Objective 4: Compare definitions with term use and make recommendations
Ecotype originally was used to describe patterns in traits (gene) combinations in particular environments. This term also tends to describe a genetic basis for ISD and is often used in genetic- and evolution-based journals during 1990 – 2019. Thus, a general consistency between the classical definition and the current use of ecotype exists. Ecotype would therefore be an obvious term for geneticists and evolutionary biologists wishing to address patterns in genes associated with particular habitats. Significant overlap in meaning exists between ecotype and reaction norms, and the nuanced differences in these terms should be carefully parsed out by authors. The classical definition of “life history” is clear and straightforward on demographic trade-offs (e.g., Stearns 1989, 1992). Life history has strong demographic connotations (Table 1), and thus the general trend for its use in organismal- and ecology-based journals during 1990 – 2019 makes sense. Life history types implies discontinuity in phenotypic expressions, whereas life history pathways (e.g., Thorpe et al. 1998) implies continuity in phenotypic expression (Table 1). The term, species pairs implies two phenotypes, whereas in some species and situations, more than one phenotype or a continuum of phenotypes, akin to life history pathways, may be expressed. Race is generally synonymous with subspecies. “Satellite species” is used for lampreys, but not for sticklebacks or PST.
Although we did not assess the use of derivative terms such as morphotypes, ecomorphotypes, ecophenotypes, these terms arguably do not offer insight into ISD, and we therefore recommend that authors either should not use these terms or use them sparingly. All attempts to describe ISD would benefit from clear definitions. In situations where the genetic and demographic components of ISD are not well understood, a conservative approach would be to forego categorization of the diversity as ecotypes or life history types / life history pathways and to call them phenotypes.
We speculate that if, for some reason, cultural, recreational, and economic roles of species were reversed so that sticklebacks and lampreys became more important than PST, that perhaps the research focus of sticklebacks and lampreys would have tended more towards demographic and life history assessments and less towards evolutionary assessments — even though these disciplines are connected (e.g., Frank and Leggett 1994; Hutchings 2000; Waples and Hendry 2008). If this were to have occurred, we wonder the extent to which the diversity within sticklebacks and lampreys might be more classified by life histories, races, and subspecies, whereas the diversity of Pacific salmon and trout might be more towards ecotypes. With this simple thought experiment, we speculate that the scope and breadth of human interests in particular taxa could influence the epistemology of how these organisms are studied, and hence the way we describe their diversity. We realize this thought experiment is somewhat overly simplified, however, in that species characteristics that can determine their commercial and recreational interests (e.g., large body size and hence longer generation times, as in Oncorhynchus spp.) also make these animals more challenging to study (Stearns and Hendry 2004). By contrast, the relatively small-bodied sticklebacks, which do not have a known commercial or recreational interest are comparatively short-lived and this therefore makes them tractable study animals. In addition, the evolutionary histories of these three fish taxa are very different, with the oldest by far being the lampreys, followed by the PST, and then sticklebacks. Although some of the same past geological events may have influenced the evolutionary trajectories of these taxa (e.g., glaciation; Bell and Andrews 1997), these events would have worked at different time frames and to different extents. The foregoing rationale suggests that terminology to describe ISD may not be directly transferable between taxa. For example, the abundant delineation of Pacific trouts to subspecies may make more sense for that taxon, given their population structure and distribution into geographically isolated streams (that can be geologically and ecologically diverse; Behnke 2002; Penaluna et al. 2016) than it would for either sticklebacks or lampreys. On the other hand, some terminology may have broader applicability — such as life history and ecotypes (Table 2).
In conclusion, understanding and preserving ISD is important for species conservation. Ecotype was originally used to describe patterns in genes and ecology, and recent studies generally use this term in a similar way. By contrast, life history includes biological parameters that affect abundance and population growth and decline, and recent studies generally use this term in a similar way. Ecotype and life history were used equally among recent studies on sticklebacks. By contrast, life history was used more frequently than ecotype among recent studies on PST and lampreys. The nature of human interests (i.e., scientific, commercial, recreational, cultural) in particular taxa could influence the epistemology of how these organisms are studied, and hence the ways in which their ISD is understood, described, and conserved.