avec une fidélité absolue les étapes de son
développement », 1916.
 Déterminisme des successions
végétales
vs.
Individualisme
Henry GLEASON (1882-1975)
« Le phénomène de la
végétation dépend complètement
des phénomènes des espèces
individuelles »
 Assemblage des végétations au
Introduction : Do ecological communities
exist and evolve ?
A
Ecological Community
Group of populations from different
species interacting with each other
and sharing the same spatio-temporal
framework.
B
C
D
Community 1
A: 25%
B: 25%
C: 25%
D: 25%
Species Richness
Relative Species Abundance
Community 2
A: 80%
B: 5%
C: 5%
D: 10%
27 different models of Species Abundance Distribution
McGill Brian J. et al., “Species abundance distributions: moving beyond single prediction theories to
integration within an ecological framework”, Ecology Letters, 10, 2007, p. 995-1015.
La diversité des communautés dépend de leur
structuration
Jared Diamond (1975) a été le premier à explorer l’idée selon laquelle
il existe des règles qui gouvernent l’assemblage des communautés:
Ressources
restantes
1. The Unified Neutral Theory of Biodiversity
and Biogeography (UNTBB)
- How to explain species biodiversity in
ecological communities ?
- How to explain the curves of relatives
species abundance ?
-Is the niche concept
necessary ?
-From case-study
rules to general
deductive laws !
Courbes SAD (Surface Abundance Distribution)
Principles of UNTBB
Definition:
Ecological community : Group of trophically similar and
sympatric species
Conservation law:
Neutrality : The assumption of a complete identity of ecological
interactions affecting community organisms, i. e. a per capita
ecological equivalence (in terms of reproduction and death rates
+ speciation) among all individuals of every species.
Dependant Variables:
Relative species abundance and species number
Parameters:
Reproduction rates, death rates, speciation rates, community size
(= number of individuals), limits to dispersal.
Principles of UNTBB (Cont’d)
Hypothesis:
« Communities are open, nonequilibrium assemblages of
species largely thrown together by chance, history and
random dispersal » (Hubbell, 2001, p.8)
Process:
Ecological drift: demographic stochasticity in a neutral
community (~ genetic drift)
Result:
Curves of relative species abundance at equilibrium
between speciation and extinction (stationary dynamics)
Markovian process
Theoretical developments
Random walk of the abundance of ith species under ecological
drift
(The initial relative species abundance is 0.5 - ergodic )case)
A brief reminder of population genetics:
how to calculate the proportion of homozygotes at
equilibrium between mutation and drift.
Theoretical developments
How to calculate the probability of drawing two
individuals from the same species at equilibrium between
speciation and ecological drift.
Let  be the speciation rate per generation and JM the number
of individuals in metacommunity M.
Theoretical developments
F2 = 1 / 1+ 2 JM 

Feq = 1 / 1 + 2Nμ
 = 2 JM 
 is named by Hubbell the fundamental biodiversity number because
this dimensionless number controls not only the equilibrium species
richness but also the equilibrium relative species abundance in the
metacommunity.
Some results and predictions of UNT on biodiversity
dynamics
By generalization, Hubbell extends this rationale to the case of 3
individuals randomly chosen in JM , etc., then J individuals
belonging to S species.
Etienne R., Alonso D., McKane A. J., « The zero-sum assumption in neutral biodiversity theory »,
Journal of Theoretical Biology, 248, 2007, p. 522-536.
Dornelas M. et al., “Coral reef diversity refutes the neutral theory of
biodiversity”, Nature, 440, 2006, p. 80-82.
