In his article, Hardin says this:
Adam Smith did not assert that this was invariably true, and perhaps neither did any of his followers. But he contributed to a dominant tendency of thought that has ever since interfered with positive action based on rational analysis, namely, the tendency to assume that decisions reached individually will, in fact, be the best decisions for an entire society. If this assumption is correct it justifies the continuance of our present policy of laissez-faire in reproduction. If it is correct we can assume that men will control their individual fecundity so as to produce the optimum population. If the assumption is not correct, we need to reexamine our individual freedoms to see which ones are defensible. [Emphasis added.]Most people will probably recognize this assumption as the one made by libertarians (yes, I know there are different kinds, already). It still amazes me that anyone seriously thinks that human society is in any way optimized without regulation. That is incredibly naive. The tragedy of the commons substantiates the problem. Individual success does not always translate into success of the group.
The tragedy of the commons develops in this way. Picture a pasture open to all. It is to be expected that each herdsman will try to keep as many cattle as possible on the commons. Such an arrangement may work reasonably satisfactorily for centuries because tribal wars, poaching, and disease keep the numbers of both man and beast well below the carrying capacity of the land. Finally, however, comes the day of reckoning, that is, the day when the long-desired goal of social stability becomes a reality. At this point, the inherent logic of the commons remorselessly generates tragedy.Here is an interview with Hardin.
As a rational being, each herdsman seeks to maximize his gain. Explicitly or implicitly, more or less consciously, he asks, "What is the utility to me of adding one more animal to my herd?" This utility has one negative and one positive component.
1) The positive component is a function of the increment of one animal. Since the herdsman receives all the proceeds from the sale of the additional animal, the positive utility is nearly +1.
2) The negative component is a function of the additional overgrazing created by one more animal. Since, however, the effects of overgrazing are shared by all the herdsmen, the negative utility for any particular decision-making herdsman is only a fraction of -1.
Adding together the component partial utilities, the rational herdsman concludes that the only sensible course for him to pursue is to add another animal to his herd. And another; and another. . . . But this is the conclusion reached by each and every rational herdsman sharing a commons. Therein is the tragedy. Each man is locked into a system that compels him to increase his herd without limit--in a world that is limited. Ruin is the destination toward which all men rush, each pursuing his own best interest in a society that believes in the freedom of the commons. Freedom in a commons brings ruin to all.
In The Social Lives of Microbes, Stuart West and his collaborators describe evidence for complex systems of cooperation, communication, and synchronization in microbes . They highlight results that demonstrate the importance in evolutionary theory of factors such as relatedness, kin discrimination, competition between relatives, and enforcement of cooperation.
One such example if the production of public goods. Public goods are products manufactured by an individual that can then be utilized by the individual or its neighbors. The public goods produced by the microbes can be used by all, even the individuals who are not themselves producers. Since production comes at a metabolic cost, and thus a cost in fitness, it pays to be a cheater. A cheater is a kind of parasite, living off the public goods producing individuals, without contributing anything to the group. Since the cheaters have higher fitness, the whole group would soon fall into the trap of the tragedy of the commons. The question then is how this is avoided.
The answer is that such a system is regulated by negative frequency-dependent selection. When one kind becomes rare, it gains a fitness advantage, and therefore rebounds to become more frequent again. When cheaters become frequent, the amount of public goods is decreased, and suddenly cheaters are at a disadvantage. This relies on some spatial structure that makes sure that the public goods are on average closer to the non-cheating microbes.
The attentive reader might already have seen the similarity to certain aspects of human society. Criminals, for example, are cheaters who live off the production of the rest of society. However, society can tolerate some amount of crime, obviously, just as in the microbial world freeloaders can exist at low frequencies. Where the tragedy of the commons in humans has yet to play out, is in the case of population growth, as discussed by Hardin. In many parts of the world, there is no incentive for the individual to be the one that does not have more children. The result is population growth, and if that growth isn't stopped sooner than later, the result is overexploitation of resources. This is turn leads, as is already all too evident, to the disappearance of many human resources (i.e., other species), and if we're unlucky, to the eventual disappearance of humans, too. Incentives needs to be created for all humans not to have many children, such that population growth can be reversed, and the ultimate result of the tragedy of the commons can be avoided.
Here is, again, the incomparable Hans Rosling on how to do that:
 Garrett Hardin (1968). The Tragedy of the Commons Science, 162 (3859), 1243-1248 DOI: 10.1126/science.162.3859.1243
 West, S., Diggle, S., Buckling, A., Gardner, A., & Griffin, A. (2007). The Social Lives of Microbes Annual Review of Ecology, Evolution, and Systematics, 38 (1), 53-77 DOI: 10.1146/annurev.ecolsys.38.091206.095740