One life history model useful for gerontologists is the concept of r and K selection that was formally proposed by Robert MacArthur and Edward Wilson MacArthur and Wilson, ; Pianka, ; Austad, b.
Even though the r and K selection model is widely recognized as a simplification, it can be useful to interpret certain life history events. In brief, r- selection is the density-independent component of natural selection, which in practice refers to reproductive rate, while K- selection is density dependent, referring to the biggest population resources can sustain.
Organisms in hazardous environments will maximize reproduction and thus be r -selected while organisms in non-hazardous environments will maximize their performance under crowded conditions and thus be K -selected. Therefore, r -selection will favor rapid development, small body sizes, and a short lifespan while K -selection will favor delayed development, larger body sizes, and a longer lifespan Austad, b. For instance, humans, whales, or elephants are K -selected while mice and voles are r -selected.
If we consider the wide range of lifespans among animals including mammals , as well as factors correlating with longevity , r and K selection provide a useful model to begin understanding such variation. The evolutionary theory of aging is supported by abundant experimental evidence reviewed in Rose, This way, the force of natural selection would no longer decrease with age and, as predicted by the theory, lifespan was extended and aging delayed.
A more recent experiment revealed that selection for longevity also affected reproductive effort, supporting the antagonistic pleiotropy theory Hunt et al. Also in accordance with the theory, Steven Austad observed that opossums, a North American marsupial, living in a predator-free island reproduced later than animals of the same species on the more hazardous mainland.
Evolution of ageing
As determined by collagen elasticity, these animals appeared to age slower than the continental opossum Austad, ; Austad, a. Overall, for the majority of species studied, the classical models of Medawar and Williams based on the fading force of natural selection appear to explain the observations. As detailed below, however, there are some exceptions. One extreme example among life history strategies are animals that reproduce only once, as mentioned earlier.
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Semelparous species appear to fit life history theory as examples of ecological adaptation to certain life history conditions. For example, if due to high extrinsic mortality attaining reproductive maturity is unusually difficult and not likely to be repeated more than once Austad, a , p.
Another scenario is one in which mortality is significantly lower in juveniles than in adults--e. On the other hand, there is evidence that some cases of semelparity might be a result of group selection. The idea that aging evolved for a purpose goes against classical evolutionary models of aging, but perhaps some cases could be considered as such reviewed in Bowles, ; Goldsmith, ; Longo et al. For instance, adult moths mimic the movements of the juvenile forms presumably to attract predators and there are a few documented cases of insects in which the offspring eats its mother Hayflick, , pp.
Therefore, while evidence of group selection is largely absent for most species, such as mammals, we cannot dismiss it from playing a role in the evolution of aging of a subset of short-lived species. There is some experimental evidence against the evolutionary theory of aging. Although Medawar suggested that aging was controlled by a few, key physiological processes Medawar, , modern evolutionary theory of aging argues that aging is multifactorial Rose, ; Kirkwood and Austad, In other words, the evolutionary theory of aging postulates that numerous small-effect genes, rather than a few strong-effect ones, are involved in aging.
Hence, the way single gene knock-outs have been shown to delay aging in animals, which is detailed elsewhere , was in contradiction with at least some aspects of the evolutionary theory of aging reviewed in Johnson, In addition, some genetic manipulations appear to delay aging while not affecting reproduction Dillin et al. Eusocial animals like ants that have a single reproductive female also provide evidence against trade-offs between longevity and reproduction; one study even found that mating increases longevity in ant queens Schrempf et al.
Lastly, it was shown in guppies that animals with higher extrinsic mortality rates evolved earlier maturity and invested more in reproduction, as expected, but do not have an earlier onset of demographic or reproductive aging, which contradicts the evolutionary theory of aging Reznick et al. In addition, the classical evolutionary theory of aging does not explain why aging, a phenotype that escaped natural selection, is so similar among mammals, as described before. One of the most intriguing phenotypes in the biology of aging are animals that appear not to age, as previously discussed.
Studies conducted both in captivity and in the wild have shown that several species of fishes, amphibians, and reptiles, to name only vertebrates, fail to show signs of aging.
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Of course, these animals have only been studied for a limited amount of time. Still, it is surprising to find that, in a year study, female Blanding's turtles increased both survivorship and reproductive output with age Congdon et al.
Another year field study suggested that older female Painted turtles, when compared to younger animals, feature increased reproductive output and offspring quality while maintaining survivorship Congdon et al. Classical evolutionary models of aging predict that all species eventually age Hamilton, It has also been argued that since bacteria age, all other organisms must age Stewart et al.
This idea seems a bit counter-intuitive since it assumes that complex species must age if less complex species age; it seems more logical that highly complex species have a greater capacity to replace their components, such as cells, and hence avoid aging. Nonetheless, the observations clearly suggest some species may not age, which is in contradiction with the evolutionary theory of aging.
It is also difficult to reconcile the disposable soma theory with these observations of increased reproduction and survival with age. Moreover, some have argued that germ cells in mammals could originate in somatic cell precursors Bukovsky et al. In conclusion, the evolutionary theory of aging offers a theoretical framework that explains many--perhaps most--observations and remains a major theoretical landmark in gerontology. The theory offers clues as to the evolutionary mechanisms and the events leading to the evolution of aging, yet it does not offer a complete picture on the evolution of aging across different species.
Moreover, the evolutionary theory of aging can be harmful by imposing limitations on aging studies Gavrilov and Gavrilova, As it stands, the evolutionary theory of aging cannot be safely used to make predictions on the biology of aging Le Bourg, Evolutionary theories of aging are not predictive, they are descriptive. For instance, it has been argued that non-aging animals, especially those that increase size and fertility with age, may be favored by natural selection, thus contradicting the classical evolutionary theory of aging Vaupel et al.
In species with a high infant mortality and long generation times, an adult animal is precious and worth preserving; if reproductive output increases with age, natural selection will favor preservation rather than immediate reproduction. The impact of intergenerational transfers--e. Therefore, new evolutionary models of aging continue to be proposed and the evolutionary theory of aging will certainly continue to evolve.
Besides, a major open question concerns the precise genetic mechanisms and specific genes underlying the evolution of aging and species differences in aging, as discussed elsewhere. Since humans are mammals, of special interest is the evolution of mammalian aging. As mentioned before , the aging phenotype of mammals has some common features that may be a result of unique evolutionary events.
What follows is a particular model that aims to explain the evolution of aging in mammals based on the classical evolutionary models of aging. Figure 2: Overview of the evolution of mammals and closely-related taxa. Lines are not to scale. Adapted from Hedges, ; Maddison and Schulz, Fish and Wildlife Service. Mammals evolved from reptiles Fig. On the contrary, and as described before , all known mammals age. In fact, the intensity and incidence of aging appears to be higher in mammals than in reptiles.
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