  Most of the DNA that composes a complex organism is non-coding and defined as
junk. Even the coding part is composed of genes that affect the phenotype
differently. Therefore, a random mutation has an effect on the specimen fitness
that strongly depends on the DNA region where it occurs. Such heterogeneous
composition should be linked to the evolutionary process. However, the way is
still unknown.
  Here, we study a minimal model for the evolution of an ecosystem where two
antagonist species struggle for survival on a lattice. Each specimen possesses
a toy genome, encoding for its phenotype. The gene pool of populations changes
in time due to the effect of random mutations on genes (entropic force) and of
interactions with the environment and between individuals (natural selection).
  We prove that the relevance of each gene in the manifestation of the
phenotype is a key feature for evolution. In the presence of a uniform gene
relevance, a mutational meltdown is observed. Natural selection acts quenching
the ecosystem in a non-equilibriumstate that slowly drifts, decreasing the
fitness and leading to the extinction of the species. Conversely, if a specimen
is provided with a heterogeneous gene relevance, natural selection wins against
entropic forces, and the species evolves increasing its fitness. We finally
show that heterogeneity together with spatial correlations is responsible for
spontaneous sympatric speciation.
