Serial homology is the concept that initially existing structures were gradually modified via discrete intermediary steps until such time as an evolutionary novelty (e.g., jaws) appeared.
Many examples of serial homology, e.g. the body segments of many animals (vertebrates, arthropods etc), are examples of gene duplication on regulatory genes such as homeobox genes, followed by evolution differentiating the duplicated genes.
See also: Homology
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