MegalocerosIrish Elk

Orthogenesis vs Hypermorphosis in Irish Elk (Megaloceros)


    The 13' antler rack in the Irish Elk (Megaloceros) [right] was formerly argued to be an example of Orthogenesis: continued evolutionary increase in size of a structure past the point where it is adaptive, due to an innate tendency of organisms. The extinction of dinosaurs was explained by suggesting they simply became "too big" to survive.

    Paleontologist Stephen J. Gould showed instead that the size of antlers in deer displays positive hypermorphosis with respect to body size: antlers develop proportionately faster than body size. Antler size in
Megaloceros is the expected size for a species of its height (left), and therefore requires no special evolutionary explanation. The same relationship holds within species, such that larger Megaloceros have proportionately larger antlers than smaller [right]. This is one example of the general phenomenon of developmental Allometry, where different body parts grow at different relative rates.

Homework: From the five points in the right-hand graph, calculate the linear equation (Y = mX + B) to relate Antler Size (Y) as a function of Length of Skull (X).


Text material © 2021 by Steven M. Carr