Species as a Heuristic: Reconciling Theory and Practice

Abstract

Species are crucial to most branches of biological research, yet remain controversial in terms of definition, delimitation and reality. The difficulty of resolving the “species problem” stems from the tension between their theoretical concept as groups of evolving and highly variable organisms and the practical need for a stable and comparable unit of biology. Here we suggest that treating species as a heuristic can be consistent with a theoretical definition of what species are and with the practical means by which they are identified and delimited. Specifically, we suggest that theoretically species are heuristic since they comprise clusters of closely related individuals responding in a similar manner to comparable sets of evolutionary and ecological forces, whilst they are practically heuristic because they are identifiable by the congruence of contingent properties indicative of those forces. This reconciliation of the theoretical basis of species with their practical applications in biological research allows for a loose but relatively consistent definition of species based on the strategic analysis and integration of genotypic, phenotypic and ecotypic data.

Revised Species Delimitation in the Giant Water Lily Genus Victoria (Nymphaeaceae) Confirms a New Species and Has Implications for Its Conservation

“…We applied a heuristic species concept ( Wells et al, 2021 ) in which morphological, field and horticultural observations were used iteratively to develop initial hypotheses of species limits that we further tested using rigorous phylogenomic and population genomic analyses. This study was an Anglo-Bolivian collaboration instigated simultaneously by both parties with the goal of conducting research into Victoria in an equitable manner ( McAlvay et al, 2021 ).…”

“…Since Darwin first linked the phenomenon of speciation to that of evolution, systematic biologists have largely conceived of species mechanistically, equating them with separately evolving lineages equivalent to branches of the ‘Tree of Life’ ( de Queiroz, 2007 ; Padial and De la Riva, 2021 ), with the logical consequence that the basis and process of species delimitation centres on assigning individuals to a phylogenetic lineage ( de Queiroz, 1988 , 1999 , 2007 ; Mayo, in press ). However, others argue that lineage divergence alone is not sufficient to delimit species ( Freudenstein et al, 2017 ; Wells et al, 2021 ; Lavin and Pennington, in press ). For example, Lavin and Pennington provide several examples in plants of mechanisms that yield paraphyletic species.…”

“…Templeton in his ‘Cohesion Species Concept’ instead applies explicitly evolutionary criteria to define species as, “the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability” ( Templeton, 1989 , p. 181). The heuristic approach, which is an extension of this idea, aims to reconcile the theory and practice of species delimitation ( Wells et al, 2021 ). This proposes species as the outcome of their constituent individuals responding to similar suites of ecological and evolutionary forces in the same way and recognizes them in practice due to congruence in properties shaped by these forces.…”

“…This proposes species as the outcome of their constituent individuals responding to similar suites of ecological and evolutionary forces in the same way and recognizes them in practice due to congruence in properties shaped by these forces. In addition, by advocating the application of multiple categories of data, we believe that this approach to delimiting species best reflects both the quality and the breadth of observations available ( Monro, in press ), whilst also overcoming the limitations of lineage-based approaches ( Wells et al, 2021 ).…”

Lineage and role in integrative taxonomy of a heterotrophic orchid complex

“…Equating species with historical lineages as identified by genomic data forms the foundation of multispecies coalescent (MSC) delimitation models (Rannala & Yang, 2020; Yang & Rannala, 2010). Recent studies have questioned the relevance of the lineage as the sole criterion for recognizing species, however, and some methods designed to delimit species, for example, the MSC (Freudenstein et al, 2017; Sukumaran et al, 2021; Sukumaran & Knowles, 2017; Wells et al, 2021), because they may not circumscribe the units we care about. Arguments against lineage‐only approaches centre around observations that such methods may tend to delimit population structure rather than species and are sensitive to biases in range‐wide sampling (Chambers & Hillis, 2020; Hillis et al, 2021; Mason et al, 2020; Sukumaran et al, 2021; Sukumaran & Knowles, 2017).…”