the major transitions in evolution eörs szathmáry collegium budapest eötvös university münchen
TRANSCRIPT
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The major transitions in evolution
Eörs Szathmáry
Collegium Budapest Eötvös University
München
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Units of evolution
Some hereditary traits affect survival and/or fertility
1. multiplication
2. heredity
3. variability
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John Maynard Smith (1920-2004)
• Educated in Eaton• The influence of J.B.S.
Haldane• Aeroplane engineer• Sequence space• Evolution of sex• Game theory• Animal signalling• Balsan, Kyoto,
Crafoord prizes
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The major transitions (1995)
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* These transitions are regarded to be ‘difficult’
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Recurrent themes in transitions
• Independently reproducing units come together and form higher-level units
• Division of labour/combination of function
• Origin of novel inheritance systems Increase in complexity
• Contingent irreversibility
• Central control
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Difficulty of a transition
• Selection limited (special environment)
• Pre-emption: first come selective overkill
• Variation-limited: improbable series of rare variations (genetic code, eukaryotic nucleocytoplasm, etc.)
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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)
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A crucial insight: Eigen’s paradox (1971)
• Early replication must have been error-prone
• Error threshold sets the limit of maximal genome size to <100 nucleotides
• Not enough for several genes• Unlinked genes will compete• Genome collapses• Resolution???
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Molecular hypercycle (Eigen, 1971)
autocatalysis
heterocatalytic aid
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Parasites in the hypercycle (JMS)
parasite
short circuit
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Gánti’s chemoton model (1974)
ALL THREE SUBSYSTEMS ARE AUTOCATALYTIC
template copying
metabolism
membrane growth
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The latest edition: OUP 2003
• After several editions in Hungarian
• Two previous books (the Principles and Contra Crick) plus one essay
• Essays appreciating the biological and philosophical importance
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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.
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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
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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 Chromosomes!
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Egalitarian and fraternal major transitions (Queller, 1997)
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The simplest cells are bacterial
• THUS we want to explain the origin of some primitive bacterium-like cell
• Even present-day bacteria are far too complex• The main problem is the genetic code
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The eukaryotic cell is very complex—too complex!
These cells have endosymbiont-derived organelles
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LECA and phagocytosis
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Most forms of multicellularity result from fraternal transitions
• Cells divide and stick together
• Economy of scale (predation, etc.)
• Division of labour follows
• Cancer is no miracle (Szent-Györgyi)
• A main difficulty: “appropriate down-regulation of cell division at the right place and the right time” (E.S. & L. Wolpert)
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The royal chamber of a termite
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Division of labour
• Is advantageous, if the “extent of the market” is sufficiently large
• If it holds that a “jack-of-all-trades” is a master of none
• Not always guaranteed (hermaphroditism)
• Morphs differ epigenetically
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Hamilton’s rule
b r > c• b: help given to recipient• r: degree of genetic relatedness between altruist and
recipient• c: price to altruist in terms of fitness• Formula valid for INVASION and MAINTENANCE• APPLIES TO THE FRATERNAL TRANSITIONS!!!
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The origin of insect societies• Living together must have some advantage in the
first place, WITHOUT kinship• The case of colonies that are founded by
UNRELATED females• They build a nest together, then…• They fight it out until only ONE of them
survives!!!• P(nest establishment together) x P(survival in the
shared nest) > P(making nest alone) x P(survival alone)
• True, even though P(survival in the shared nest) < P(survival alone)
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Why is often no way back?
• There are secondary solitary insects
• Parthenogens arise again and again
• BUT no secondary ribo-organism that would have lost the genetic code
• No mitochondrial cancer
• No parthenogenic gymnosperms
• No parthenogenic mammals
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Contingent irreversibility
• In gymnosperms, plastids come from one gamete and mitochondria from the other: complementary uniparental inheritance of organelles
• In mammals, so-called genomic imprinting poses special difficulties
• Two simultaneous transitions are difficult squared: parthenogenesis per se combined with the abolishment of imprinting or complementary uniparental inheritance
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Central control
• Endosymbiotic organelles (plastids and mitochondria) lost most of their genes
• Quite a number of genes have been transferred to the nucleus
• The nucleus controls organelle division
• It frequently controls uniparental inheritance, thereby reducing intragenomic conflict
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What makes us human?
• Note the different time-scales involved• Cultural transmission: language transmits itself as
well as other things• A novel inheritance system
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Evolution OF the brain
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The coevolutionary wheel
Intermediate capacities: e.g. analogical reasoning