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proteins in other bodies. The mechanism of blind production of variant plus selec-
tion is able to \optimize" this assembly of organisms. This optimization is always at
short term: the competition (red queen) forbids the development or maintenance of
\accessories" that may be useful in the future, or that may lead to the development
of new functions. Selection tends to prefer quick-and-dirty solutions. d
We have to consider that the phenotype is often best understood as the result
of a pool of genotypes, not as a characteristic of an individual. For instance, an
ant or termite nest is the result of the cooperation of several individuals, and of
many generations. A multicellular body is the result of the cooperation of many
cells. What these cells have in common is the sharing of large portions of their
genome (all for multicellular bodies). In other cases (symbionts, larger communities
like human societies), cooperation tends to favor assemblies of rather unrelated
individuals. How cooperation can arise from the \selshness" of genes, is one of the
most interesting outcome of evolution.
We have spoken mainly of bacteria, and of asexual reproduction. However, many
eukaryotes reproduce sexually. Sex poses a puzzling problem [5, 6]. In true sexual
reproduction (not just exchange of genetic material like in some bacteria), the male
does oer only genetic information, in form of sperm. The female adds her half of
genetic material, plus all the body. The production rate of ospring is therefore
proportional to the abundance of females. The sex rate at birth is generally about
50%, and in comparisons with an asexual replicator or a parthenogenic female, the
eciency is just the half. In spite of this, many organisms that are able to reproduce
both sexually and parthenogenycally (many plants, some insects and amphibians)
maintain the sexual habits.
Sex induces also dangerous habits. Especially in large animals, a kind of runo is
observed: one sex develops a phenotypic characteristic, like the tail of peacock, and
the other sex nds that the partners with that characteristic are more interesting,
and therefore reproduce more. The following generations enhance those character-
istics, until the limit (trade-o between survival and exaggeration of the character)
is reached.
15.2.2. The sequence space
Let us start our investigation by considering \standard" living forms, based on
nucleic acid. The storage of information is an one-dimensional structure, possibly
fragmented into many chromosomes. We shall call this information the genome,
and in usual living beings it is formed by one or more laments of DNA or RNA.
Such laments are formed by a sequence of symbols (basis), and therefore are digital
pieces of information.
d This eect is particularly evident in the fact that birds living in islands with no predator tend
to lose the capacity of ying: a non-ying relative that can invest more energy in egg production
tends to be selected, even if when predators reappear it would be useful to have working wings.
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