Health: An Underappreciated Factor in Darwinian Evolution

animalsOne of the most surefire ways to upset an evolutionary biologist is to bring up the phrase “survival of the fittest”, and on the basis of that saying, make the inferential claim that evolution is a race in which it is always the most physically fit organisms that are in the lead. If you do that, no evolutionist will take you seriously. The idea that it’s always the most physically fit or healthiest organisms that are at the front in the evolutionary arms race we call life is one of the most common misconceptions about evolution. As any Darwinist will tell you, the “currency” of evolution is not health or physical fitness, but rather Reproductive Success (RS). You can be the strongest, fastest, and healthiest organism on the planet; it won’t “matter” if you’re not able to pass on your genes.

With that said, health is not irrelevant in the context of Darwinian evolution. Far from it. I would argue that it’s a lot more important than most evolutionists think…

Darwinian evolution: A never-ending process

You don’t have to have a PhD in evolutionary biology to understand that there is a link between health and RS. Organisms that are weak, fragile, or sick are typically less likely to survive and reproduce than organisms that are physically fit, strong, and robust, at least if the organisms in question live in a natural environment.

Most modern humans do not live in a natural environment; we live in a built/manufactured one; hence, the aforementioned principles don’t apply as strongly to us. You don’t have to be physically fit to survive in the modern world. It doesn’t require much strength or stamina to be able to drive down to the grocery store to pick up some food. Even if you’re sick and unable to move around much, you shouldn’t have a problem surviving in an industrialized environment, seeing as you have access to motorized transportation, modern medicine, and social care services.

One of the consequences of all of this is that many chronic health conditions that would compromise a person’s survival in a natural environment get to persist in contemporary human populations. The selective pressures acting against genes associated with disorders such as myopia and obesity are quite weak as a result of modern innovations such as glasses, contact lenses, motorized vehicles, and so forth. Hence, we can’t expect natural selection to eliminate these conditions anytime soon.

The fact that many of the selective pressures acting upon traits associated with health and physical fitness have been weakened by cultural evolution/environmental change is generally believed to mean that natural selection doesn’t act as strongly on humans today as it did in the past. Some people have even gone as far as to say that the biological evolution of our species has basically come to a halt, due to the fact that we today have access to medicine and other tools that – in some instances – diminish the force of natural selection, and that we have designed our environment in such a way that it’s not necessary to be physically fit or strong to survive.

This is obviously not correct. Darwinian evolution hasn’t stopped. It’s true that we, via the process of building and transforming our environment, have changed the interaction that takes place between man and evolutionary forces; however, we haven’t cut ourselves off from the rest of the living world and taken ourselves out of the evolutionary arms race. We’re still a part of nature. We’re just another species. We’re not elevated above the rest of the organisms on this planet.

Natural selection may not act as “strongly” upon us – contemporary populations of industrialized humans – as it did upon many of our ancestors; however, it’s still acting. Actually, in some places of the world, such as in developing countries where infectious illness is a widespread problem, the pace of Darwinian evolution is quite rapid.

Also, it’s very important to note that the fact that you don’t have to be particularly strong or healthy to survive in a modern, built environment doesn’t mean that natural selection isn’t at work. Not all sick babies can be saved by modern medicine and not all life-threatening illnesses can be cured. Moreover, perhaps more importantly, the fact that a large number of people survive into reproductive age (industrialized societies) doesn’t mean that everyone gets an equal number of surviving offsprings. “Equal” survival is not synonymous with equal RS. Survival is a prerequisite for RS, but it’s obviously not a guarantee for RS. Some people get more children than others.

The link between health and reproductive success is stronger than most people think

Over the past couple of years it has become increasingly clear to me that health probably plays a much more important role in Darwinian evolution than generally believed. Obviously, it’s not the currency that counts; reproductive success is. However, health can have a major impact on survival and reproduction; thereby affecting Darwinian fitness.

Perhaps needless to say, I’ve “always” known that an organism’s ability to survive and reproduce is affected by its health and physical fitness condition. However, recently it has become clear to me that this link between health/physical fitness and evolutionary fitness is probably much stronger than what most textbooks on biology and evolution say.

As pointed out earlier, an organism that lives in a natural environment and that has to compete with other organisms for limited resources (e.g., food), has to be somewhat physically fit and healthy to survive. If it’s sick or unable to move around, it will have a hard time getting a hold of food and protecting itself from predators; hence, it will have a hard time reproducing. This is true regardless of whether we’re talking about a fox, a human hunter-gatherer, or any other organisms that lives in the wild.

Domesticated animals and most contemporary humans are largely exempt from this rule, given that we live in a built environment and have easy access to food and shelter. However, that doesn’t mean that our health has no impact on our reproductive success.

The health and physical fitness condition of a person living in the industrialized world will likely not have much of an impact on his ability to survive until reproductive age, unless of course he suffers from a life-threatening illness; however, it can affect his reproductive success via other means.

