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energy and matter. We can simulate this in a lab, by picking a drop of the culture,
and putting it into a fresh medium. In this way, the survival from a test tube
to another is a random process, with a vanishing probability from the point of
view of individuals. However, when viewed by genes, there are many individuals
that share the same genetic information, while other ones may have a mutated
version. The genetic selection favors the genes that are present in a larger number
of copies, and have therefore more chances to survive. The growth of bacteria just
after being inoculated into a fresh medium is exponential, so those genomes that
allow a faster replication of individuals will be present in more copies. This is the
winning strategy of most bacteria. The speed is not the only quantity that can be
optimized. One could replicate more slowly, but produce some toxic chemical to
which it is immune. Or, if the refreshing occurs lately, one could prefer to develop
ways of surviving starvation. Finally, if there are oscillations of temperature, acidity,
or in the presence of predators, a better strategy could be that of developing ways
to overcome these accidents. All these additional instruments need more proteins,
more DNA coding, and therefore a slower replications speed. How can the optimum
compromise between speed and complexity be reached?
The main idea of evolution is that this is a self-organized process. Mutations
produce variety, and selection prunes it. We have to stress that selection is not
an absolute limit, it depends on the rest of the environment. The probability of
a given genome to pass to next \ask" generation (the tness) does not depend
on how fast in absolute it replicates, but on how fast it replicates in comparisons
with the other genotypes present in the environment. And, as soon as the \best"
genotype, possibly recently arisen by a mutation, is picked up and colonizes a new
ask, its comparative advantage vanishes. Due to selection, copies of it become more
numerous, and the competition becomes harder. This eect, named \red queen" is
the main motor of evolution.
But let us dig more on this subject. The way in which an individual if perceived
by others, or inuences the environment is called its \phenotype". It is a product
of proteins (mainly) and therefore of the genes in its genome. It may depend also
on the age of the individual, on past experiences, and on the interactions with
other genomes (often very similar, as we shall see). A genome is a \bag of genes",
extending the \denition" (not very mathematical) of gene to all genetic elements
that have a recognizable persistence along generations, and that inuence in some
way their phenotype.
A gene is successful and tends to spread among the population if it confers a
selective advantage to the individuals that carry it, and this happens through its
expression, i.e., the phenotype. The phenotype is in general a rather complex func-
tions of the genes that constitute it. Genes very rarely control directly some aspect
of the phenotype. What a gene does, is to produce a protein, that participates to
the intricate biochemical machinery of the cell, and in multicellular organisms to
the development of the organism. Variants of the gene (alleles) produces similar
