The “Evolutionarily Significant Unit” concept and its applicability in biological conservation

Abstract

Although most conservationists claim to protect "species", the conservation unit actually and practically managed is the individual population. As resources are not unlimited, we need to focus on a restricted number of populations. But how can we select them? The Evolutionarily Significant Unit (ESU), first conceptualised by Ryder in 1986, may offer some answer. Several definitions have been proposed for the ESU, but all make reference to units "whose divergence can be measured or evaluated by putting differential emphasis on the role of evolutionary forces at varied temporal scales". Thus, an ESU might be fully identical with a "species", or a "species" could be composed of multiple ESUs. On the other hand, an ESU might comprise single/multiple populations exchanging a degree of gene flow, such as meta-populations. In an attempt to show strengths and weaknesses of ESU concepts, we present here, among several others, some case studies on the myrmecophilous butterflies of the genus Maculinea. In particular, we analyse the apparently everlasting debate about Maculinea alcon and M. rebeli, whose separation into separate species has been accepted by many authors, on mainly ecological criteria, but has not been fully supported by molecular analyses. We also discuss how the tight association with host ants may have driven selection for increasingly more strictly adapted Maculinea populations, arguably deserving specific taxonomic identity. Finally we discuss how current DNA analyses may fail to detect critical information on differences between taxa recently originated by the action of separate adaptive processes, which non-molecular studies can sometimes reveal. We conclude by discussing some current and often conflicting taxonomic trends, in their relationships with conservation policies.

Direct and indirect effects of weather variability in a specialist butterfly

“…Climatic models suggest that in the near future the proportion of the area climatically suitable for M. alcon will undergo strong reductions, with a potential loss of almost 70% of the range currently available for this species. In this framework, high-altitude populations, such as those we have studied, represent different conservation units, showing that, in assuring long-term species conservation, it is important to adopt a population-centred approach rather than a species-centred approach (Casacci et al, 2013). Our results strongly support these predictions and offer some potential mechanistic explanation.…”

Fish conservation in the land of steppe and sky: Evolutionarily significant units of threatened salmonid species in Mongolia mirror major river basins

“…ESUs can consist of multiple allopatric populations and can cover extensive geographic regions depending on the species and its ecology (Moritz, 1994;Palsbøll, Bérube, & Allendorf, 2006). Units are usually defined based on neutral and sometimes adaptive genetic variation, which represent the effects of both historical spatial processes and environmental selection (Casacci, Barbero, & Balletto, 2014;Crandall, Bininda-Emonds, Mace, & Wayne, 2000;Funk et al, 2012;Moritz, 1994). For the initial demarcation of an ESU, researchers have focused on genetic markers including maternally transmitted, slowly evolving mtDNA, but also biparentally transmitted, quickly evolving microsatellites, as both yield relevant information on complementary spatiotemporal scales (O'Connell & Wright, 1997;Vogler & DeSalle, 1994).…”

Contrasting genetic structure in cuckoo and predatory Maculinea butterflies

“…Fraser andBernatchez 2001, de Guia andSaitoh 2007). Moreover, conservation units are expected to be adaptively differentiated from each other (Funk et al 2012, Casacci et al 2014). Since the level and pattern of genetic differentiation are different in the two Maculinea species, the delineation of CU needs to differ between them.…”

“…Since the level and pattern of genetic differentiation are different in the two Maculinea species, the delineation of CU needs to differ between them. Variation in host ant use is known to be a strong diversifying force in the cuckoo Maculinea species (Nash et al 2008;Casacci et al 2014). In Polish populations of M. rebeli ('cruciata type' of M. alcon) this process resulted in genetic structure specific to the host ant (Sielezniew et al 2012).…”