For example, if he’s very overweight and chronically inflamed, his libido may be low and his sexual function impaired; something that could obviously affect his fertility and fecundity. Not only may he have problems reproducing, but the children he potentially brings into life may be in poor health, due in part to “bad genes” they’ve inherited or epigenetic modifications left on their genome as a result of the actions and behaviors of their father. This in turn may impair their ability to successfully pass on their genes to future generations.

As you can see, there’s an obvious link between health and RS here.

At this point you may be asking yourself: Is this something that really happens? Or is it just a hypothetical example that has little relevance to the real world?

The short answer is that it can – and does – happen. Over the most recent decades a long range of studies linking health status with reproductive capabilities has been published (1, 2, 3, 4, 5, 6, 7, 8, 9). Many of these studies point to the same thing, namely that immune status has a potent impact on reproductive health and fitness.

The inflammatory Western lifestyle: A libido killer

Virtually all chronic illnesses known to man have been linked with chronic inflammation. Inflammation tends to closely correlate with health (It’s a proxy for health). The correlation is negative, in the sense that more inflammation translates to worse health.

Chronic, systemic inflammation is involved in everything from heart disease to obesity to osteoporosis. Even if you’re not chronically sick, chances are the levels of proinflammatory cytokines in your body are slightly higher than what would be ideal. This is particularly true if you’re overweight, don’t exercise much, and/or eat an unhealthy diet.

The fire that is burning inside the body of the obese, burger-eating modern man affects all aspects of his body and health. It negatively affects his mood, cognitive function, energy levels, and cardiovascular health, among other things. Moreover, it probably suppresses his desire for sex and impairs his capability to perform sexually. These statements are supported by a large body of evidence implicating inflammation in the pathogenesis of chronic depression, sexual dysfunction, heart disease, and many other health problems (4, 5, 6, 7, 8, 9, 10).

Given that a lot of contemporary humans are chronically inflamed, it’s not really surprising that sexual-related health problems are so common in our society today. I strongly suspect that very few people alive today are maxing out their potential in terms of sexual health and vitality. It has become so common to experience low libido, sexual dysfunction, and other issues associated with sexual health that a multi-billion dollar industry that delivers drugs and services that specifically address these types of health problems has emerged.

From an evolutionary perspective, it makes absolutely no sense that so many people experience these types of sexual problems. After all, sex is what drives evolution “forward” and produces much of the genetic variability that natural selection acts upon. Without sex, the evolution of sexually reproducing organisms such as ourselves stops.

Natural selection favors organisms that desire sex, like sex, and are able to perform sexually. We are sexual creatures. We have to be, or else the process of biological evolution comes to a halt.

The fact that many contemporary people experience little or no sex drive and/or are not capable of performing sexually, can’t be explained by “bad genes”, rather, it can be explained by novel interactions between genes and environment. Our genes find themselves in an environment they have little evolutionary experience with. The environmental forces acting upon the genome of the modern man differ markedly from the ones that acted upon the genome of the Paleolithic man. Many of the modern forces produce inflammation and disease; and consequently, impaired sexual health. An inflamed body doesn’t prioritize reproduction and sexual functions, rather its priority is to turn out the fire that’s burning inside it.

So far I’ve talked mostly about males. Don’t take that to mean that the things discussed so far don’t apply to women. They do! Women may actually in some ways be even more susceptible to some of the aforementioned problems, due to the fact that it’s women that actually bring children into life. Inflammation may not have the same impact on a woman’s sexual performance as it will have on a man’s; however, it will affect her libido, and perhaps more importantly, her ability to bring healthy offspring into life. Unhealthy, chronically inflamed females are more likely to experience pregnancy and birth-related problems (e.g., miscarriage) and are less likely to bring healthy children into life than healthy mothers are (4, 11, 12, 13).

Three conditions that are closely associated with chronic inflammation are obesity, insulin resistance, and hyperinsulinemia. These disorders are extremely common in industrialized societies, due in part to the widespread consumption of highly processed foods in these societies, and are known to undermine reproductive and sexual health. Here’s what the book Evolutionary Medicine And Health: New Perspectives  had to say about the role these conditions play in the reproductive health of women:

High and increasing levels of obesity, insulin resistance, and insulin in Western and Westernizing populations have potentially harmful effects on ovarian and reproductive function in women. In particular. hyperinsulinemia is associated with hyperandrogenism, irregular mestrual cycles, reduced fecundity, and PCOS. Thus it is appropriate to consider hyperandrogenism, irregular mestrual cycles, PCOS, and associated low levels of fecundity in women as problems caused by the evolutionarily novel Western environment, and as conditions that are likely to be experienced at particularly high levels in populations in transition. (14)

The things discussed in this section obviously also apply to hunter-gatherers. However, since hunter-gatherers tend to be lean, physically fit, and in fairly good health, the link between inflammation/health and sexual dysfunction isn’t as relevant when talking about them as when talking about industrialized people.

