Summary: Heterochrony, which is evolutionary change in a lineage in the timing of development, is a topic commonly treated in collegiate-level textbooks for introductory biology or evolution. For modern biologists and teachers, the term was popularized by Gould (1977) and later reaffirmed in numerous works, such as Alberch et al (1979), Bonner (1988), McKinney and McNamara (1991), Niklas (1994), Raff (1996), McNamara (1997) and Schlichting and Pigliucci (1998).
Heterochrony is one kind of developmental explanation to a basic question in biology: how can two related organisms be quite different in appearance? Other developmental explanations exist. The production of new innovations is a second explanation (and is not discussed further in this paper). A third explanation is heterotopy, an evolutionary change in a lineage in the spatial pattern of development. Heterotopy is increasingly considered to be of equal importance to heterochrony (Zelditch and Fink 1996; Schlichting and Pigliucci 1998). Indeed, the biological literature seems to be littered with examples and potential examples of heterotopy, despite the infrequent use of the term (Schlichting and Pigliucci 1998). In a sample I examined of 10 recently published introductory textbooks for biology courses, none include any mention of heterotopy, though 8 include coverage of heterochrony. To date heterotopy is yet to become an established topic in textbooks.
In this paper, I describe an inexpensive, simple
method using balloons to demonstrate heterochrony and heterotopy in the
classroom. The method can be used to supplement lectures and assigned
readings for a course, especially in the case that heterotopy is not treated
in the readings.