The defeat of the highlanders
Computer simulation indicates that growing old can be an adaptive advantage
In the movies about the character, Highlander, a Scottish warrior who becomes immortal at the start of the sixteenth century, the main character Connor MacLeod goes through the ages fighting evil-doers and bringing about justice without feeling the weight of passing time. The hero never ages and his body never degenerates. The only way for him to die is to be decapitated by an enemy. In the face of this fictional plot, common sense leads us to conclude that an army made up exclusively of Highlanders would be virtually unbeatable for an equivalent force of mortals.
However, a series of computer simulations carried out by a Brazilian researcher suggests that becoming old may be an evolutionary advantage for the population, if the process of natural selection takes place in an environment characterized by change. In this situation, the group whose members grow old is likely to come out on top in the struggle for survival and provoke the extinction of the band of immortals. The explanation for the victory of the population that grows old lies in its greater capacity to adapt to habitat changes and to produce offspring adapted to the environment faster than those competitors who are endowed with a biology that is immune to the effects of aging.
This study, the findings of which appear to be paradoxical or at the very least counter-intuitive, was conducted by the theoretical physicist André Martins, from the University of São Paulo’s School of Arts, Sciences and Humanities (EACH-USP). “Although it has a negative impact on the individuals and entails an evolutionary cost, the aging process may in itself be a benefit,” states Martins, who published the study, which he undertook by himself, without any collaborators, on September 16 of last year in the scientific journal PLoS ONE. “It enables new offspring to adapt more rapidly to changes in living conditions.”
In simulations in which the individuals from the two armies were exposed to environmental changes and genetic mutations, the team of mortals won 39 out of 50 battles against the immortals. However, this was not the case when the virtual scenario in which the dispute took place was diametrically opposed. In a static environment in which the groups of individuals did not experience DNA changes, the Highlanders won every encounter, generally after disputes carried on for 220 generations.
“Different intensities in terms of mutation and environmental change were tested as well as changes in the age at which death due to ageing occurred,” explains the physicist from USP. “Those who aged came out victorious in a broad band of variations in these parameters, while the immortals were the victors only in environments where change was very slow or non-existent.”
In nature, it is common for the conditions in which different populations or species compete change from time to time. Therefore, according to Martins, the results of his simulations favor the interpretation that becoming older may have been an adaptive advantage that was naturally selected by the evolutionary process. “I believe that this may well be the case, although it contradicts established theory,” comments the American researcher Joshua Mitteldorf, a theoretician specializing in evolution and computer models at Temple University, in Philadelphia. “Over the course of the last 50 years, various experiments have shown that aging is an adaptation while theories say that this cannot be the case.”
Genes linked to the aging process appear to be well preserved in many species of animals. In 2008, researchers from the University of Arkansas increased the lifespan of the worm C. elegans, one of biology’s model organisms, 10-fold, by introducing a mutation in a single gene. Instead of dying after two weeks, examples of the nematode lived for a period of six months, with some living as long as nine months. Calorie-restricted diets have also shown themselves to be useful for increasing the lifespan of various species in laboratory experiments. This evidence, coupled with the fact that the organisms contain mechanisms of programmed cellular death, such as apoptosis, is usually cited by those who argue in favor of the idea that the capacity to become old was a characteristic chosen by natural selection.
However, over the last few decades, the main evolutionary theories on aging have been going in the opposite direction. Broadly speaking, these theories argue that aging is a type of collateral effect, a cost to be charged over the long term, on account of the gains obtained during youth. According to this line of reasoning, the first few years of life are the crucial ones for a species, when its specimens are most able to reproduce and to perpetuate descendants. Because in nature few individuals reach an advanced age due to the pressures of the environment, natural selection did not prioritize beneficial traits for old age.
First conceived at the end of the 1950s, the theory of antagonistic pleiotropy argues, for instance, that beneficial genes in youth, such as those that increase fertility, end up having a negative effect in old age. In this case, senility would be a debt acquired as the result of a benefit enjoyed during youth. More recently, in the second half of the 1970s, the English biologist Tom Kirkwood, currently a professor at the University of Newcastle, put forward another theory on aging, namely, the “disposable soma” theory.
According to this point of view, the organism regulates its quantity of energy and channels it, as a matter of priority, to the functions that are essential during the first few decades of life, such as reproduction. Repairing the organism so that it can maintain itself for a long period is of secondary importance, as this has a very high cost for the metabolic system. Therefore, progressive deterioration of the individual sets in, leading to aging.
To test the hypothesis that aging may be an adaptive advantage in itself, the physicist André Martins constructed a computer model using the software NetLog, which works with a programming language that can create virtual environments. The simulations are fed by an equation with variables that, in sum, govern a dispute between two populations in a closed two-dimensional scenario.
