Transcript The Major Transitions in Evolution

Dynamical coexistence of
molecules
Eörs Szathmáry
Collegium Budapest (Institute for
Advanced Study)
The major transitions (1995)
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* These transitions are regarded to be ‘difficult’
Difficulty of a transition
• Selection limited (special environment)
• Pre-emption: first comer  selective
overkill
• Variation-limited: improbable series of rare
variations (genetic code, eukaryotic
nucleocytoplasm, etc.)
Difficult transitions are ‘unique’
• Operational definition: all organisms
sharing the trait go back to a common
ancestor after the transition
• These unique transitions are usually
irreversible (no cell without a genetic code,
no bacterium derived from a eukaryote can
be found today)
A common theme: origin of
higher levels of evolution
1. multiplication
2. heredity
3. variation
hereditary traits affecting
survival and/or
reproduction
Increase in complexity
(a) Duplication and divergence
(b) ‘symbiosis’
(c) epigenesis
Egalitarian and fraternal major
transitions (Queller, 1997)
The formose ‘reaction’: noninformational replication
formaldehyd
e
autocatalysi
s
glycolaldehyde
Butlerow, 1861
Von Kiedrowski’s replicator
Classification of replicators
Limited
heredity
Holistic
formose
Modular
Von
Kiedrowski
Unlimited
heredity
genes
Limited
types)
(# of individuals) >,  (# of
Umlimited
(# of individuals) << (# of types)
Eigen’s paradox (1971)
• Early replication must have been errorprone
• Error threshold sets the limit of maximal
genome size to <100 nucleotides
• Not enough for several genes
• Unlinked genes will compete
• Genome collapses
• Resolution???
Molecular hypercycle (Eigen,
1971)
autocatalysis
heterocatalytic
aid
Parasites in the hypercycle
(Maynard Smith, 1979)
short circuit
parasite
Population structure is necessary!
• Good-bye to the well-stirred flow reactor
• Adhesion to surface or compartmentation
• Hypercycles (with more than 4 members)
spiral on the surface and resist parasites,
BUT
• Are not resistant to short-circuits
• Collapse if the adhesive surface is patchy
• Only compartmentation saves them
The stochastic corrector model
for compartmentation
Szathmáry, E. &
Demeter L. (1987)
Group selection of early
replicators and the
origin of life. J. theor
Biol. 128, 463-486.
Grey, D., Hutson, V. &
Szathmáry, E. (1995) A
re-examination of the
stochastic corrector
model. Proc. R. Soc.
Lond. B 262, 29-35.
The stochastic corrector model
(1986, ’87, ’95, 2002)
metabolic
gene
replicas
e
membrane
Dynamics of the SC model
• Independently reassorting genes
• Selection for optimal gene composition between
compartments
• Competition among genes within the same
compartment
• Stochasticity in replication and fission generates
variation on which natural selection acts
• A stationary compartment population emerges
Group selection of early
replicators
• Many more compartments than templates
within any compartment
• No migration (fusion) between
compartments
• Each compartment has only one parent
• Group selection is very efficient
• Selection for replication synchrony
Bubbles and permeability
We do not know where lipids
able to form membranes had
come from!!!
A ‘metabolic’ system on the
surface (2000)
A cellular automaton simulation
Metabolic
Replication
Grey sites: neighbourhood
Black: empty site
X: potential mothers
• Reaction: template
replication
• Diffusion (ToffoliMargolus algorithm)
• Metabolic
neighbourhood
respected
Parasite on metabolism
• Parasites do not kill
the system
• Can be selected for
to perform useful
function
Nature 420, 360-363 (2002).
Maximum as a function of
molecule length
• Target and
replicase
efficiency
• Copying fidelity
• Trade-off among
all three traits:
worst case
Evolution of replicases on the
rocks
• All functions coevolve
and improve despite
the tradeoffs
• Increased diffusion
destroys the system
• Reciprocal altruism on
the rocks
‘Stationary’ population
efficient
replicases
parasites