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\begin{document}

\title{The genetics-morphology-behaviour trifecta: unravelling the single
greatest limitation affecting our understanding of chondrichthyan
evolution}



\author[1]{Joel Gayford}%
\affil[1]{Imperial College London - Silwood Park Campus}%


\vspace{-1em}



  
  \date{May 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Sharks, rays and chimaera form the clade Chondrichthyes, an ancient
group of morphologically and ecologically diverse vertebrates that has
played an important role in our understanding of gnathostome evolution.
Increasingly studies have are seeking to investigate evolutionary
processes operating within the chondrichthyan crown group, with the
broad aim of understanding the driving forces behind the vast phenotypic
diversity observed amongst its constituent taxa. Genetic, morphological
and behavioural studies have all contributed to our understanding of
phenotypic evolution yet are typically considered in isolation in the
context of Chondrichthyes. I argue that integrating these core fields of
organismal biology is vital if we are to understand the evolutionary
processes operating in contemporary chondrichthyan taxa, and how such
processes have contributed to past phenotypic evolution. In light of the
global extinction crisis facing this clade, this understanding is
crucial if we are to successfully conserve rapidly declining
chondrichthyan populations.%
\end{abstract}%



\sloppy


\textbf{Title: The genetics-morphology-behaviour trifecta: unravelling
the single greatest limitation affecting our understanding of
chondrichthyan evolution}

Joel H. Gayford\textsuperscript{1,2}

\selectlanguage{ngerman}¹ Department of Life Sciences, Silwood Park Campus, Imperial College
London,

\textsuperscript{2} Shark Measurements, London, United Kingdom

Corresponding author email address: joelgayford@yahoo.co.uk

Corresponding author mailing address: 6 New Road Lewes,U.K., BN71YW

ORCID: 0000-0002-0839-3940

\textbf{Statements and declarations}

\textbf{Author contributions}

JHG conceptualised the study and prepared the entirety of the
manuscript.

\textbf{Funding}

The author declares no funding sources for this study.

\textbf{Competing interests}

The author declares no competing interests.

\textbf{Ethics and animal welfare}

No experimental work was carried out at any point during this study,
either involving animals or otherwise.

\textbf{Data availability}

There is no data to be archived associated with this study

\textbf{Abstract:}

Sharks, rays and chimaera form the clade Chondrichthyes, an ancient
group of morphologically and ecologically diverse vertebrates that has
played an important role in our understanding of gnathostome evolution.
Increasingly studies have are seeking to investigate evolutionary
processes operating within the chondrichthyan crown group, with the
broad aim of understanding the driving forces behind the vast phenotypic
diversity observed amongst its constituent taxa. Genetic, morphological
and behavioural studies have all contributed to our understanding of
phenotypic evolution yet are typically considered in isolation in the
context of Chondrichthyes. I argue that integrating these core fields of
organismal biology is vital if we are to understand the evolutionary
processes operating in contemporary chondrichthyan taxa, and how such
processes have contributed to past phenotypic evolution. In light of the
global extinction crisis facing this clade, this understanding is
crucial if we are to successfully conserve rapidly declining
chondrichthyan populations.

\textbf{Key words:} natural selection -- Elasmobranchii -- shark --
ecomorphology -- quantitative genetics

\textbf{Overview}

Sharks, rays, and chimaera form the clade Chondrichthyes, a group of
cartilaginous fishes that first arose during the Palaeozoic and has
subsequently survived at least five mass extinction events (Stiassny et
al., 2004; Soldo, 2013). Over the course of its long history the clade
has undergone major transitions in diversity (Grogan et al., 2012;
Kriwet and Benton, 2004; Kriwet and Klug, 2008), however remains a
morphologically and ecologically diverse component of modern ecosystems
(Cailliet et al., 2005; Compagno, 2008; Kolmann et al., 2022; Stein et
al., 2018), performing various important ecological functions (Flowers
et al., 2021; Heupel et al., 2014; Navia et al., 2010). Chondrichthyan
taxa have long been influential in studies of vertebrate, and
particularly gnathostome evolution (Gillis et al., 2009; Smith, 2003).
As the basal-most extant crown gnathostomes chondrichthyans are
frequently used to represent the ancestral gnathostome condition when
considering evolutionary transitions in specific
morphological/developmental characters (Gillis et al., 2013; Mallatt,
1996). Chondrichthyan taxa have also been used as case studies for
phenomena such as genetic conflict (Crespi and Semeniuk, 2004;
DiBattista et al., 2008). Unfortunately, chondrichthyans are currently
facing a global extinction crisis (Stein et al., 2018), with over 300
species vulnerable to extinction (Dulvy et al., 2021). For this reason,
it has never been more important to improve our understanding of
chondrichthyan evolution -- particularly in the context of contemporary
populations.

