Genetics and Evolution of Aging
While this apparent paradox has puzzled evolutionary biologists for decades, we now have a few models that explain it. Genetic diseases that affect children and young adults are reduced to a very low frequency within the population because they severely impact fitness and reproductive capability. Once an individual has reproduced, they have fulfilled their evolutionary goal. To test this model, researchers devised a study on two populations of fruit flies to determine if one would live longer if they were experimentally selected to breed at a young or old life stage.
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The answer lies in the fact that counteracting the process of aging was under strong positive selection in the old-reproducing population of fruit flies, as they needed to survive until the point of reproduction being granted to them. While Mutation Accumulation is a passive mechanism that explains how evolution condones the enrichment of late-acting mutations that accelerate the process of aging, an extrapolation of this model shows us how late-acting mutations could be actively selected for. Think about it like buying a new car. But the acceleration and speed this car possesses may consequently lower the safety of the driver.
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In the context of aging, the AP model holds that deleterious late-onset mutations are more likely to accumulate in the population if they are beneficial in early life; for example, a gene that increases reproductive rate at the cost of cellular maintenance later in life Medawar, The AP model of aging may explain several late-onset human diseases, including:.
Further development of these models underpinning the evolution of aging elucidated that there is a fundamental balance that must be struck for allocation of finite resources between cellular maintenance and sexual reproduction. There is thus a trade-off between the investment of resources in reproduction, and the survival time of the soma.
At a stroke, this solves the problem of different rates of aging in different species, because those that develop and reproduce fast also have short life spans, and those that develop and reproduce slowly have long life spans. Thus, aging can be defined as the eventual failure of maintenance. This model of aging states that a finite amount of nutrients can be extracted from the food we eat and the environment we live in, which must be allocated effectively to optimize our evolutionary fitness Kirkwood, In nature, reproduction is energetically costly and risky for survival.
There are time, energy and monetary for humans costs of searching for a viable partner. There are disease risks when interacting with others as well as mating injury risks that must be considered, including the strange phenomenon of female consumption of unsuspecting males post-copulation, or more relevantly domestic violence for humans. Based on this model of aging, species that experience a higher extrinsic mortality rate i. To visualize this idea, consider two mammals with similar body weights but very different lifespans: mice and bats.
Mice will typically expend their energy reproducing at the earliest possible stage before they die or are killed. This occurs at the expense of cellular maintenance later in life. Mice having an average lifespan of years even when taken out of the wild and grown in a controlled environment.
The Evolutionary Theory of Aging and Life History Theory
Meanwhile, bats have very few natural predators. They have much lower selection pressure to reproduce as soon as possible early in life, but rather have evolved their genetic machinery to allocate resources towards survival, having an average lifespan of approximately 30 years! To attract potential partners, male crickets rub their hind legs against their abdomen to produce a mating call that we have become accustomed to hearing in the evenings. This call is not only metabolically costly, but alerts the presence of male crickets to predators. A Nature study Hunt et al.
The energy-intensive night calling efforts of male crickets result in lower longevity even in the absence of predation. However, if this resource allocation results in finding a mate to reproduce with, this represents a successful strategy and their shorter lifespan becomes evolutionarily beneficial!
In contrast, adult males reared on a high-protein diet died sooner than those on low-protein diets because they invested more energy in calling during early adulthood.
Moving forward, evolutionary medicine has the potential to deepen our understanding of disease development and prevention, by combining individual genetic information with broad evolutionary hypotheses. Austad, S. Diverse aging rates in metazoans: targets for functional genomics. Mechanisms of Ageing and Development , 1 , Carter, A.
Antagonistic pleiotropy as a widespread mechanism for the maintenance of polymorphic disease alleles. BMC Medical Genetics , 12 1 , Eskenazi, B.
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Medical Hypotheses , 69 6 , Gann P. Journal of the National Cancer Institute. Gluckman, P. How evolutionary principles improve the understanding of human health and disease.
Hunt, J. High-quality male field crickets invest heavily in sexual display but die young. Nature , , Rose, M.
Why haven't we evolved immortality? The answer is in our genes
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