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Digest: A decoupling between size and shape in Indo‐Pacific shore fishes

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Santiago Herrera-Alvarez at University of Chicago
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To what extent has body size driven body shape disparity across fish species? Friedman et al. (2019) found that, for Indo‐Pacific shore fishes, body size accounts for a low fraction of variation, suggesting that there is a very weak relationship between body size and shape in this group. This article is protected by copyright. All rights reserved
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DIGESTS
doi:10.1111/evo.13822
Digest: A decoupling between size and
shape in Indo-Pacific shore fishes∗
Santiago Herrera- ´
Alvarez1,2
1Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637
2E-mail: sherrera2@uchicago.edu
Received May 24, 2019
Accepted July 5, 2019
To what extent has body size driven body shape disparity across fish species? Friedman et al. found that for Indo-Pacific shore
fishes, body size accounts for a low fraction of variation, suggesting that there is a very weak relationship between body size and
shape in this group.
Understanding the processes that underlie morphological evolu-
tion is especially important when addressing how the present di-
versity of organic forms (i.e., disparity) has come to be. Research
on disparity has given special attention to body size because it
affects nearly every aspect of a species’ biology. Evolutionary
changes in size have major impacts on physiology and morphol-
ogy, which in turn affect behavioral and life-history traits.
The relationship between body shape and size, known as al-
lometry, is a classic example of how changes in body size can alter
overall organismal design (Gould 1966). Allometry describes how
traits scale with body size, resulting in changes to an organism’s
shape as well as the relative contribution of body size in gener-
ating disparity (Mirth et al. 2016). For example, morphological
diversity among insect orders is largely produced by changing the
size of the wings relative to the body (Frankino et al. 2005; Mirth
et al. 2016).
Allometries are thought of as the result of different traits
being under common growth regulation during development,sug-
gesting that morphological disparity is subject to a size-dependent
constraint. For instance, 50% of morphological disparity at the
phylum level within metazoans is explained by body size varia-
tion (Deline et al. 2018). An important question that remains is
how general such a trend is at different phylogenetic levels. Thus,
understanding the evolution of allometric relationships across
clades has become a prominent focus in evolutionary biology.
∗This article corresponds to Friedman, S. T., C. M. Martinez, S. A. Price,
and P. C. Wainwright. 2019. The influence of size on body shape diversi-
fication across Indo-Pacific shore fishes. Evolution. https://doi.org/10.1111/
evo.13755.
In this issue, Friedman et al. (2019) investigate the extent to
which body size drives disparity in a group of Indo-Pacific shore
fishes that display high diversity in both body size and shape.
Using geometric morphometrics, the authors found that changes
in body size only explain a low fraction (2.9%) of variation in
shape across the entire fish assemblage, contrasting markedly
with previous results for other vertebrates: for example, 80% of
variation in skull and beak shape are explained by body size in
birds of prey (Bright et al. 2016), and 34% to 46% of skull shape
is explained by body size in mammals (Cooper and Purvis 2009;
Fig. 1). Moreover, they found substantial variation in the effect of
body size on generating shape disparity across families, ranging
from 3% to 50%. Thus, the results suggest that variation in shape
between fish species is decoupled from size evolution, providing
the first case of a release from allometric constraints.
Interestingly, the authors also found a (weak) negative rela-
tionship between rates of shape evolution and body size, such that
larger fishes evolve more slowly in shape disparity (although they
may have higher overall disparity than smaller fishes). There is a
well-established negative relationship between rates of molecular
evolution and body size; however, identifying the genetic com-
ponents of complex traits such as body shape and linking rates
of molecular evolution to morphological evolution remains an
exciting and promising endeavor.
The results of the present paper show that for Indo-Pacific
fishes, disparity is not constrained by size, suggesting that lineage-
specific dynamics have dominated the evolution of form in this
group. Future research may investigate the extent to which allom-
etry has influenced macroevolutionary patterns of diversity, and
most importantly, how allometries themselves evolve.
2028
C2019 The Author(s). Evolution C2019 The Society for the Study of Evolution.
Evolution 73-9: 2028–2029
DIGEST
Figure 1. Bar plot showing the average amount of variation in shape explained by body size in six vertebrate groups. For birds and
mammals, shape was measured for skull-beak and skull-mandible, respectively. For fishes, data of shape corresponds to overall body
form. Data from Bright, et al. (2016), Cooper & Purvis (2009), and Friedman et al. (2019). Images are labeled for noncommercial use with
modification.
DATA ARCHIVING
The doi for data is https://doi.org/10.1073/pnas.1602683113.
LITERATURE CITED
Bright, J. A., J. Marug´
an-Lob´
on, S. N. Cobb, and E. J. Rayfield. 2016. The
shapes of bird beaks are highly controlled by nondietary factors. Proc.
Natl. Acad. Sci. USA 113:5352–5357
Cooper, N., and A. Purvis. 2009. What factors shape rates of phenotypic evo-
lution? A comparative study of cranial morphology of four mammalian
clades. J. Evol. Biol. 22:1024–1035.
Deline, B., J. M. Greenwood, J. W. Clark, M. N. Puttick, K. J. Peterson, and P.
C. J. Donoghue. 2018. Evolution of metazoan morphological disparity.
Proc. Natl. Acad. Sci. USA 115:E8909–E8918.
Frankino, W. A., B. J. Zwaan, D. L. Stern, and P. M. Brakefield. 2005. Natural
selection and developmental constraints in the evolution of allometries.
Science 307:718–720.
Friedman, S. T., C. M. Martinez, S. A. Price, and P. C. Wainwright. 2019. The
influence of size on body shape diversification across Indo-Pacific shore
fishes. Evolution 73:1873–1884.
Gould, S. J. 1966. Allometry and size in ontogeny and phylogeny. Biol. Rev.
41:587–640.
Mirth, C. K., W. A. Frankino, and A. W. Shingleton. 2016. Allometry and
size control: what can studies of body size regulation teach us about the
evolution of morphological scaling relationships? Curr. Opin. Insect Sci.
13:93–98.
Associate Editor: K. Moore
Handling Editor: Mohamed A. F. Noor
SUBMIT A DIGEST
Digests are short (500 word), news articles about selected original
research included in the journal, written by students or postdocs. These
digests are published online and linked to their corresponding original
research articles. For instructions on Digests preparation and submission,
please visit the following link: https://sites.duke.edu/evodigests/.
EVOLUTION SEPTEMBER 2019 2029
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Understanding the causes of body shape variability across the tree of life is one of the central issues surrounding the origins of biodiversity. One potential mechanism driving observed patterns of shape disparity is a strongly conserved relationship between size and shape. Conserved allometry has been shown to account for as much as 80% of shape variation in some vertebrate groups. Here, we quantify the amount of body shape disparity attributable to changes in body size across nearly 800 species of Indo‐Pacific shore fishes using a phylogenetic framework to analyze 17 geometric landmarks positioned to capture general body shape and functionally‐significant features. In marked contrast to other vertebrate lineages, we find that changes in body size only explain 2.9% of the body shape variation across fishes, ranging from 3–50% within our 11 sampled families. We also find a slight but significant trend of decreasing rates of shape evolution with increasing size. Our results suggest that the influence of size on fish shape has largely been overwhelmed by lineage‐specific patterns of diversification that have produced the modern landscape of highly diverse forms that we currently observe in nature. This article is protected by copyright. All rights reserved
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