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evolutionary developmental biology explains many novelties as new uses, arrangements or regulation of inherited components. A new organ need not require an entirely new set of genes: changes to a gene regulatory network can deploy existing activities at a new place, time or strength during biological development. Novelty in the resulting phenotype is therefore compatible with continuity of the underlying materials.

One route is gene co-option. A regulator already controlling a useful cellular program can acquire a new domain of gene expression, bringing downstream activities into a different structure. The important unit may be an entire interacting module, rather than a single gene. In the classic ectopic-eye experiment, targeted gene expression of eyeless in developing fruit-fly tissues induced eye structures in abnormal locations. This shows that competent tissues can deploy an existing developmental program when a regulatory input changes; it does not show that a regulator alone constructs an eye independently of its cellular context.^[ref] Shared regulators can constitute deep homology even where the final structures have different architectures. Sharing one regulator, however, is not sufficient to establish that two complete organs are homologous.

cis-regulatory evolution supplies a way to change one deployment while preserving others. A transcription factor may function in several tissues, giving coding changes extensive pleiotropy. A tissue-specific enhancer can instead change the spatial pattern of gene expression. In sticklebacks, recurrent deletion of a pelvic enhancer of Pitx1 contributes to pelvic reduction while leaving other deployments of the gene available. Mapping, enhancer assays and transgenic restoration connected the regulatory region to the skeletal outcome.^[ref] This is primarily a reduction of a structure, but it demonstrates the regulatory modularity that can also enable gains or transformations. Not every enhancer is tissue-specific, and regulatory changes can themselves have extensive pleiotropy.

Developmental transformations can also involve heterochrony, a change in timing or duration, and heterotopy, a change in location. Extending a growth phase can alter a structure's proportions; moving a signaling source can alter a pattern. Changes in Hox genes or their regulation can alter regional identities along the body axis, while changes in downstream targets can modify one segment without changing every segment. serial homology supplies repeated structures that can subsequently specialize. These routes modify inherited developmental processes, but the resulting functional organization can still be new. The size of an adult change is not a reliable measure of the number of underlying mutations.

gene duplication makes another form of reuse possible. Initially the copies may share a function. subfunctionalization can partition ancestral deployments between them; neofunctionalization can give one copy a new function while the other preserves an old one. Changes in regulatory connections and protein sequence can then accumulate with fewer immediate costs. Duplication is not guaranteed to generate novelty: most extra copies may be lost or change gene dosage, and retaining a copy requires an account of its inheritance and effects on genotype fitness. Similarly, exaptation describes recruitment of an existing character to a new use; it is a functional historical claim, whereas gene co-option names a developmental mechanism that may underlie that history.

Reuse can also generate a new biochemical function. Comparative sequence analysis traced Antarctic fish antifreeze glycoproteins to an ancestral trypsinogen gene: retained ancestral segments and amplification of a short sequence helped construct a new repetitive coding region.^[ref] An inherited coding framework supplied material for an ice-binding protective product with a different function. This is a molecular novelty rather than a new organ, but it illustrates how reuse can build a new activity rather than merely preserve or remove an old one.

Development also changes which variants are accessible. A gene regulatory network can buffer many perturbations but respond sharply to a particular change, so mutation does not sample all imaginable forms equally. developmental bias can make similar outcomes recur in independent lineages. natural selection then filters the variants that actually arise; developmental accessibility and selective advantage are distinct explanations that can operate together. Repeated involvement of the same gene might reflect its useful regulatory position, an unusually high local mutation rate, or inheritance of the same ancestral allele. Distinguishing these requires comparative and functional evidence.

The quotation should not be interpreted as denying new sequence or new function. de novo gene birth can produce a newly translated product from previously non-coding DNA; mutations can create a new regulatory binding site; and a genuinely new interaction can arise between old components. Such cases challenge the narrower assertion that every new gene is a modified pre-existing coding gene. They still involve changes to existing DNA, cellular machinery and developmental context, rather than construction without material antecedents. Failure to detect a distant sequence relative is also insufficient evidence for de novo gene birth, because rapid divergence and incomplete sampling can hide ancestry.

A convincing evolutionary developmental account therefore combines a phylogenetic tree establishing ancestral states, comparative measurements at matched developmental stages, and perturbations that connect the proposed change to the new outcome. For a candidate enhancer, compare ancestral and derived sequences in the same integration context, include an inactive construct and the normal sequence as controls, and test replacement or rescue at the endogenous genetic locus. A reporter alone tests regulatory potential, not the necessity of the sequence in its native context. Novelties commonly arise by modifying inherited developmental systems, but their new organization and function are real, and the origin of new genes cannot be reduced to duplication alone.

A precursor of a digestive enzyme. Portions of an ancestral trypsinogen gene contributed to the origin of an Antarctic fish antifreeze glycoprotein; this does not mean that the new product retained the original digestive function.