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
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
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)
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
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
Cell count in a nematode
Bell and Mooers, 1997
Organism size and number of cell types (Bell)
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?
Genome size and gene number
Genome size and gene number II
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:
Protein functions
Genes for various functions
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
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)
Number of transcriptional activator families
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
Temporal complexity - yeast
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)
Increase in genetic complexity
(a) duplication and divergence(b) symbiosis(c) epigenesis
Animal phylogeny
* sequenced genomes
Hox gene duplications
ParaHox evolution
Some vertebrate proteins assembled from modules