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.