  The evolution of species habitat range is an important topic over a wide
range of research fields. In higher organisms, habitat range evolution is
generally associated with genetic events such as gene duplication. However, the
specific factors that determine habitat variability remain unclear at higher
levels of biological organization (e.g., biochemical networks). One widely
accepted hypothesis developed from both theoretical and empirical analyses is
that habitat variability promotes network modularity; however, this
relationship has not yet been directly tested in higher organisms. Therefore, I
investigated the relationship between habitat variability and metabolic network
modularity using compound and enzymatic networks in flies and mammals. Contrary
to expectation, there was no clear positive correlation between habitat
variability and network modularity. As an exception, the network modularity
increased with habitat variability in the enzymatic networks of flies. However,
the observed association was likely an artifact, and the frequency of gene
duplication appears to be the main factor contributing to network modularity.
These findings raise the question of whether or not there is a general
mechanism for habitat range expansion at a higher level (i.e., above the gene
scale). This study suggests that the currently widely accepted hypothesis for
habitat variability should be reconsidered.
