Heterotopy

Heterotopy is an evolutionary change in the spatial arrangement of an animal's embryonic development, complementary to heterochrony, a change to the rate or timing of a development process. It was first identified by Ernst Haeckel in 1866 and has remained less well studied than heterochrony.

The concept of heterotopy, bringing evolution about by a change in the spatial arrangement of some process within the embryo, was introduced by the German zoologist Ernst Haeckel in 1866. He gave as an example a change in the positioning of the germ layer which created the gonads. Since then, heterotopy has been studied less than its companion, heterochrony which results in more readily observable phenomena like neoteny. With the arrival of evolutionary developmental biology in the late 20th century, heterotopy has been identified in changes in growth rate; in the distribution of proteins in the embryo; the creation of the vertebrate jaw; the repositioning of the mouth of nematode worms, and of the anus of irregular sea urchins. Heterotopy can create new morphologies in the embryo and hence in the adult, helping to explain how evolution shapes bodies.

In terms of evolutionary developmental biology, heterotopy means the positioning of a developmental process at any level in an embryo, whether at the level of the gene, a circuit of genes, a body structure, or an organ. It often involves homeosis, the evolutionary change of one organ into another. Heterotopy is achieved by the rewiring of an organism's genome, and can accordingly create rapid evolutionary change.

The evolutionary biologist Brian K. Hall argues that heterochrony offers such a simple and readily understood mechanism for reshaping bodies that heterotopy has likely often been overlooked. Since starting or stopping a process earlier or later, or changing its rate, can clearly cause a wide variety of changes in body shape and size (allometry), biologists have in Hall's view often invoked heterochrony to the exclusion of heterotopy.

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