Objective 1: Definitions
In our search for classical and authoritative definitions of ecotype and life history, it became apparent that several terms are used more-or-less synonymously (e.g., “species pairs”, “ecotypes”, and “life histories” in Taylor 1999 and “races”, “phenotype”, “types”, and “subspecies” in Brannon et al. 2004). This entanglement of phenotypic terms was noted over eight decades ago: “The questions of what is a species, or a subspecies, any classificatory category of specific or lower rank, cannot be disassociated from one another” (Ginsburg 1937).
We compiled definitions of common terms used to describe ISD (Table 1). Generic terms used to describe ISD include “form” and “type” (Table 1). Our review revealed the ambiguity of terms such as “races” and “subspecies”, and the extent to which these two terms appear to be used less by scientists now than in the past. Classification of subspecies is controversial among taxonomists (Haig et al. 2006; Patten 2015; de Queiroz 2020), and a common accepted definition of the subspecies remains elusive (Haig et al. 2006). Subspecies have recently been defined as components of a species that are incompletely speciated (Patten 2015; de Queiroz 2020; Table 1). We also found an overlap in definitions between subspecies and ecotypes, and a striking similarity between the terms, ecotype and reaction norms. Ecotype was originally used to describe patterns in traits (genes) and ecology in the early 1900s (Turesson 1922; Gregor 1944). Life history, by contrast, includes biological parameters that affect abundance and population growth or decline, including parameters related to birth, survival, reproductive timing, reproductive investment, and mortality (Stearns 1992). The terms paired species/species pairs have been used to describe a dichotomy of two different phenotypes such as benthic versus limnetic sticklebacks or freshwater resident kokanee versus anadromous sockeye salmon (O. kisutch ; Taylor 1999) and freshwater resident, non-feeding brook lampreys vs. anadromous and parasitic lampreys (Salewski 2003; Docker 2009; Docker and Potter 2019). The term, satellite species is generally used to describe situations in which one lamprey species yields two or more derived species (Vladykov and Kott 1979; Salewski 2003; Docker 2009). Evolutionary Significant Units (ESUs) have been used primarily for PST.