One of the timeless (get it?) questions in biology is why did we evolve to age? What benefit is there to getting older and deteriorating before we die? (Image Credit: medienluemmel, Pixabay licence, Image Cropped)

Evolution favours aging in populations with assortative mating and in sexully dimorphic populations (2018) Lenart, P. et al., Scientific Reports, 8, https://doi.org/10.1038/s41598-018-34391-x

The Crux

We as humans are familiar with aging as the slow deterioration of our bodies and minds over time, and we can see this in other animals as well (think of the old family dog with white around its muzzle). The interesting thing is that not every species ages in the way that we do, that is to say that they stay forever “young” until they die. In a biological sense that means that while these organisms can and do die, their risk of death remains the same throughout the course of their lives. This would be akin to your grandparents, in their old age, having the same risk of death as you during the prime of your life. Or, conversely, you being just as likely to die in your sleep as a senior citizen.

The authors of this study note that, while theories for the evolution of aging abound in the scientific literature, they are not broadly applicable and some of them even require the existence of aging for the evolution of aging to even happen. They wanted to find out in what situations aging individuals could outcompete non-aging individuals, and vice-versa.

What They Did

The authors used an Agent-Based Model to simulate competition between aging and non-aging organisms. These models are kind of like a video game where you program every aspect of your “organism” and the “environment” that it lives in. You control how long it lives, what it does, and what kind of conditions it experiences. Because you can’t really grab a bunch of different organisms, control every aspect of their lives and environment, and then get results without an ENORMOUS amount of effort and a non-existent study system, these models are a great way to figure out what may have driven certain evolutionary processes.

In addition to simulating aging and non-aging species, the authors also simulated sexually dimorphic (species where males and females look markedly different) reproducing species against self-fertilizing, hermaphroditic species. Both of these organisms types (aging/non-aging and dimorphic/hermaphroditic) competed in situations where their fecundity (how much they reproduce), mating preferences (if they mated with each other or the opposite aging type), and parasites differed.

Did You Know: Immortal Species Some organisms are unique even in the world of non-aging animals, not only do they not age, but they can also live for an incredible length of time. One special example of this is the immortal jellyfish (Turritopsis dohrnii), which is thought to be an “immortal” animal. It is able to transition freely between its polyp (juvenile) and medusa (free-swimming adult) forms by changing the function of its cells, and in theory this process can go on indefinitely.

What They Found

Not surprisingly, organisms with a higher initial fecundity were able to outbreed and eventually dominate their less fecund competitors. These results held no matter which of the species types (aging or non-aging) had the higher initial fecundity, and this higher birth rate could even overcome a fitness disadvantage (such as suffering more from parasites).

In the scenarios where the two aging types were given choices between mating with either their same type or the opposite, due to some inherent preference for one or the other, the aging phenotype also wins out in cases where the preference for mating with the same type was stronger than the preference for mating with the opposite type. The aging type also outcompeted the non-aging in cases with sexually dimorphic species.

Any Problems?

Due to the nature of the model, the different scenarios presented are extreme compared to what you would find in nature. For example, the aging species in the model age very quickly, and they are competing with species that don’t age at all. They mention that the model was set up in this way so that it would be easier and more straightforward, not only to interpret, but also to set up the model itself. The authors note that if these fast aging species can outcompete the non-aging species, we are also likely to see “normal aging” species outcompete “slowly aging” species.

So What?

The authors of this study were able to show that, under a variety of simulated scenarios and circumstances, aging was the preferred species type over non-aging species. This comes down to the basic rule of natural selection: if you have some trait that lets you survive more than some other species, and if that trait is passed on to your offspring, you will produce more offspring and eventually come to dominate the environment.

Like many things in ecological research, the answer to a question (such as is aging or non-aging preferred?) depends on the context. This study showed that in several situations aging is favored over non-aging, and the aging individuals will outcompete (outbreed) the non-aging individuals. Several of those situations occur in some populations, while others occur in almost every population. This seems to support what we see in nature, where the majority of species age throughout the course of their lives. But, due to this variation, we have the rare and extraordinary examples like the Greenland shark (400+ years old) and the various, still-living trees thought to be 3500 years old.