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TESTING HYPOTHESES ABOUT THE EVOLUTION OF ON TOGENY
Ontogenetic scaling and heterochrony are the most frequently studied and reported
kinds of modifications of ontogeny (e.g. Shea, 1983b, 1992; McKinney, 1986; Wayne, 1986;
McKinney and McNamara, 1991; German et al., 1994; Neige et al., 1997; Cronier and
Courville, 2003; Lieberman et al., 2007; Galatius et al., 2011; Gerber, 2011; Piras et al.,
2011 ). That is partly because the definition of “heterochrony” was broadened to the point
that any and all modifications of ontogeny qualify as “heterochrony” (see, especially
McKinney and McNamara, 1991 ). But even if we discount studies that make heterochrony
synonymous with the evolution of ontogeny, we are still left with an enormous number of
studies that support the hypothesis of either heterochrony or ontogenetic scaling. Many of
these use traditional morphometric data, so we first consider some of the methodological
issues posed by these data and whether methodology might explain why heterochrony
and scaling appear to be the predominant modes of evolutionary change in ontogenies.
However, we would also point out many studies use ontogenetic scaling primarily as a
criterion of subtraction. The idea is to identify the traits that do not evince scaling rather
than to support a hypothesis of scaling.
Numerous studies do reject a hypothesis of heterochrony or scaling in favor of parallel
ontogenies. For example, parallel trajectories have been inferred for postnatal facial ontog-
eny of several hominins (e.g. Ponce de Leon and Zollikofer, 2001; Ackermann and Krovitz,
2002; Zollikofer and Ponce de Leon, 2010 ), for postnatal mandibular ontogeny of common
and pygmy chimpanzees ( Boughner and Dean, 2008 ), for postnatal scapular development
of anthropoids ( Young, 2008 ) as well as for postnatal development of the postcranial
skeleton for callitrichines ( Falsetti and Cole, 1992 ) and anthropoids ( Jungers and Cole,
1992 ). Additionally, this pattern has been detected in post-weaning craniofacial ontogeny
of seven pairs of congeneric sigmodontine rodents ( Voss and Marcus, 1992 ) and for
the evolution of giant damselfishes ( Frederich and Sheets, 2010 ). Some of these cases,
however, have been vigorously challenged, most notably that of postnatal hominin facial
ontogenies (e.g. O'Higgins et al., 2001; Strand Vioarsdottir et al., 2002; Cobb and
O'Higgins, 2004; Bastir et al., 2007 ) on the grounds that hypothesis of a conserved ontoge-
netic trajectory can be rejected when tested statistically.
The question raised by the case of postnatal hominin facial ontogenies is whether
methods commonly used to infer shared ontogenies fail to detect divergent ones even
when trajectories do in fact diverge. Because many studies have used traditional mor-
phometric data, and these data remain valuable for the reasons discussed above, we first
consider the methodological issues raised by them.
Framing Hypotheses About the Ontogeny of Shape in Terms of Size Variables
Both the clock model and the Alberch et al. scheme framed the hypothesis of hetero-
chrony (and scaling) in terms of shape. Thus, to test the hypotheses using traditional
morphometric data, the hypotheses must be translated into expectations for size variables.
Sometimes this step is skipped, even intentionally, on the grounds that shape is merely a
derivative of size, making analysis of shape change just a comparison of relative sizes
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