The fields of genetics, morphology and behaviour have each played
important roles in our understanding of evolution. Phylogenetics has
revolutionised our understanding of phenotypic evolution and
interrelationships (Lee and Palci, 2015; Naylor et al., 2005), whereas
morphological and behavioural studies have both provided insight into
how organisms interact with other components of the ecosystem
(Wainwright, 1994; Wainwright, 1996) and the evolutionary processes
operating within natural populations (Davies et al., 2012; Le Roy et
al., 2019; Owens, 2006). Whilst important in isolation, it is the
interplay between genetics, morphology and behaviour that is most
significant for understanding evolutionary processes, their phenotypic
consequences, and how such phenotypes engage with the wider ecological
community (Lewontin and Krimpas, 2000; Owens, 2006; Wainwright, 1994).
Regrettably studies of chondrichthyan evolution rarely unify these
concepts, fundamentally limiting our ability to identify and understand
the nature of evolutionary processes operating in this clade.

Here I explain how this failure to integrate between core fields of
organismal biology has significantly hampered our understanding of
chondrichthyan evolution, both past and present. I provide reasons for
such failures as well as potential solutions and examples of their
application in other taxa. I suggest that integration of genetics,
morphology and behavioural studies is crucial to our understanding of
phenotypic evolution in both past and present chondrichthyan
populations.

\textbf{Linking morphology to genetics}

Comparative morphological studies of vertebrate anatomy and
morphogenesis have contributed significantly to our understanding of
evolutionary history and taxonomic interrelationships (Lee and Palci,
2015), and this is no different in Chondrichthyes (Naylor et al., 2005;
Shirai, 1996). Despite this, the interrelationships of various
chondrichthyan subclades have long been debated (Klug, 2010; Naylor et
al., 2005; Qiao et al., 2016), and it is only relatively recently, upon
the development of molecular phylogenies with high taxonomic coverage
(Naylor et al., 2005; Stein et al., 2018) that these controversies have
been resolved. Phylogenetics is likely the most universally applicable
integration of genetics and morphology, however increasingly
evolutionary-developmental (evo-devo) studies and evolutionary genetics
are being used to uncover the genetic basis of morphological traits
(Abzhanov et al., 2002; Mallarino and Abzhanov, 2012) and the selective
regimes under which they have evolved (Ho et al., 2017; Rolland et al.,
2018). Evo-devo studies targeting chondrichthyan taxa are present in the
literature, yet these target the expression of a small number of
morphological characters in a minute proportion of extant species
(Gillis et al., 2009; Gillis et al., 2013). Besides this, we have
extremely minimal knowledge of the genetic and developmental
underpinnings of morphological variation in Chondrichthyes.

This of course represents a knowledge gap in itself, but also
fundamentally constrains our ability to understand morphological
evolution within Chondrichthyes. Most such studies rely on an
ecomorphological approach (Gayford et al., 2023) under the assumption
that ecological selection is dominant and has shaped the evolution of
morphological structures (Andrew-Barr, 2018). Crucially, these studies
typically negate the potential role of constraint in morphological
evolution (Gayford et al., 2023). The importance of these constraints to
morphological evolution in other taxa is well known (Beldade et al.,
2002; Wagner, 1996), however such an understanding relies upon knowledge
of the genetic architectures or gene regulatory networks underlying
morphology (Davidson and Erwin, 2006; Hegmann and Possidente, 1981).
Evolutionary constraints such as genetic correlations can substantially
alter the pace of evolution by modulating the response to selection
(Crespi, 2000; Greenbury et al., 2016), whilst others such as lack of
additive variance can make `optimal' genotypes effectively unattainable
(Hansen et al., 2003) or result in maladaptive evolution (Crespi, 2000).
There is debate surrounding the extent to which short-term genetic
constraints influence long-term evolution (Dooren, 2020; Hadfield et
al., 2007), however even if we ignore mounting evidence regarding the
importance of such constraint to past evolution (Futuyma, 2010), the
relevance of these concepts to the immediate future of chondrichthyans
and their contemporary evolution is unquestionable: in light of their
dire conservation status (Dulvy et al., 2021; Stein et al., 2018), low
fecundity, and long generation times (Cailliet et al., 2005), an
understanding of the extent to which constraint may influence future
morphological adaptation to environmental change should be one of the
key priorities of contemporary chondrichthyan evolutionary research.

