A novel rriethod that allows us to study the emergence of modularity
for genotype-phenotype mapping in the course of Darwinian evolution is described. The evolutionary method used is based on cornposite chromosomes with two parts; One is a binary genotype whereas the other corresponds to the mapping of genes onto phenotype characters. For such generalized chromosomes the modularity is determined by the following intuitive way: The genes are divided into two subgroups; simultaneously with this decomposition also an accompanied decomposition of the set of phenotype characters is defined. We expect that for chromosomes with rnodular structures the genes frorn one group are rnapped onto characters from the respective group, an appearance of “crosslink” mappings is rnaximally suppressed. A fundamental question for the whole evolutionary biology (and also for evolutioriary algorithms and connectionist cognitive science) is the nature of mechanism of evolutionary emergence of modular structures. An idea of effective fitness is used in the presented explanatory simulations. It is based on the rnetaphor of Hinton and Nowlan theory of the Baldwin eífect, and was ušed as an effective idea for generalization of evolutionary algorithms. The effective fitness reflects not only a static concept of the phenotype, but also its ability to be adapted (learned) within a neighborhood of the respective chromosome. The chromosomes determined in the presented paper inay be understood as objects with the type of plasticity. The rnetaphor of the Baldwin effect (or effective fitness) applied to evolutionary algorithms offers an evolutionary tool that is potentially able to produce the emergence of modularity.