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