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14. Of BIGS and BUGS or Why Size Does Matter

Updated: Jun 9, 2021

For professors who teach evolutionary biology, the question will always arise: Have humans adapted to the environment? Answer: Yes, of course they have. Humans, like any species, have a long unbroken history of successfully adapting to the stresses and challenges of the environment. We show those adaptations in a number of physical attributes, but especially in our biochemistry and immune systems. But are humans adapting to the current environment? That question is more difficult and the answer is, no, not really.

The question usually comes up during conversations about epidemics such as HIV/AIDS and whether humans will become resistant or immune to the HIV infections. The answer is not really one of whether we are capable of evolving resistance (yes, we are), but whether it will actually happen and that answer is no, it won’t. What is far more likely is that HIV will adapt to us for the reasons explained in the previous chapter.

The explanation of why we won’t adapt requires going over some basics about evolutionary biology. Essentially, the problem is this: we just don’t have the time to adapt to HIV, or a coronavirus, or any other pathogen in our environment. The rate of evolution, that is, the time necessary for a population to adapt to a particular stress in the environment, depends on several variables.

First, we have to possess, somewhere in the population, the genetic variation needed to tolerate a stress that is highly lethal. That variation will be a mutation that a number of people unknowingly carry with them, buried in their chromosomes, waiting for the opportunity to be expressed. As an aside, a mutation for survival cannot be present in only one person or even two people. Many people must possess it because the survivors of a highly lethal stress must produce the next generation and the parent population cannot be too small or too closely related.)

Do mutations for HIV resistance exist in the human population? Yes, at least one such gene for HIV resistance has already been identified.[i] Ideally, such a mutation would be a pre-existing gene and, even better, it would be several different mutations but for the same thing. For example, there are a number of sickle-cell and thalassemia mutations conveying some degree of resistance to malaria infection throughout Africa and the Mediterranean and Indian Ocean regions.

Second, there has to be an environmental stress that is sufficiently strong to favor those individuals with the mutation while eliminating those without it. I’m sure you know what I mean and I’ll get to that next. Third, the rate of adaptation depends primarily on the length of a generation. That is, evolution depends on how quickly the parents with the genes for tolerating the stress can produce offspring that are also able to tolerate the stresses faced by the parents.

One common estimate of the time necessary for adaptation of new traits in an entire population is 100 generations. This is the rub. For humans with a generation time of about 15-20 years, something on the order of 1000-2000 years might be needed for an adaptation to become common throughout the population. There are some ways around the time constraints for the rate of adaptation, but those alternatives aren’t very pleasant.

As I mentioned previously, it is important to note that, as a species, population, or culture, we really do not want to adapt. Not at all. Adapting is not fun. The requirement of having a stress in the environment that is strong enough to effect genetic change in a population has a rather critical requirement attached to it. That requirement is this: most of the population has to die in a relatively short period of time and the stress has to exist for several generations to eliminate the weak genotypes and to favor the strong ones to the point that they dominate the gene pool. That is, adaptation requires death, a lot of it, and for a long time. If the death rate is lower, the time for adaptation is longer; for adaptation to occur very quickly, the death rate must be extremely high.

As a population experiences such an intense stress, the death rate would likely be very high among the elderly but, unfortunately, for rapid adaptation to occur it is far more important for the juveniles and the younger adults in the population to experience very high mortality rates. For adaptation to occur in any species, the individuals without the necessary protective genes would be removed from the population, and that must happen before those individuals can reproduce and pass on their weaker (non-adaptive) genes. That means mortality must occur prior to adulthood, which can either mean before reproductive maturity or that reproduction itself would fail.

The death of adults who have already produced offspring does not affect the makeup of the population in a meaningful way because those adults have already passed on their genes. Only the death of their offspring before they have a chance to produce their own offspring matters in the course of adaptation. Those individuals with the mutation for resistance to the stress are the survivors and the successful reproducers. Their offspring come to repopulate the depleted population.

Over the course of many generations, only those exhibiting resistance to the environmental stress, such as a lethal pathogen, will survive in the population. That population has now adapted to the stress, but it took several traumatic generations. So, in short, humans are very slow to adapt and, in fact, we will do anything to avoid it.

If we think about what sorts of organisms adapt very quickly, we should think of very small, rapidly reproducing sorts of things. Let’s use the generic term “bugs”. Bugs, from bacteria to houseflies to cockroaches, can produce prodigious numbers of offspring in a very short amount of time and those offspring can be producing their own offspring very shortly after that.