There is just one difference between the two sides: in one group, that of the immortals, the individuals never grow old (therefore, they do not die from old age) and can only be eliminated by means of internal competition or by members of the rival population; in the other group, the individuals die on account of competition and those who survive this dispute for a long period will, sooner or later, pass away as result of the aging process.
The graphic representation of the scenario of the evolutionary dispute is a square made up of 2,600 pixels in a smaller simulation version and of 10,200 pixels in an expanded variation. The points are painted green when they are empty, if they are not occupied by either of the two populations. They become blue if they are controlled by the army of those who age, and they take on a reddish color when they are controlled by the Highlanders. There is only space in each pixel for a single individual of one population or of the other. “Whenever there are two or more individuals on a single point, whether they are of the same population or of the opposing group, only one will survive,” explains Martins. It is literally a life-or-death struggle.
At the beginning of the game, the conditions are identical for both sides. Both start off with an equal number of participants, distributed at random across the battlefield. The two populations start off with the same capacity in terms of adapting to the environment, experience the same rate of mutation from generation to generation, and, over the course of the virtual experiment, reproduce at the same moment and at the same rate. Each individual produces one descendant, who inherits from his parents the trait of being either mortal or immortal. Among the team of those who grow old there is an added variable in the equation, responsible for causing the death of the individuals whenever, by chance, they reach a certain age. However, the majority, though not all, succumb to the competitive environment before they get old and have the chance to die of old age.
Death of the aged accelerates adaptation
In a non-static environment, in which the natural conditions change and the successive generations of a population undergo genetic mutations, the dynamics of the virtual dispute created by Martins tend to be the same. Two types quickly take shape in the simulations, setting battalions of red soldiers, the Highlanders, against the blue combatants, consisting of those who can grow old. In the initial stages of the game, the red population becomes larger, with almost double the number of members, and it seems clear that this group will win the dispute. There is an obvious reason for the Highlanders’ advantage: they can only die as a result of the competition, whereas the other side perishes due to the pressure of the environment as well as old age.
However, in almost 80% of the simulations, the blue team managed to turn the game around and exterminate the other population. Death due to old age of the oldest individuals in this team, particularly of those who were far away from the zones occupied by the opposing group, created more empty spaces inside the territory occupied by the blues. This phenomenon seems to favor the development of levies of individuals who are better adapted to the environment among the population of those who can grow old. “The new generations of the two opposing groups adapt to the environmental changes, but this effect occurs more quickly among the population that can grow old,” comments Martins. It is this detail that would cause it to win the majority of the disputes, creating a paradox: aging is bad for the individual, but beneficial for a group.
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In the computer model, the numerical variable that measures the average rate of adaptation of the two populations tends to rise sharply among the blues at a certain moment, leaving the reds behind. The program paints the agents of each army in tones proportional to their individual degree of adaptation to the environment. The darker the color of a member of the two populations, the greater his level of adaptation to the environment. In order to certify that those who were best suited to winning the struggle for life were concentrated on the side of the mortals, Martins also ordered the program to color the members of the two groups with the same color, in this case green. In this way, it was possible to see that the soldiers in the darker tones of green, i.e., those who were the best adapted, were individuals from the population that grew old.
Still, one must be careful when interpreting simulations. For the biologist Diogo Meyer, from USP, a specialist in evolution, it is important that the models have “freedom” to explore a variety of scenarios. However, it is also essential that these scenarios be based on reality. “Is the variation in longevity that exists between the populations and the degree to which natural selection can alter it compatible with the parameters used in the simulations?” asks Meyer. “Perhaps we lack data on this, but this question is a natural outcome of the results obtained with computer models.”
The theoretical physicist Roberto Kraenkel, from Paulista State University (Unesp), who is a specialist in the use of mathematical models in biology of populations, added a technical reservation to the work of his colleague, the theoretical physicist Martins. According to Kraenkel, there is a variable in the equation that governs the competition between the two populations, which is labeled ‘d’, whose concept appears a little mysterious and vague. “I am not really sure which biological concept this variable represents,” said Kraenkel. “However, it is fundamental to ensure the final result of the model.” Martins admits that no model is perfect and that of course it will always be a simplification of the struggle for survival of the species. “The question that I wanted to answer was whether or not the aging process could be an adaptive advantage,” he says. “The simulation shows that, in some cases, it may have been so.” Perhaps this is why nobody has ever seen a species similar to the Highlanders, that never age.
Martins, A.C.R. Change and aging senescence as an adaptation. PLoS ONE. Sept. 16, 2011.