There are several explanations for the lack of previous integration
between morphological and genetic studies of chondrichthyan evolution.
In the case of evo-devo studies, research effort appears to be the
primary limitations, with only a relatively low number of morphological
structures considered from a handful of species (Gillis et al., 2009).
Conducting such studies in a greater range of taxa would increase our
understanding of the genetic basis of morphological variation within
Chondrichthyes and the extent to which evolutionary constraints relating
to gene regulatory networks appear to be present (Figure 1).
Quantitative genetic studies of chondrichthyan populations have
previously been limited by the costs of genome sequencing however
technological advances are rapidly increasing the accessibility of
sequencing technologies (Mardis, 2017). The requirement of pedigree
information for quantitative trait loci (QTL) and genome wide
association studies (GWAS) also provides a limitation to their usage in
wild populations (Day-Williams et al., 2011). Recent advances have
softened this requirement, with several routes available for
quantitative genetic analyses in wild populations where pedigree
information is absent (Johnston et al., 2022). Finally, where the
genetic basis of morphological traits is known, signatures of past
selection can be detected through phylogenetic analysis (Vitti et al.,
2013). These methodologies are not mutually exclusive, and should all
contribute to future research, through which we can improve our
understanding of past morphological evolution and future `evolvability'
of morphological traits in chondrichthyan taxa.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1: Methodologies that can be utilised to increase
integration between genetic, behavioural, and morphological studies of
chondrichthyan evolution}

\textbf{Linking behaviour to genetics}

Whilst there is no consensus definition of behaviour in the literature
(Levitis et al., 2009), behavioural traits do not differ fundamentally
from any other class of traits, and many have been shown to have some
degree of genetic underpinning (Bleakley et al., 2010; Fitzpatrick et
al., 2005). Studying behaviour in extinct species is challenging (Hsieh
and Plotnick, 2020), and understanding the relevance of behavioural
studies to past evolutionary events can be difficult. Nevertheless,
integrative studies combining genetics and behavioural ecology are
essential to our understanding of the adaptive value (or lack thereof)
of behaviours (Penke et al., 2007). This is of paramount importance to
our understanding of evolution both past and present given that
behaviour is the suite of traits by which ecological interactions (both
inter and intraspecific) are directly mediated. Unfortunately, our
understanding of chondrichthyan behaviour (Bres, 1993; Guttridge et al.,
2009), let alone the genetic basis of behaviour is severely limited.
This hampers our understanding of chondrichthyan evolution in much the
same way as a lack of integration between morphology and genetics:
without an understanding of the genetic architectures and adaptive
landscapes underlying behavioural traits we are fundamentally
constrained in our ability to understand behavioural evolution, how it
has contributed to the evolution of phenotypic diversity observed in
extant taxa, and how it may influence organismal evolution in the face
of rapid environmental change. Of particular importance to contemporary
populations, it is not possible to evaluate the posited `special' role
of behaviour in evolution without an understanding of the genetic basis
of behavioural traits (Levis and Pfennig, 2016; McGlothin and Brodie
III, 2009).

The importance of behaviour to evolution has long been understood
(Corning, 2014), however recent integrative studies combining genetics
and behavioural ecology point towards two phenomena of particular
importance to our understanding of contemporary chondrichthyan
evolution. Indirect genetic effects (IGEs) occur the genotype of one
individual alter the phenotype of another (Wolf et al., 1998). These
effects -- which are often cryptic and difficult to detect -- are
important in an evolutionary context as they can modulate the response
to selection (McGlothin and Brodie III, 2009), thus acting as
`pacemakers' of adaptive evolution (Bailey et al., 2018). The other
phenomenon of particular importance is plasticity first evolution -- the
proposition that phenotypic plasticity (including behaviour, which is
intrinsically plastic) may precede and facilitate adaptive evolution by
providing the `raw material' upon which selection can act in the absence
of de novo, mutation-based adaptations (Levis and Pfennig, 2016; Perry
et al., 2018). This, like IGEs, could increase the rate at which
adaptive evolution occurs, although debate exists regarding the validity
of such hypotheses (Levis and Pfennig, 2016). As many chondrichthyan
populations have low effective population sizes (Pazmi\selectlanguage{ngerman}ño et al., 2017)
and long generation times (Cailliet et al., 2005), both IGEs and
plasticity first adaptations could play a major role in determining the
vulnerability of contemporary populations to rapid environmental change,
yet until know both of these phenomena have been ignored in the context
of chondrichthyan evolution.