Here’s the irony. By changing our environment in an attempt to make our lives more comfortable, we’ve largely eliminated the requirement to be physically fit in order to survive; however, we have inadvertently compromised our evolutionary fitness in other ways, such as by filling our world with novel environmental factors that impair our sexual health.

It’s impossible to estimate how big of a role these things actually play in the process of Darwinian evolution. They undoubtedly do play a role, but it’s difficult to say how much of an impact inter-individual variability in health status has on inter-individual variability in RS.

We’re attracted to healthy people

So far, we’ve mostly talked about how our health and physical fitness condition affect our ability to survive, our desire for sex, and our ability to perform sexually and bring healthy children into life. What we haven’t talked about is how our health and physical fitness condition affect our ability to attract members of the opposite sex.

Perhaps needless to say, a person’s ability to attract a healthy mate is affected by his health, body composition, and looks. We’re attracted to healthy-looking people, which isn’t surprising, given that healthy people are more likely to produce healthy, robust offspring than unhealthy folks.

A lean body, glowing skin, nice teeth, and beautiful hair are some of the many physical features that we are attracted to. All of these characteristics are symbolic of good health.

Many of the physical traits that are generally perceived as attractive are not heritable, but are rather shaped by the behaviors of the organism in question. For example, the amount of fat mass you carry is largely determined by what you eat and how much you exercise. Heritable factors also play a role, but they aren’t the end all be all when it comes to how you look. I would argue that the environment is in many respects even more important than genes in this context. With that said, heritable factors can indirectly affect our body composition and health status by shaping our behavior.

Obviously, there are some things you can’t change, such as the color of your eyes; however, most things are modifiable, at least to some extent.

In the case where the attractive physical characteristic in question is not primarily a result of heritable genetic information; it’s not sexual selection that is at play. Rather, it could be the behaviors and actions of the person in question that are responsible for producing the physical characteristic and elevating his attractiveness to members of the opposite sex, and hence, perhaps improving his reproductive success. These behaviors and actions don’t necessarily just affect his body, but also the trillions of microorganisms that he carries with him. This is important to note, because the microbes that colonize our bodies affect who we are attracted to, how attractive other people find us, and who we attract. But that’s a topic for another day.

Key takeaways

Health matters in evolution. Its role in Darwinian evolution is a lot more important than most evolutionists recognize. It’s not the primary factor responsible for “fueling” the process of Darwinian evolution; however, it does play an indirect role, in the sense that it affects many aspects of organismal survival and reproduction. This is very important to acknowledge, because it has major implications for how we view and approach health and medicine.

To summarize, there are three primary ways that the health status of an organism can affect its Darwinian fitness:

  1. It can affect the organism’s ability to survive (e.g., get a hold of food, evade predators, combat infections)
  2. It can affect the organism’s sexual and reproductive health (e.g., libido, ovarian function, menstrual cycles)
  3. It can affect the organism’s sexual attractiveness (e.g., body composition, skin complexion)


  1. Eve Fain says:

    Darwin inheritance changes the population ONLY if it decreases reproductive fitness. It is a fallacy of thinking to believe it can change any other part of the population. Beware of using “fitness” as though you can personally see who is more fit in the population. If they can produce an offspring who can in turn reproduce, their genes are flowing through history.

    • Hi Eve,

      That’s not correct. Natural selection-induced changes in the gene pool of a population occur as a result of inter-organismal variation in reproductive success. Alleles increase or decrease in prevalence as a result of their fitness value in the current environment. Obviously, both traits that are adaptive and deleterious in the context of survival and reproduction are under selection.

  2. Alessio says:

    I think the devil is in the detail.
    I’m a strong supporter of the impact of being in physical fitness on RS. It’s consistent with the observation of nature. I still haven’t seen a video of a limping tiger or a zebra with love handles.
    I think that some selection took place at the dawn of agriculture where people unable to reproduce with the new foods were doomed.
    On the other hand, it was the first time in human history where fisical fitness was became unrelated with reproductive fitness.
    Sugar availability was one of the main drivers of fertility in nature.
    Since carb rich food were not available all year round, our ancestors were not always fertile, but just seasonally.
    With the advent of agriculture, we became chronically fertile but we paid the consequence later in life.
    Selective pressure toward fitness and health became more relaxed since you don’t have to be so fit and healthy to raise a crop in a patriarchal family system where you have a hubris of son who make a huge working strenght.
    Neither you have to age well because elderly people are not essential in such economical structure and fitness after reproductive age is not necessary.
    I think that despite a small level of selection is still working, it doesn’t make a great contribute to an overall change of gene pool.
    We are too many in the world and no more isolated and pressure is very low.
    Yes, we may be attracted by cute people, but since we have less and less of them among the average Joe and Mary, people put up with mating with anyone not to be alone.
    We may go toward some kind of important selection when almost nobody will be able to reproduce due to a severe infertility rate.
    What do you think?


  1. […] certainly correct that fitness, not health, is what natural selection is concerned about; however, as I’ve pointed out before on the site, organismal fitness is closely linked with organismal health. Sick, weak, and/or fragile organisms […]

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