Turtles, monsters and the revised evolutionary theory
Evolutionary theory is incomplete. That claim has been made
several times since Darwin’s 1859 publication of On the
Origin of Species—specially between those studying paleon-
tology and development (e.g., Alpheus Hyatt, Richard Owen,
and more recently Stephen J. Gould; Amundson, 2005;
Gould, 1977; Pfeifer, 1965)—but recently a growing
movement has gained space on scientific and general public
forums calling for an Extended Evolutionary Synthesis of
Evolution (Laland et al., 2015; Pigliucci & Finkelman, 2014).
Its supporters argue that the Modern Synthesis—developed
during the first half of the 20th century—leaves out of the
picture certain processes related to ontogenetic development
(but not only those; for a summary see Laland et al., 2015),
that have the potential to causally explain some evolutionary
events poorly addressed by the microevolution centered view
of the Modern Synthesis.
In this context, Olivier Rieppel’sTurtles as Hopeful
Monsters joins the movement to increase those claims. The
title may be slightly misleading: the book is not about turtles
—although they play a very important role in the argument,
only the last two chapters are “turtle-focused”—but about the
last concept, the “Hopeful Monsters.”The author provides a
very detailed historical and philosophical account of how
early critics of Darwin’s theory saw its two basic processes,
mutation and natural selection, as insufficient to explain all
The term Hopeful Monster was coined by Richard
Goldschmidt, a German–Jewish geneticist who spent the
last part of his career at the University of California,
Berkeley, studying development, genetics, and evolution. A
“monstrosity,”according to Goldschmidt (1940), is a
phenotypic alteration that develops in a mutant individual,
such as a Manx cat with truncated tail vertebrae. For this
individual, the mutation does not generate an advantage
(although it may not be disadvantageous, also), hence, it is
just a monster. However, the same monstrosity in a non-bird
dinosaur, say an Archaeopteryx mutant, may result in a
rearrangement of the tail feathers, improving its flight
capacity and its fitness. The mutant Archaeopteryx is, thus,
a hopeful monster.
This kind of mutation should occur early in ontogeny,
reprogramming the developmental pathway while the embryo
is not well-differentiated yet, resulting in a distinct adult
morphology. As such, when comparing adults they appear to
emerge in a “single step,”that is, shortened tail offspring
generated by normal tailed parents. This creates a gap
between the “ancestral”—long tail—and the “descendant”—
short tail—forms. The explanation of the emergence of novel
structures by reprogramming developmental pathways is
called “emergentist paradigm”by Rieppel (p. 124). In
contrast, the Modern Synthesis embraces the so-called
transformationist paradigm, which explains morphological
evolution in a gradual transition between small steps. There
can be no gaps in this process.
These contrasting views recall the distinction between
structuralists and functionalists well developed by Ron
Amundson (2005). In Rieppel’s book, the emergentist
paradigm sees structures coming before—and, hence,
determining—functions, whereas in for the transformation-
ists the opposite is true: function shapes structures. On
Amundson’s account, those supposedly incommensurable
views were contrasted several times before Darwin’sOrigin,
and succeeded each other in dominating the predominant
view of nature. Along the exciting and easy reading 216
pages, Rieppel presents his view on how this transforma-
tionist versus emergentist explanations also shaped the debate
about morphological evolution after Darwin’s publication,
using the turtle shell as the prime example.
Briefly, two competing hypotheses have been raised to
explain the unique body plan of turtles (Rieppel, 2001). The
first one considers the carapace (the dorsal portion of the
shell) as an endoskeletal structure, derived from the
cartilaginous precursors of the dorsal ribs and vertebrae.
The second posits an exoskeletal nature for it, in which the
ribs and vertebrae would fuse to overlying dermal
ossifications, the osteoderms. The latter view is related to
what Rieppel calls a Polka Dot Turtle Ancestor. This would
be a hypothetical ancestor, that would explain the transition
from a shell-less reptile to a turtle, through a gradual
accumulation of osteoderms on its back. This hypothetical
ancestor has been called to bridge the gap in the fossil
record a couple of times during the last three decades (e.g.,
Joyce et al., 2009; Lee, 1996).
Present address of Gabriel S. Ferreira is Hölderlinstraße 12, room: 407,
D-72074 Tübingen, Germany.
Evolution & Development. 2017;19:227–228. wileyonlinelibrary.com/journal/ede © 2017 Wiley Periodicals, Inc.
The big problem is that, since at least the end of the 19th
century, studies on the development of the carapace (e.g.,
Rathke, 1848) show that the ribs and vertebrae in the turtle
trunk never fuse with dermal ossifications, supporting the first
hypothesis. That means that, even with evidence to the
contrary, several researchers have favored the more main-
stream transformationist view.
Rieppel’s new book presents a view of “science facts”as
social constructs, building on Ludwik Fleck’s concept of
thought style to explain why sometimes scientists favor one
view in spite of another and how it refrains the community of
thinking outside this box of knowledge. The Modern
Synthesis can be viewed as one of these boxes (or circles,
in Fleck’s terminology), limiting our thoughts about how
morphological evolution is realized through small and
gradual steps, even when the sudden emergence of a new
structure—like the mutant Manx cat short tails or the turtle
carapace—seems more likely. According to Rieppel (p. 161)
“there is no question that Darwinian evolution through
variation and natural selection [...] does occur, but the
question remains whether this is the only possible way of
evolutionary transformation.”The book highlights that it may
be time to embrace the hopeful monsters out there and revisit
the “dogmas”of Evolutionary Theory.
Gabriel S. Ferreira
Biology Department, Faculty of Philosophy, Science, and
Letters at Ribeirão Preto, University of São Paulo,
Ribeirão Preto, Brazil
Senckenberg Center for Human Evolution and
Palaeoenvironment (HEP) at Eberhard Karls Universität,
Fachbereich Geowissenschaften, Eberhard Karls
Universität, Tübingen, Germany
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