"No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species" Charles Darwin (1859)

The field of systematic biology has long been focused on understanding the relationships between different living organisms and how they are classified within the tree of life. The main objectives of systematics have been to discover monophyletic groups (clades) and relationships within them at all hierarchical levels above species, and to discover lineages (species) at lower levels. The delimitation of species is often difficult and is based on a qualitative assessment rather than a quantitative one. To address this problem, Sites and Marshall (2003) proposed an empirical way of delimiting species. Since then, a variety of methodologies have been developed to tackle the problem of species delimitation. These include phylogenetic species concepts, which focus on the evolutionary history of organisms, as well as molecular and morphological approaches, which rely on genetic and physical characteristics to define species. Additionally, the integration of multiple lines of evidence, such as biogeography, behavior and ecology, have been used to support the identification of species.

De Queiroz (2007) emphasized the distinction between a general lineage concept (GLC = metapopulation lineages that evolve independently, or more specifically, segments of such lineages) and the secondary biological attributes or properties of organisms that allow for the empirical quantification of a species' state. This distinction is important because it highlights the complexity of species delimitation, especially when organisms are found in so-called "gray zones" (see Fig 1). These gray zones refer to cases where organisms are morphologically or genetically similar, yet have evolved independently and have different ecological or behavioral characteristics. In such cases, traditional methods of species delimitation may not be sufficient, and a more holistic approach is needed. This approach must take into account not only the physical and genetic characteristics of the organisms but also their ecological, biogeographical and behavioral characteristics, in order to provide a more accurate representation of the organism's evolutionary history and its place within the tree of life. The development of new tools and methodologies, such as those based on molecular and phylogenetic approaches, has greatly improved our ability to understand the complex relationships between different organisms and the diversity of life on Earth.

Wherever two species of blue mussels coexist, they hybridize and can form hybrid zones between Mytilus species. In Mytilus, hybridization occurs due to recent divergence among species, and is facilitated by the external fertilization that these mollusks possess. However, one of the interesting aspects of Mytilus hybridization is that not all inter-species interactions have the same outcome. For example, in some places there may be extensive hybridization and introgression, while in others extensive hybridization may be accompanied by limited introgression. This suggests that there are other factors at play that affect the outcome of hybridization. Furthermore, the implications of these different outcomes on the ecology and evolution of Mytilus populations is an area of ongoing research. Understanding the dynamics of hybridization in Mytilus can provide insights into the mechanisms of speciation, as well as the potential impacts of anthropogenic activities on marine biodiversity. In Chile, an important hybrid zone has been described in the Strait of Magellan (Oyarzún et al. 2016). Hybridization of endemic species with exotic species was found, with a focus on the impact of native biodiversity. The Strait of Magellan is a unique ecosystem with a high diversity of marine organisms, many of which are endemic to the region. However, the introduction of non-native species can have severe consequences for the native biodiversity. The study found that the hybridization of native and exotic species in the Strait of Magellan can lead to the loss of genetic diversity and the homogenization of populations. This has important implications for the conservation and management of the native biodiversity in the region. Additionally, it highlights the need for better understanding of the potential impacts of non-native species on marine ecosystems and the importance of taking action to prevent their introduction. The Strait of Magellan can be considered as a model of the consequences that can occur in marine ecosystems as a result of human activities and the importance of taking actions to prevent it.