Posted by: paulgarner | November 9, 2010

Heterochrony in evolution and the fossil record

Changes in the timing of developmental events (or heterochrony) are thought to have played a major role in evolution, leading either to paedomorphosis (the retention of juvenile traits in the adult) or peramorphosis (in which traits go on developing ‘beyond’ the adult form). Some have argued, for example, that the domestic dog originated from the wolf as the product of artificial selection for juvenile behaviour and the morphological traits that came with it. In fact many organisms are complex mosaics of paedomorphic and peramorphic traits, for example the ostrich which has downy feathers like a chick but also very powerful, muscular legs.

Creationists haven’t written much about heterochrony, although Leonard Brand has a section on it in Faith, Reason, and Earth History (Brand 2009, pp.194-202). He suggests that although heterochrony might be helpful in explaining how significant morphological change occurred rapidly within lower taxonomic groups, it would not produce new body plans or other major changes. Of course, evolutionary biologists propose that heterochrony has led to some major evolutionary transitions. The evolution of chordates from a sessile filter-feeding ancestor, for instance, is said to have occurred when the motile larval form of the ancestor became sexually mature before metamorphosis. The role that heterochrony has played in morphological evolution (and the possible limits to it) hasn’t really been explicated by creationists so far.

Also, what are creationists to make of the many cases where organisms appear to display paedomorphic or peramorphic trends in the fossil record? For instance, the brachiopod Tegulorhynchia shows a trend towards a larger pedicle opening and smaller umbonal angle through the Cenozoic, both interpreted as paedomorphic changes (McNamara 1983). Similarly, Lower Cambrian oryctocephalid trilobites in China show a paedomorphic decrease in thoracic segments and increase in pygidial segments as one moves up the stratigraphic section (McNamara et al. 2006). From a creationist perspective, examples in the Cenozoic might be explained in terms of post-Flood intrabaraminic diversification, but that explanation won’t work for those in Palaeozoic or Mesozoic sediments (which are thought to have been laid down during the Flood). Is it possible to explain pre-Cenozoic examples in terms of Flood depositional processes, perhaps preserving ecological (and hence morphological) clines from the pre-Flood world? Clearly creationists have a lot more thinking to do in this area.

References

Brand L. 2009. Faith, Reason, and Earth History: A Paradigm of Earth and Biological Origins by Intelligent Design. Second Edition. Andrews University Press, Berrien Springs, Michigan.

McNamara K. J. 1983. The earliest Tegulorhynchia (Brachiopoda: Rhynchonellida) and its evolutionary significance. Journal of Paleontology 57(3):461-473.

McNamara K. J., Yu F., Zhiyi Z. 2006. Ontogeny and heterochrony in the Early Cambrian oryctocephalid trilobites Changaspis, Duyunaspis and Balangia from China. Palaeontology 49(1)1-19.

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