Contemporary biological research has suggested that some host–microbiome multispecies systems (referred to as “holobionts”) can in certain circumstances evolve as unique biological individual, thus being a unit of selection in evolution. If this is so, then it is arguably the case that some biological adaptations have evolved at the level of the multispecies system, what we call hologenomic adaptations. However, no research has yet been devoted to investigating their nature, or how these adaptations can be distinguished from adaptations at the (...) species-level (genomic adaptations). In this paper, we cover this gap by investigating the nature of hologenomic adaptations. By drawing on the case of the evolution of sanguivory diet in vampire bats, we argue that a trait constitutes a hologenomic adaptation when its evolution can only be explained if the holobiont is considered the biological individual that manifests this adaptation, while the bacterial taxa that bear the trait are only opportunistic beneficiaries of it. We then use the philosophical notions of emergence and inter-identity to explain the nature of this form of individuality and argue why it is special of holobionts. Overall, our paper illustrates how the use of philosophical concepts can illuminate scientific discussions, in the trend of what has recently been called metaphysics of biology. (shrink)

Holobionts are symbiotic assemblages composed by a host plus its microbiome. The status of holobionts as individuals has recently been a subject of continuous controversy, which has given rise to two main positions: on the one hand, holobiont advocates argue that holobionts are biological individuals; on the other, holobiont detractors argue that they are just mere chimeras or ecological communities, but not individuals. Both parties in the dispute develop their arguments from the framework of the philosophy of biology, in terms (...) of what it takes for a “conglomerate” to be considered an interesting individual from a biological point of view. However, the debates about holobiont individuality have important ontological implications that have remained vaguely explored from a metaphysical framework. The purpose of this paper is to cover that gap by presenting a metaphysical approach to holobionts individuality. Drawing upon a conception of natural selection that puts the focus on the transgenerational recurrence of the traits and that supports the thesis that holobionts are units of selection, we argue that holobionts bear emergent traits and exert downward powers over the entities that compose them. In this vein, we argue, a reasonable argument can be made for conceiving holobionts as emergent biological individuals. (shrink)

Holobionts are symbiotic assemblages composed by a macrobe host plus its symbiotic microbiota. In recent years, the ontological status of holobionts has created a great amount of controversy among philosophers and biologists: are holobionts biological individuals or are they rather ecological communities of independent individuals that interact together? Chiu and Eberl have recently developed an eco-immunity account of the holobiont wherein holobionts are neither biological individuals nor ecological communities, but hybrids between a host and its microbiota. According to their account, (...) the microbiota is not a proper part of the holobiont. Yet, it should be regarded as a set of scaffolds that support the individuality of the host. In this paper, we approach Chiu and Eberl’s account from a metaphysical perspective and argue that, contrary to what the authors claim, the eco-immunity account entails that the microorganisms that compose the host’s microbiota are proper parts of the holobiont. Second, we argue that by claiming that holobionts are hybrids, and therefore, not biological individuals, the authors seem to be assuming a controversial position about the ontology of hybrids, which are conventionally characterized as a type of biological individual. In doing so, our paper aligns with the contemporary tendency to incorporate metaphysical resources to shed light on current biological debates and builds on that to provide additional support to the consideration of holobionts as biological individuals from an eco-immunity perspective. (shrink)