Evolutionary mismatch


Evolutionary mismatch is the evolutionary biology concept that a previously advantageous trait may become maladaptive due to change in the environment, especially when change is rapid. It is said this can take place in humans as well as other animals.
Environmental change leading to evolutionary mismatch can be broken down into two major categories: temporal or spatial. Since environmental change occurs naturally and constantly, there will certainly be examples of evolutionary mismatch over time. However, because large-scale natural environmental change – like a natural disaster – is often rare, it is less often observed. Another more prevalent kind of environmental change is anthropogenic. In recent times, humans have had a large, rapid, and trackable impact on the environment, thus creating scenarios where it is easier to observe evolutionary mismatch.
Because of the mechanism of evolution by natural selection, the environment determines which traits will persist in a population. Therefore, there will be a gradual weeding out of disadvantageous traits over several generations as the population becomes more adapted to its environment. Any significant change in a population's traits that cannot be attributed to other factors will be responsive to a change in that population's environment; in other words, natural selection is inherently reactive. Shortly following an environmental change, traits that evolved in the previous environment, whether they were advantageous or neutral, are persistent for several generations in the new environment. Because evolution is gradual and environmental changes often occur very quickly on a geological scale, there is always a period of "catching-up" as the population evolves to become adapted to the environment. It is this temporary period of "disequilibrium" that is referred to as mismatch. Mismatched traits are ultimately addressed in one of several possible ways: the organism may evolve such that the maladaptive trait is no longer expressed, the organism may decline and/or become extinct as a result of the disadvantageous trait, or the environment may change such that the trait is no longer selected against.

History

As evolutionary thought became more prevalent, scientists studied and attempted to explain the existence of disadvantageous traits, known as maladaptations, that are the basis of evolutionary mismatch.
The theory of evolutionary mismatch began under the term evolutionary trap as early as the 1940s. In his 1942 book, evolutionary biologist Ernst Mayr described evolutionary traps as the phenomenon that occurs when a genetically uniform population suited for a single set of environmental conditions is susceptible to extinction from sudden environment changes. Since then, key scientists such as Warren J. Gross and Edward O. Wilson have studied and identified numerous examples of evolutionary traps.
The first occurrence of the term "evolutionary mismatch" may have been in a paper by Jack E. Riggs published in the Journal of Clinical Epidemiology in 1993. In the years to follow, the term evolutionary mismatch has become widely used to describe biological maladaptations in a wide range of disciplines. A coalition of modern scientists and community organizers assembled to found the Evolution Institute in 2008, and in 2011 published a more recent culmination of information on evolutionary mismatch theory in an article by Elisabeth Lloyd, David Sloan Wilson, and Elliott Sober. In 2018 a popular science book appeared by evolutionary psychologists on evolutionary mismatch and its implications for humans.

Mismatch in human evolution

Neolithic Revolution: transitional context

The Neolithic Revolution brought about significant evolutionary changes in humans; namely the transition from a hunter-gatherer lifestyle, in which humans foraged for food, to an agricultural lifestyle. This change occurred approximately 10,000–12,000 years ago. Humans began to domesticate both plants and animals, allowing for the maintenance of constant food resources. This transition quickly and dramatically changed the way that humans interact with the environment, with societies taking up practices of farming and animal husbandry. However, human bodies had evolved to be adapted to their previous foraging lifestyle. The slow pace of evolution in comparison with the very fast pace of human advancement allowed for the persistence of these adaptations in an environment where they are no longer necessary. In some human societies that now function in a vastly different way from the hunter-gatherer lifestyle, these outdated adaptations now lead to the presence of maladaptive, or mismatched, traits.

