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.