Genetic load and biological changes to extant humans

Abstract

Extant humans are currently increasing their genetic load, which is informing present and future human microevolution. This has been a gradual process that has been rising over the last centuries as a consequence of improved sanitation, nutritional improvements, advancements in microbiology and medical interventions, which have relaxed natural selection. Moreover, a reduction in infant and child mortality and changing societal attitudes towards fertility have led to a decrease in total fertility rates (TFRs) since the 19th century. 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. It has been argued that the average human may carry ~250–300 mutations that are mostly deleterious, as well as several hundred less-deleterious variants. These deleterious alleles in extant humans mean that our fitness is being constrained. 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. Saying this, our current knowledge on these fitness compromising alleles is still lacking.

Parental care results in a greater mutation load, for which it is also a phenotypic antidote

“…Indeed, Crow [2] made the argument originally in the context of environmental improvements in human societies and their effect on genetic variation. Consistent with his predictions, recent comparative genomic analyses have revealed a greater incidence of genetic pathologies in western industrialized populations than in traditional, pre-industrial human societies which are more exposed to natural selection [3,[36][37][38][39].…”

Anatomical variations of the flexor carpi ulnaris in the fetal period

“…However, other factors related to human evolution must be taken into account. For example, Saniotis et al (2021) point out that the directional factors of evolution are mutations and natural selection. According to the authors of this publication, it cannot be excluded that the observed variation in selected muscles is the result of a single mutation that disrupts or alters the process of fetal development.…”

Parental care results in a greater mutation load, for which it is also a phenotypic antidote

“…Better quality housing, improved diets, and benevolent social activities, such as a welfare state or the universal provision of medical care, are suggested to have been particularly influential in preventing natural selection from purging deleterious mutations in human populations [1, 2, 4, 5]. Consistent with this suggestion, recent comparative genomic analyses have revealed a greater incidence of genetic pathologies in western industrialised populations than in traditional, pre-industrial human societies which are more exposed to natural selection [4, 5, 7-9]. Nevertheless, it is impossible to demonstrate that a more benign physical and social environment, in which selection is relaxed, has caused this difference.…”

“…Benevolent social activities, such as the widely available provision of medical care, are suggested to have been particularly influential in preventing natural selection from purging deleterious mutations [2][3][4] . Consistent with this suggestion, recent comparative genomic analyses have revealed a greater incidence of genetic pathologies in western industrialised populations than in more traditional human societies [3][4][5][6][7] -though it is impossible to demonstrate that benevolent social behaviours have caused this difference.…”

“…Together, these analyses support all three of our predictions, and demonstrate that the benign social environment created by parental care can weaken selection sufficiently to enable the accumulation of genetic mutations within populations. In the short-term, such genetic mutants are overwhelmingly likely to be recessive and mildly deleterious [1][2][3][4][5][6][7] . It would be interesting to determine in future work whether this sets in motion a positive evolutionary feedback loop between the extent of cooperation and mutation load, due to a greater reliance on cooperative behaviour to compensate for any genetic deficiencies.…”