Refining species boundaries in algae

For centuries, biologists have struggled with “the species problem”(i.e., the question of what comprises a species). Both the nature of species, and the criteria used to define them, have been intensely debated. The fundamental issue of whether species are real entities or the products of our propensity to classify biodiversity seems to have largely been settled in favor of the former view. Species are real, but perhaps not “real” in the sense that many would like. First, Darwin’s evolutionary theory essentially killed the typological view of species and paved the way for a genealogical concept of species as groups of organisms linked by historical lines of descent. During the Modern Synthesis and under the influence of Dobzhansky (1935) and Mayr (1942), however, species became equivalent to groups of interbreeding organisms (the biological species concept). This view significantly promoted the idea of species as “fundamental units in biology,” but differed significantly from Darwin’s original view (Mallet 2010). A general lineage concept of species, which regards them as separately evolving metapopulation lineages in which mutation, selection, migration, and drift are acting independently (Wiley 1978), shifted the emphasis back to the genealogical nature of species. This resulted from making a clear and explicit distinction between the theoretical concept of species and the operational criteria that are used to diagnose taxa (De Queiroz 2007). The concept that species are separately evolving metapopulation lineages, abetted by theoretical progress in phylogenetic and population genetic analyses, has led to the development of a wide array of methods aimed at identifying such lineages and hence delimiting species (Camargo and Sites 2013). Gene trees inferred from loci that bridge intraspecific and interspecific variation are vital to understanding the process of speciation and thus defining species boundaries. Whereas gene trees were in the past often used to delimit species on some kind of (often) subjective threshold (e.g., genetic distance), new model-based methods offer the promise to make species delimitation based on DNA sequence data more effective and objective. Several of these methods also provide statistical support regarding species boundaries, which is important in lineages that have recently diverged, or are in the process of speciation. In such recently diverging species complexes, species delimitation based on a single marker has progressively made way for methods incorporating multiple unlinked loci, which enable accounting for confounding processes like incomplete lineage sorting or hybridization (Dupuis et al. 2012). Crucial in this paradigm shift regarding species delimitation, is that a general consensus is growing among systematic biologists that all other information that may aid as diagnostic characters for species is subordinate to DNA sequence data. In other words, species may or may not be morphologically or ecologically distinct or even reproductively isolated. Because species evolve separately, or because adaptation to specific niches drives their divergence, species are often phenotypically distinct, and in many cases these differences may thus serve as evidence relevant for species delimitation. However, morphological/ecological divergence and reproductive isolation is often lagging, making species delimitation using phenotypic data problematic in such cases. This is particularly true for recently diverged species, and in taxonomic groups that are morphologically simple, such as many algae (John and Maggs 1997, Leliaert et al. 2014). The study by Montecinos et al. (2016), published in this issue, is an excellent example of how phycology has embraced this new trend in species delimitation. The paper focuses on the brown filamentous seaweed Ectocarpus, a genus with a notoriously difficult taxonomic history. Ectocarpus is widely distributed in intertidal pools and subtidal habitats of temperate regions in both hemispheres. Although one of the species, E. siliculosus, has become a genetic and genomic model organism for the brown algae (Cock et al. 2010), species diversity and the geographic distribution of this genus are not fully documented. The long and confounded taxonomic history of Ectocarpus has resulted in the description of nearly 500 species, subspecies, varieties and forms, many of which have later been synonymized or have an uncertain status. J. Phycol. 53, 12–16 (2017) © 2016 Phycological Society of America DOI: 10.1111/jpy.12477