Extant humans are currently increasing their genetic load, and this is informing present and future human microevolution (You & Henneberg, Reference You and Henneberg2016, Reference You and Henneberg2017; Lynch, Reference Lynch2016). Genetic load may be defined as an accumulation of harmful genetic mutations in the gene pool. In other words, a population with a genetic load may have too many ‘bad’ genes in the gene pool in comparison to a ‘standard’ population with minimal deleterious genes. In theory, a standard population has the capacity to contain an optimal genotype. Since most random mutations are negative (fitness reducing) rather than positive (fitness enhancing) – the Probable Mutation Effect – there is a higher probability of genetic load accumulating in a human population where natural selection has been relaxed (Brace, Reference Brace1964).

This is precisely what has been happening since the Industrial Revolution. The Industrial Revolution (circa 19^th century) eventually led to the introduction of sanitation, public health measures, medical technologies and nutritional improvements. The last two centuries also saw many scientific and technological advancements and an understanding of the causation of infectious diseases. Consequently, natural selection, which had operated to ‘weed out’ non-fit individuals since the Paleolithic period, has reduced. This trend was enhanced in the 20th century with the advent of antibiotics in the 1940s. There is no doubt that antibiotics enabled millions of individuals to survive the onslaught of communicative diseases that had been the bane of countless generations of humans. A case in point is tuberculosis, which has interacted with human populations for at least 5000 years (Holloway et al., Reference Holloway, Henneberg, de Barros Lopes and Henneberg2011) and produced major morphological malformations (Armocida et al., Reference Armocida, Böni, Rühli and Galassi2016).

Unfortunately, the great influenza pandemic of 1918–1919, which killed ~20–40 million individuals, was a stark reminder of the ever-looming presence of the microbial world. The modern phenomena of Avian Flu, Ebola, Zika Disease and COVID-19, and the rise of antibiotic-resistant strains of tuberculosis, exemplify our inescapable ‘arms race’ with pathogens.

Reduction in infant and child mortality, coupled with increased consumption of industrially produced goods, have changed attitudes to fertility, reducing the numbers of children born to women during their lifetime: the total fertility rates (TFRs) in industrial countries fell from around 7 in the mid-19^th century to less than 2 in a third of countries in the world at the beginning of the 21^st century. Effective birth control reduced differential fertility in two ways: a small number of offspring for normally fecund people, and promoted by assisted reproduction techniques, fertility of individuals or couples who are naturally infecund.

Figure 1 shows the TFRs of a selection of countries at the beginning of the 21^th century. In many countries the premature death of infants and toddlers has declined considerably. From 4 Ma ago to the Middle Ages the odds of neonates surviving to adulthood and reproducing were ~0.20–0.30. It was only at the close of the 20^th century that the odds of neonates surviving into adulthood and reproducing reached around 0.99 (Saniotis & Henneberg, Reference Saniotis and Henneberg2011). This meant that nearly all individuals in developed nations could reach adulthood and experience an extended post-reproductive period. Generally speaking, decreases in differential fertility and mortality have meant that there is less opportunity for natural selection to eliminate deleterious mutations from the human gene pool. Although there exists some opportunity for natural selection this is probably <1% in the developed world, compared with ~ 50% in the pre-industrial age.

Figure 1. Total fertility rates at the beginning of the 21^st century. Source: UN Population Council data.

Figure 2 shows the change in the opportunity for selection in Homo sapiens over time. It has been hypothesized that the average human may carry ~250–300 mutations, most of which are deleterious, as well as several hundred less-deleterious variants (Agrawal & Whitlock, Reference Agrawal and Whitlock2012). These deleterious alleles in extant humans means that our fitness is being constrained (Agrawal & Whitlock, Reference Agrawal and Whitlock2012). While such alleles are viewed as reducing human fitness, they may also have had an adaptive function in the past, such as assisting in genetic complexity, sexual recombination and diploidy (Otto & Goldstein, Reference Otto and Goldstein1992; Keightley & Otto, Reference Keightley and Otto2006; Agrawal & Whitlock, Reference Agrawal and Whitlock2012).

Figure 2. Changes in the opportunity for selection in Homo sapiens. The index is taken from You and Henneberg (Reference You and Henneberg2016), as applied to data originally used for Figure 3 in Saniotis and Henneberg (Reference Saniotis and Henneberg2011).

Little is known about these fitness-compromising alleles. However, initial studies of all countries of the world (N≅190) by You and Henneberg (Reference You and Henneberg2016, Reference You and Henneberg2017) have suggested a deleterious effect of the accumulation of such mutations due to relaxed selection, by showing a negative correlation between a country’s opportunity for natural selection and the prevalence of type 1 diabetes and the incidence of fifteen basic types of cancer.