The discovery of the genes that control the development of Drosophila provided biologists with tools to investigate the development of other organisms. For example, when scientists used homeobox DNA as a hybridization probe (see Key Concept 14.4) to search for similar genes elsewhere, they found the homeobox sequence in many genes in other organisms. This and other discoveries that followed revealed similarities in the molecular events underlying morphogenesis in organisms ranging from flies to fish to mammals. These findings suggested that just as the forms of organisms evolved through descent with modification from a common ancestor, so did the molecular mechanisms that produce those forms. Biologists started to ask new questions about the interplay between evolutionary and developmental processes, a field of study called evolutionary developmental biology, or evo-
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Common developmental pathways are involved in embryonic development of many animals, such as the formation of eyes in both insects and vertebrates.
DNA sequences and regulatory proteins common in developmental pathways comprise a “toolkit.” Genetic switches control how the toolkit is expressed.
Heterochrony has significant importance, as illustrated by the extreme length of a giraffe’s neck.
As organisms have evolved, there are changes in where (heterotopy), when (heterochrony), and to what extent (heterometry) developmental genes are expressed.
Spatial differences in the expression of a developmental gene are known as heterotopy.