Obesity and diabetes

Human bodies are predisposed to maintain homeostasis, especially when storing energy as fat. This trait serves as the main basis for the "thrifty gene hypothesis", the idea that "feast-or-famine conditions during human evolutionary development naturally selected for people whose bodies were efficient in their use of food calories". Hunter-gatherers, who used to live under environmental stress, benefit from this trait; there was an uncertainty of when the next meal would be, and they would spend most of their time performing high levels of physical activity. Therefore, those that consumed many calories would store the extra energy as fat, which they could draw upon in times of hunger.
However, modern humans have evolved to a world of more sedentary lifestyles and convenience foods. People are sitting more throughout their days, whether it be in their cars during rush hour or in their cubicles during their full-time jobs. Less physical activity in general means fewer calories burned throughout the day. Human diets have changed considerably over the 10,000 years since the advent of agriculture, with more processed foods in their diets that lack nutritional value and lead them to consume more sodium, sugar, and fat. These high-calorie, nutrient-deficient foods cause people to consume more calories than they burn. Fast food combined with decreased physical activity means that the "thrifty gene" that once benefited human predecessors now works against them, causing their bodies to store more fat and leading to higher levels of obesity in the population.
Obesity is one consequence of mismatched genes. Known as "metabolic syndrome", this condition is also associated with other health concerns, including insulin resistance, where the body no longer responds to insulin secretion, so blood glucose levels are unable to be lowered, which can lead to type 2 diabetes.

Osteoporosis

Another human disorder that can be explained by mismatch theory is the rise in osteoporosis in modern humans. In advanced societies, many people, especially women, are remarkably susceptible to osteoporosis during aging. Fossil evidence has suggested that this was not always the case, with bones from elderly hunter-gatherer women often showing no evidence of osteoporosis. Evolutionary biologists have posited that the increase in osteoporosis in modern Western populations is likely due to our considerably sedentary lifestyles. Women in hunter-gatherer societies were physically active both from a young age and well into their late-adult lives. This constant physical activity likely lead to peak bone mass being considerably higher in hunter-gatherer humans than in modern-day humans. While the pattern of bone mass degradation during aging is purportedly the same for both hunter-gatherers and modern humans, the higher peak bone mass associated with more physical activity may have enabled hunter-gatherers to resist osteoporosis during aging.

Hygiene hypothesis

The hygiene hypothesis, a concept initially theorized by immunologists and epidemiologists, has been proved to have a strong connection with evolutionary mismatch through recent studies. The hygiene hypothesis states that the profound increase in allergies, autoimmune diseases, and some other chronic inflammatory diseases is related to the reduced exposure of the immune system to antigens. Such reduced exposure is more common in industrialized countries and especially urban areas, where the inflammatory chronic diseases are also more frequently seen. Recent analysis and studies have tied the hygiene hypothesis and evolutionary mismatch together. Some researchers suggest that the overly sterilized urban environment changes or depletes the microbiota composition and diversity. Such environmental conditions favor the development of the inflammatory chronic diseases because human bodies have been selected to adapt to a pathogen-rich environment in the history of evolution. For example, studies have shown that change in our symbiont community can lead to the disorder of immune homeostasis, which can be used to explain why antibiotic use in early childhood can result in higher asthma risk. Because the change or depletion of the microbiome is often associated with hygiene hypothesis, the hypothesis is sometimes also called "biome depletion theory".

Human behavior

Behavioral examples of evolutionary mismatch theory include the abuse of dopaminergic pathways and the reward system. An action or behavior that stimulates the release of dopamine, a neurotransmitter known for generating a sense of pleasure, will likely be repeated since the brain is programmed to continually seek such pleasure. In hunter-gatherer societies, this reward system was beneficial for survival and reproductive success. But now, when there are fewer challenges to survival and reproducing, certain activities in the present environment exploit this system, leading to addictive behaviors.

Anxiety

Anxiety is another example of a modern manifestation of evolutionary mismatch in humans. An immediate return environment is when decisions made in the present create immediate results. Prehistoric human brains evolved to assimilate to this particular environment; creating reactions such as anxiety to solve short-term problems. For example, the fear of a predator stalking a human, causes the human to run away consequently immediately ensuring the safety of the human as the distance increases from the predator. However, humans currently live in a different environment called the delayed-reaction environment. In this environment, current decisions do not create immediate results. The advancement of society has reduced the threat of external factors such as predators, lack of food, shelter, etc. therefore human problems that once circulated around current survival have changed into how the present will affect the quality of future survival. In summation, traits like anxiety have become outdated as the advancement of society has allowed humans to no longer be under constant threat and instead worry about the future.