increase in complexity in evolution (questions, answers, research programme) eörs szathmáry...
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Increase in complexity in evolution (questions, answers,
research programme)
Eörs Szathmáry
Collegium Budapest Eötvös University Budapest
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What are we interested in?
• Genetic basis of organismic complexity
• What is organismic complexity?
• Complex morphology?
• Complex behaviour?
• How do you quantify complexity for the different cases?
• An intuitive feel for complexity is widespread
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Programme complexity
• S (spatial): storage space needed• T (temporal): execution time• P (programme): the size of the shortest
programme with given input and output, given an agreed language
• Partly independent• Short programmes with complicated
dynamics (chaos, cellular automata)
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Complexity II• Kolmogorov: entirely random sequence has
the highest complexity
• Another problem: in general one cannot prove that a given programme is the shortest possible
• A string is random if the minimal programme producing it is about as long as the string
• Randomness cannot be contracted
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The number of cell types in an organism (Bonner)
• Countable at our present state of knowledge
• Can be refined with molecular techniques (microarrays)
• Fits the intuition rather well
• In the animal world there is a correlation between number of cell types and organism size, hence between size and complexity
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Cell count in a nematode
Bell and Mooers, 1997
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Organism size and number of cell types (Bell)
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Does complexity correlate with the number of genes?
• A few years ago this seemed to be the case• There is no a priori reason why this should
be so• Algorithmic complexity: the length of the
minimal programme, written in a specified language, that solves a particular problem
• Why should tinkered programmes be minimal?
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Genome size and gene number
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Genome size and gene number II
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Gene number is not so good
• There is a correlation with complexity, but rather weak…
• Although there is an interesting pattern in the fraction of genes devoted to various functions:
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Protein functions
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Genes for various functions
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Interaction density among genes is better (Szathmáry et al. 2001
Science)• Cell types need genes to be switched on and
off in an orderly manner
• Genes regulate other genes
• Once a gene is set, this state can be passed on to offspring in cell division
• Epigenetic inheritance (Jablonka & Lamb, 1995) systems
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Complexity related to network properties of interacting genes?
• Networks are fashionable, but this by itself does not render them uninteresting
• Other areas in biology have a vast experience with network properties
• Food web theory in ecology• Connectance = (number of existing
links)/(number of possible links)
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Number of transcriptional activator families
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Egy gén számos más gént szabályozhat
• Az X gén terméke egy transzkripciós faktor
• Ez a fehérje az érintett gének szabályozó régiójához kötődik
• Aktiválás és gátlás egyaránt lehetséges
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Temporal complexity - yeast
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Complexity must be characterized slightly (?) better
• Delegated complexity: a generative system (genes, chemistry, language) can be launched with a finite number of discrete entities
• Immune and nervous systems: excellent examples• Information carrying capacity of those systems
should be quantified and combined• Plants do not have a nervous/immune system, they
use secondary metabolites, which must be coded explicitly (25,498 genes in Arabidopsis)
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Increase in genetic complexity
(a) duplication and divergence(b) symbiosis(c) epigenesis
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Animal phylogeny
* sequenced genomes
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Hox gene duplications
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ParaHox evolution
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Some vertebrate proteins assembled from modules