Evolution of ageing

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Bathtub curve

Evolution of ageing refers to the gradual changes in the biological structures and functions of organisms that lead to ageing, a process characterized by a decline in reproductive fitness and an increase in mortality with advancing age. The theories surrounding the evolution of ageing are diverse and have been a subject of extensive scientific study and debate. This article explores the key concepts, theories, and mechanisms proposed to explain how ageing has evolved across different species.

Theories of Ageing

Several theories have been proposed to explain the evolutionary origins of ageing. These theories are not mutually exclusive and may interact in complex ways.

Mutation Accumulation

The Mutation Accumulation theory, first proposed by Peter Medawar in 1952, suggests that ageing is the result of mutations that cause deleterious effects in older age. According to this theory, natural selection is weaker at removing mutations that affect an organism later in life, after it has reproduced. As a result, these mutations can accumulate over generations, leading to the ageing process.

Antagonistic Pleiotropy

The Antagonistic Pleiotropy theory, introduced by George C. Williams in 1957, posits that some genes have multiple effects, some beneficial and some detrimental. A gene that enhances reproductive success in early life but causes harm in later life can be favored by natural selection, despite its negative effects on longevity. This trade-off between fertility and longevity is a key concept in understanding the evolution of ageing.

Disposable Soma Theory

The Disposable Soma Theory, proposed by Thomas Kirkwood in 1977, suggests that organisms allocate resources between reproduction and maintenance of the body (soma). Because resources are finite, investing more in reproduction can lead to less investment in somatic maintenance, leading to ageing. This theory emphasizes the trade-off between reproduction and the maintenance of the body's integrity over time.

Mechanisms of Ageing

The evolution of ageing is also linked to specific biological mechanisms that contribute to the ageing process. These include:

• DNA Damage and Repair: Accumulation of DNA damage over time, and the body's decreasing ability to repair this damage, is a key factor in ageing.
• Telomere Shortening: Telomeres protect the ends of chromosomes. With each cell division, telomeres shorten, eventually leading to cell senescence or apoptosis, contributing to ageing.
• Oxidative Stress: Reactive oxygen species (ROS) can damage cells and tissues, leading to ageing. The balance between ROS production and the antioxidant defenses is crucial for lifespan regulation.
• Cellular Senescence: Cells may enter a state of permanent growth arrest (senescence) in response to various stressors, contributing to ageing and age-related diseases.

Evolutionary Perspectives

Understanding the evolution of ageing provides insights into why ageing occurs and why it varies so much among different species. For example, some species like certain turtles and lobsters show negligible senescence, living for centuries without significant decline in their physiological functions or reproductive capabilities. In contrast, other species have very short lifespans and age rapidly.

The evolutionary perspective on ageing also highlights the importance of the environment and life history strategies in shaping the ageing process. Species that face high levels of predation or harsh environmental conditions may evolve to prioritize early reproduction at the cost of longevity.

Conclusion

The evolution of ageing is a complex and multifaceted topic that intersects with genetics, ecology, and evolutionary biology. While significant progress has been made in understanding the mechanisms and theories behind ageing, many questions remain. Continued research in this field promises to not only deepen our understanding of biological ageing but also inform strategies for promoting health and longevity in humans.

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