Senescence

by

Roberto Salguero-Gómez, Mark Roper, Connor Bernard, Jacques Deere

• LAST REVIEWED: 27 October 2021
• LAST MODIFIED: 27 October 2021
• DOI: 10.1093/obo/9780199830060-0236

Introduction

Researchers are now beginning to unravel the variation in patterns of whole-organism senescence across the tree of life, yet the existence of senescence in nature itself is an evolutionary conundrum. This document introduces the literature on explanations for whole-organism senescence, starting with the early classical theories developed in the 20th century. This piece reviews extensions to these classical theories before touching on the wealth of data now available that challenges their predictions. The manuscript concludes by considering current research on more modern theories of whole-organism senescence, as the field attempts to bridge the gap between current theoretical predictions and empirical observations.

Cellular, Physiological, and Demographic Senescence

Senescence is the decline of vitality with age, from maturity until the organism dies. Underlying losses of vitality are driven by physiological breakdowns (i.e., dysfunction of functional and regulatory properties of tissues and the endocrine system) that may ultimately emerge from cellular breakdowns (Comfort 1964). If these mechanisms upscale to the whole individual, senescence can then be quantified at the demographic level as the decline in reproductive outputs and/or increase of mortality risks with age. It is noteworthy highlighting that demographic senescence is not necessarily the result of cellular senescence, as cellular declines with age may or may not upscale to demographic effects on an individual organism (Shefferson, et al. 2017). The extent to which lower-level, age-mediated breakdowns translate into demographic effects—thus influencing an individual’s mortality and reproductive schedule—is not firmly resolved and remains an active area of research (Finch 1990; see also Modern Theories of Senescence—Growth Forms). This section focuses on the theories and current evidence of the drivers of the evolution of senescence at the whole-organism level, as well as the (a)biotic drivers that allow organisms to postpone its onset or even escape from it.

• Comfort, A. 1964. Ageing: The biology of senescence. Rev. ed. London: Routledge & Kegan Paul.

  Comfort’s textbook surveys the knowledge about ageing from cellular mechanisms to comparative population biodemography. This is a classical text in the broad-scale study of senescence and the study of ageing as mediated by different mechanisms and translating across different scales of biological organization. Comfort does not cover a number of theoretical and empirical developments that are addressed by the more recent textbooks Finch 1990 and Shefferson, et al. 2017.

• Finch, C. E. 1990. Longevity, senescence, and the genome. Chicago: Univ. of Chicago Press.

  This textbook is a seminal review that covers ageing from the ground up—starting with cells and walking through the levels at which age-correlated dysfunctions manifest. Finch ably navigates the partitions between different levels of biological organization, drawing on expertise in the domain of the cellular level through the individual level, as well as among species. The first several chapters of Finch’s book offer a concise and clear review of the semantic distinctions surrounding senescence.

• Shefferson, R. P., O. R. Jones, and R. Salguero-Gómez, eds. 2017. The evolution of senescence in the tree of life. Cambridge, UK: Cambridge Univ. Press.

  This textbook provides an overview of senescence research focusing on biological diversity in ageing. The authors provide a clear distinction amongst different types and levels of ageing. In the introductory chapter of the book, Shefferson and colleagues discuss the terminology of senescence and subfields that tackle physiological, mathematical, and evolutionary dimensions of the “senescence problem.” This resource provides a general overview of the current theories on the evolution of senescence and the status quo of the understanding of senescence across different taxonomic groups.