For many if not most small things, a large number of generations can elapse during the period of the stress and that results in quick adaptation to the stress. For example, humans can apply a stress to a bug’s environment over the course of a single year, but a year is an enormous amount of time for bugs. For houseflies, a new generation can be produced in about two weeks and one female fly can lay up to 500 eggs. At that rate, a pair of flies could produce 100 million million (1.9x1014) offspring in a single summer if all offspring were to survive long enough to reproduce.

If a sufficiently intense stress were present, such as an insecticide that quickly and effectively eliminated non-resistant flies, this rate of reproduction meets all the requirements for rapid adaptation. If a few insecticide resistant individuals are present in the very large fly population, the offspring of those individuals will generate a resistant population very quickly. It would be a small population initially, but with the capacity to grow very rapidly. In general, organisms that behave in this manner and can adapt rapidly to new stresses we can refer to as BUGS.

In contrast, “Big” species like humans, cats, and sharks, cannot quickly adapt to lethal environmental stresses because the period of intense stress is typically much shorter than the amount of time needed for producing new and adapted generations. That is, in a highly stressful environment, the individuals die quickly and before each new generation can produce their offspring.

For example, a pandemic such as the Spanish Flu of 1918 killed at least 50 million people worldwide, but was already waning only six weeks after the main outbreak began. That is, the stress was gone in a very short amount of time relative to the generation time of humans. (And the death toll was about 2.5% of the world population which is not sufficient to bring about a significant shift in global human genetics. Similarly, COVID-19 will also not result in a shift in human genetics.)

If a mutation exists in the population that confers resistance to the epidemic, those with the mutation will survive at much higher rates, but there are some additional difficulties. First, the resistant survivors would have to be of reproductive age, not too young and not too old, to begin rebuilding the population. Second, the resistant survivors would have to mate and produce resistant offspring and it’s well known that (for humans) the process takes about a year.

Third, the resistant offspring require no less than 15 years to produce offspring of their own. The population would recover very slowly, but perhaps more importantly, as in the Spanish Flu example, the stress that caused the increase in mortality would have long since faded from the environment and the adaptation for resistance would be of no particular use unless the flu had a very short return period and was always very deadly.

While the human species harbors a large number of random mutations (one estimate has it at about six per person), but random implies that the vast majority of mutations will be of little use in a flu epidemic. And human populations within a particular region that might be affected by a specific stress are typically rather small populations compared to those of BUGS.

In contrast, BUG populations tend to be very large and are likely to harbor a very large number of mutations, many of which may be appropriate for tolerating new stresses. If a severe stress eliminates a large proportion of a large population of BUGS, the survivors are still very numerous and very capable of rebuilding the population very quickly. In other words, in environments where short-lived and very intense stresses occur (such as diseases), BUGS can adapt easily and BIGS cannot.

So, if adapting takes so long, how did all of the BIGS in the world (especially humans) manage to stay alive for millions of years? Let’s accept as a truism that no living species has ever faced a stress that was too intense to be accommodated through adaptation. Of course, that’s circular reasoning; if a species exists, then it must have tolerated all stresses it faced through history because otherwise it wouldn’t exist. And that isn’t an entirely correct statement either; species are made up of locally adapted and widely distributed populations and it is very common for a population to succumb to a local stress, but this does not mean the entire species disappeared.

It is also true that most stresses that arise in an environment are not so lethal that every individual dies before the population is able to adapt to the stress. If we assume that any species (ourselves included) is faced with literally dozens of different environmental stresses that potentially can lower the life expectancy of different individuals in different ways, but which don’t kill outright, then we see that most stresses are not wiping huge proportions of the populations. Those populations have longer periods of time to adjust and adapt to those different stresses. Thus, all BIGS have adapted throughout their history, but only to relatively low intensity stresses or to stresses that did not affect the entire species simultaneously.


[i] Zrinka Biloglav et al. 2009. Historic, Demographic, and Genetic Evidence for Increased Population Frequencies of CCR5Δ32 Mutation in Croatian Island Isolates after Lethal 15th Century Epidemics. Croatian Medical Journal 50:34-42. And incidentally, the CCR5-Δ32 allele was the focus of the attempt by Chinese researcher He Jiankui to create HIV resistant children in 2018.


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