Epistemological questions:
Ecology and its recurrent « Physics envy »
 In search for laws, regularities and generality
 The concept of generality: from universality
(spatio-temporal unboundedness) to invariance (Van
Fraassen, Laws and Symmetry, 1991)
 Invariance as a conservation law (when a
certain value is conserved through time despite the
changes of the system)
 Neutral models as a quest for invariance in
ecology
 Generality should enhance unification
2. The (epistemological) non-neutrality of
neutral models
- Neutrality and falsity (in biology)
Conservation law in physics (sub-atomic level) vs. conservation law in
ecology (macroscopic world)
 In particle physics, conservation laws (i. e. conservation of energy)
give rules to make sense of the data and to define the ontology of
particles.
 In ecology, the ontology is already defined but the rules are not
known. They are interpreted in light of the discrepancy between
expected patterns and empirical data.
Problems:
1. Theoretical: the underdetermination of theories
2. Empirical: the confirmation of the neutral model
3. Neutrality and the unification of
evolutionary theory
Neutral theories in genetics and in ecology
 Comparative approach of neutrality
in genetics and in ecology
Hu X.-S., He F. & Hubbel S. P. (2006), “Neutral theory in macroecology and population genetics”,
Oikos, 113, 3, p. 548-56.
NeutralNatural
theory selection
of evolution
Neutral Theory of Biodiversity
Community
Evolver
Niche ???
Evolver
Espèce
Species
Espèce
Interactor
Individu
Individu
Individual
Type
Class
of d’Allèle
Alleles
Type
d’Allèle
Replicator
-Interactor
Allèle
Allele
Allèle
Species
Class of individuals
Individual
Replicator
-Interactor
UNT and phylogeny
Last consequence of Hubbell ’s theory for phylogeny (but
not the least), UNT models show that the organization of
biodiversity at different scales is intrinsically fractal.
UNT and phylogeny
On the fractal dimension of biodiversity :
Hubbell : “ If biodiversity is a perfectly homogeneous
fractal, this implies that a satisfactory answer to the
question - how many species are there ? – cannot really be
answered except operationally.
It requires a definition of the scale of aggregation that we
call species! » (p.263)
 This implies a pluralist or arbitrary fixation of the
level of “ ranking ”for the species.
Are neutrality in genetics and ecology comparable?
Historical differences :

The neutral theory of molecular evolution appeared as a solution
to a problematic in evolutionary genetics and as a consequence of
new developments in molecular biology (high protein polymorphism).
“The theory of molecular evolution by mutation and random drift led
to Kimura’s prediction that those amino acids or nucleotide changes
that have the least consequence would evolve faster” (J. Crow,
Neutral models in biology, 1987, p. 13).
 UNTBB as an elaborated null model???
What is selection for UNT ? (Rules of niche assembly ?)
Neutrality
Neutrality in a hierarchical systems
• Neutrality relative to level L+1 (replicator <> interactor)
• Neutrality relative to level L (replicator-interactor)
Example of « contextual » neutrality: genetic drift and
transposons
« L neutrality » and « L+1 neutrality »
Level L+1
(species)
Level L
(individuals)
Yes
No
Yes
No
UNTBB
Apparent
neutrality
« free »
replicators
Species
selection
Kimura’s neutral theory was defined as a
« L+1 neutrality » with respect to natural
selection
Hubbell’s neutral theory of biodiversity
was defined as a « L neutrality » with
respect to differential replication
- « Grand Unification » or « domain unification »?
Is a neutral synthesis of evolutionary theory possible ?
Ecological drift
Incompatibility
Genetic drift
vs
deterministic “rules” of niche
assembly
Incompatibility
vs
natural selection
- Ecological drift requires non-neutral mutations
- Ecological drift would induce different speeds of genetic drift
Gould’s hierarchical
macroevolutionary theory
Vrba and Gould « The hierarchical expansion of sorting and
selection: sorting and selection cannot be equated », Paleobiology,
12, 1986, p. 217-28.