This lack of integration is driven by many of the same limitations
affecting integration between genetics and morphology, and as such can
be overcome using broadly similar methodological approaches (Figure 1).
Studies of past selection may provide valuable insights into the
evolution of behavioural traits (Eusebi et al., 2018; Grams et al.,
2015), and quantitative genetic studies are valuable not only for
uncovering the genetic basis of these traits (Bubac et al., 2020) but
for providing direct evidence of IGEs and plasticity first evolution.
The major difference is that whilst morphology is easily quantified,
relatively little is known about chondrichthyan behavioural ecology
(Bres, 1993; Guttridge et al., 2009), and most existing studies are
descriptive or qualitative in nature. This will have to be overcome
before quantitative genetic methodologies can be applied, and thus I
suggest that future studies of chondrichthyan behaviour must focus on
quantifying behavioural variation within and between populations.

\textbf{Linking morphology to behaviour}

Uncovering the genetic basis of behavioural and morphological traits and
the factors influencing their evolution is undoubtedly of great
significance, yet this only provides one half of the story; it is one
thing to establish the mechanisms of evolution and their phenotypic
consequences, but another entirely to elucidate the functional link
between them. The associations between morphology and behaviour are
perhaps the best-studied of the three integrative pathways discussed
here as in a broad sense uncovering these associations is the express
goal of functional morphology (Wainwright, 1994). Several experimental
studies have addressed the functional significance of chondrichthyan
morphology (Wilga and Lauder, 2004a). As a result, the contribution of
structures such as the dorsal, caudal, and pectoral fins to locomotor
performance in elasmobranchs is relatively well understood (Maia and
Wilga, 2013; Wilga and Lauder, 2001; Wilga and Lauder, 2004b). There are
however several major limitations of this approach that constrain
integration between morphological and behavioural studies. This
experimental approach rarely captures ecologically relevant complex
behaviours, and where it does, influence of the laboratory setting on
expressed behaviour cannot be ruled out (Moore and Biewener, 2015).
Moreover, population and species-specific morphologies (Grover, 1972;
Keeney and Heist, 2006; Sternes and Shimada, 2020) mean that there is
little reason to suggest that existing kinematic studies should be
representative of all -- or even a substantial proportion of extant
chondrichthyan diversity. The lack of studies linking morphology to
complex behaviours in wild populations fundamentally constrains our
understanding of evolution as without such studies the true adaptive
value of morphology. This in turn constrains our understanding of
ecological interactions, the evolution of complex behaviours such as
foraging strategies, and how future environmental change may influence
them. Novel technological advancements are increasingly enabling
quantitative study of chondrichthyan behaviour in wild populations
(Butcher et al., 2021; Renshaw et al., 2023) and thus with sufficient
research effort this knowledge gap is likely to decrease, however
significant further study combining experimental and observational
approaches will be required in order to achieve this goal.

\textbf{Conclusions:}

Much progress has been made in recent years towards understanding
chondrichthyan evolution and ecology. However, it is undeniable that
many major knowledge gaps remain, and in particular our understanding of
trait evolution in this clade is less robust than in many other
vertebrate radiations. I argue that the key driver of this uncertainty
is a lack of integration between genetic, morphological, and behavioural
studies. Failure to integrate these key areas of organismal biology
fundamentally constrains our understanding of phenotypic evolution, both
past and present. Most significantly, this impedes study of the genetic
architectures underlying phenotype and how selection acts upon them.
Despite this, the necessary tools to overcome this major limitation
already exist and have been applied to other taxa (Figure 1). Future
studies should focus on increasing the taxonomic breadth of existing
studies of chondrichthyan genetics, morphology, and behaviour, as well
as the implementation of more quantitative genetic approaches. Only
though this will we truly be able to understand trait evolution in
Chondrichthyes to a comparable extent to other major vertebrate
radiations, and its implications for past evolution and vulnerability in
the face of climate change.

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