Homologous structures diverge through changes in the target genes that are regulated by conserved selector genes
Schematized views of the evolution of insect hindwings (a) and vertebrate forelimbs (b). The conservation of selector gene expression in insect hindwings (Ubx) and vertebrate forelimbs (Tbx5) suggests that the ancestral hindwings of insects and ancestral forelimbs of vertebrates also expressed these genes. While the selectors regulated certain target genes (colored boxes) in the ancestral appendage (left), these sets of genes changed over the course of evolution, resulting in dramatically different morphologies of homologous limbs in modern lineages (right).
between organisms. For example, the Hox gene Ubx is expressed in all insect hindwings and the limb selector gene Tbx5 is expressed in all vertebrate forelimbs, yet these genes regulate different sets of target genes in different lineages; the result is insect and vertebrate appendage diversity (Fig. 5.17).
The flexibility of regulatory networks and the modular nature of cis-regulatory control regions allow genetic interactions to evolve without changing the number of genes or even the primary sequence of the proteins encoded by animal genomes. Evolutionary modification of the regulatory interactions between developmental transcription factors and target genes allows different animal morphologies to be built using the same genetic and structural elements. Chapter 6 will extend our discussion of regulatory evolution to include the co-option of developmental regulatory genes in new fields or to pattern new structures. The evolution of novelty will be told as a story of using "old genes to perform new tricks."
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