Biology Reference
In-Depth Information
about developmental rate and timing. The focus is on timing because the issue is the coor-
dination between periodic biological phenomena (phenology) and the environment, both
abiotic and biotic. This has become an important issue recently because one of the best
documented responses to global warming is a shift in phenologies. One of the most
intriguing (and worrying) patterns is the discordant shifts in phenologies across trophic
levels, leading to a mismatch between organisms and their foods (
Both and Visser, 2005;
Post and Forchhammer, 2008; Post et al., 2008; Both et al., 2009; Miller-Rushing et al.,
2010
). To investigate such (mis)matches, we need to relate timing of development events
(such as birth and weaning) to the environment, both thermal and biotic.
The link between ecology and developmental rate has motivated many studies of
heterochrony as well (
Gould, 1977; McKinney, 1986; Emerson et al., 1988; Schweitzer and
Lohmann, 1990
), but the theory of heterochrony does not supply a general enough frame-
work for analyzing changes in phenology because the theory of heterochrony as
developed by
Gould (1977)
and formalized by
Alberch et al. (1979)
makes specific predic-
tions about morphology. Those predictions may be wrong even if developmental rate and
timing do evolve because heterochrony predicts that only developmental rates or timings
evolve. Developmental rate and timing can evolve and so can ontogenies of shape, hence
the descendant shape might not be predictable by extrapolating the ancestral ontogeny.
That the predictions are empirically refuted for some cases does not compromise the value
of the formalism devised by Alberch et al. for the study of heterochrony, a formalism that
we discuss in detail below, because the formalism was intended to apply solely to studies
of heterochrony and it is applicable to all cases of heterochrony. The scheme is nonetheless
limited in its applicability because it applies solely to studies of heterochrony, as Gould
and Alberch et al. defined “heterochrony”. We highlight that matter of definition because
there are multiple definitions of heterochrony in the literature, which are often inconsistent
and even mathematically incommensurate. That could be considered merely a matter of
semantics (
McKinney, 1999
), but the predictions that follow from a theory, as well as the
formal representation of a theory, depend on what the theoretical terms mean.
The long-standing fascination with heterochrony has made age data seem necessary for
virtually all studies of evolving ontogenies, even when the questions are not about the link
between ontogeny and ecology. But the relationship between shape and size is no less
important than that between shape and time. Both in context of function and development,
allometry is interesting in its own right. We thus begin with a discussion of allometry, and
why it is interesting in its own right, and then how to analyze it. We first review the
formalism for the analysis of allometry using traditional morphometric variables because
some hypotheses make more sense when framed in terms of those (size) measurements.
The interpretation of the results requires scaling coefficients. We then more briefly discuss
the geometric analysis of ontogenetic allometry (which was presented in Chapter 8).
Although the regression coefficients are not readily interpretable, geometric morpho-
metrics has notable advantages for testing a range of hypotheses about the evolution of
ontogeny, the final subject of this chapter.
This chapter begins with the review of the formalism for the analysis of allometry using
traditional morphometric variables, then briefly recalls the analysis of allometry using
geometric morphometric data, and then examines a series of hypotheses that can be tested
about
the evolution of ontogenetic trajectories. As well as comparing ontogenetic
