key concept 19.5 Developmental Gene Changes Can Shape Evolution

The genetic switches that allow different structures to develop in different regions of the embryo can also give rise to major morphological differences among species. Changes in timing and position of a genetic switch (e.g., Gremlin expression) can generate morphological changes (e.g., webbed or unwebbed feet) that can be acted on by natural selection.

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  • A major developmental change can be caused by an alteration in a regulatory molecule.

  • Similar changes in gene expression in isolated populations of organisms can lead to the same evolutionary result, a process called parallel evolution.

French geneticist François Jacob suggested that evolution works like a tinker, assembling new structures by combining and modifying the available materials, and not like an engineer, who is free to develop dramatically different designs (a jet engine to replace a propeller-driven engine, for example). You have seen that morphological evolution is not usually governed by the acquisition of radically new genes, but proceeds primarily by “tinkering” with the expression patterns of existing genes. Thus developmental genes and their expression constrain evolution in two major ways:

  1. Nearly all evolutionary innovations are modifications of previously existing structures.

  2. The basic set of regulatory genes that control development is broadly conserved, changing only slowly over the course of evolution.