Stephen J. Gould’s approach
Species selection:
- Emergent species-level trait
- Emergent species-level fitness
 « True »
species selection
- Aggregate traits
- Neutral traits
- 2 Confusions :
- The nature of extinction (why dissymetry between
speciation and extinction???) (p. 669)
- Species drifting and clade drifting (p. 718)
Gould’s analogy
between micro- and
macroevolution
Gould’s analogy
between microand
macroevolution
Hiérarchies historiques et fonctionnelles
« Domain unification » in Evolution
UNTBB is both a macroecological and a macroevolutionary theory :
Evolution at the community level is ecology !!!
(The equivalent of genetics at the community level is ecology)
Macroevolution
Megaevolution
Mesoevolution
- Fauna and flora
- Geological times
-Communities et metacommunities
- Ecological times
Microevolution
Darwinian evolution
- Populations
- Adaptations
Punctuated equilibrium theory,
fractal dynamics, etc.
UNTBB
Interspecific competition between
neutral individuals
Or
Niche assembly
Natural selection
(mutation, sélection, migration)
Intraspecific competition
Molecular evolution
- Populations
- Neutral variations
Neutral theory of molecular evolution
(mutation, genetic drift)
Interallelic competition
Conclusion: 2 macro-ecoevolutionary strategies
Neutral theory:
Ecology explains
macroevolution and
phylogeny
Hierachical sorting and selection
theory
Macroevolutionary processes
changed by environment
 Iterative and formal
 Realist and causal
Alleles
Organism
Species
Genus or family
Conclusion
 3 major challenges for the UNTBB:
1. Ecology:
• “Neutral theory is limited because it is incapable of predicting
which species are rare or abundant” (Tilman, 2004)…
Frustration for disappearance of causes, forces, etc:
Opportunity for ecologists to move from typological to
population thinking!!
 Challenge of the reconciliation with the niche perspective, like
the niche construction: from idealized invariant to ceteris paribus
laws.
• Dynamics of species substitution
Conclusion
2. Evolution:
• How to test the neutral nature of species substitution at the
community level ?
• Community invasion rate as a test for species substitution???
• The unification of macro-and microevolution !!!
• More collaboration between paleoevolution and community
ecology with regard to species evolution (for a better
contribution to the emergent property/emergent fitness
debate)?????
Bibliography
- Dornelas M., Connolly S. R., Hughes T. P., “Coral reef diversity refutes the neutral theory of
biodiversity”, Nature, 440, 2006, p. 80-82.
- Etienne R., Alonso D., McKane A. J., « The zero-sum assumption in neutral biodiversity
theory », Journal of Theoretical Biology, 248, 2007, p. 522-536.
- Holt Robert D., “Emergent neutrality”, Trends in Ecology and Evolution, 21, 10, 2006, p. 531533.
- Hu X.-S., He F. & Hubbel S. P., 2006, “Neutral theory in macroecology and population
genetics”, Oikos, 113, 3, p. 548-56.
- Hubbell Stephen P., 2001, The Unified Neutral Theory of Biodiversity and Biogeography,
Princeton and Oxford, Princeton University Press.
- Leigh Jr. E. G., « Neutral theory : a historical perspective », Journal of Evolutionary Biology,
20, 2007, p. 2075-2091.
- McGill Brian J. et al., “Species abundance distributions: moving beyond single prediction
theories to integration within an ecological framework”, Ecology Letters, 10, 2007, p. 9951015.
- Sterelny, K., Local Ecological Communities, Philosophy of Science, 73, 2006, p. 215-231.
- Tilman, D., “Niche tradeoffs, neutrality, and community structure: a stochastic theory of
resource competition, invasion, and community assembly”, Proc. Natl. Acad. Sci., 101, 2004,
p. 10854–10861.
- Volkov Igor, Banavar Jayanth R., Hubbell Stephen P., Maritan Amos, “A neutral theory and
relative species abundance in ecology”, Nature, 424, 2003, p. 1035-1037.
- Volkov Igor, Banavar Jayanth R., Hubbell Stephen P., Maritan Amos, “Patterns of relative
species abundance in rainforests and coral reefs”, Nature, 450, 1 november 2007, p. 45-49.