All animals, including humans, experience the inevitable process of aging. As time progresses, bodily functions decline, making reproduction more challenging and mortality more probable.
A cornerstone of evolutionary theory suggests that natural selection doesn't prioritize mutations extending lifespan or years of fertility. This concept, proposed by scientist Peter Medawar in 1952, coined the mutation accumulation theory.
According to Medawar, organisms capable of reproducing take precedence in natural selection. After reproduction, the evolutionary pressure to prevent bodily malfunctions diminishes, accumulating detrimental mutations.
In 1957, George Williams expanded on Medawar's theory, suggesting that mutations linked to aging could still hold positive selection effects if they prompt early and abundant reproduction in an individual's youth.
This theory provides a lens through which to understand complex genetic relationships and their impact on life expectancy, fertility, and social dynamics.
Human Instances and Genetic Studies
Several human instances support these evolutionary compensations. Genetic variants associated with conditions like coronary heart disease often correlate with higher offspring numbers.
Studies, such as the Framingham heart study, highlighted a negative correlation between the number of children and life expectancy in women.
Yet, establishing clear genetic links amidst the complexity of factors influencing life expectancy and fertility remains challenging.
Erping Long and Jianzhi Zhang from the University of Michigan investigated this area and presented their findings in a recent article published in Science Advances.
They analyzed data from the U.K. Biobank, encompassing information on over 276,000 individuals. Their study revealed a link between high polygenic scores favoring reproduction and a reduced likelihood of living beyond age 76.
Moreover, an upsurge in genetic variants linked to increased offspring and shorter life expectancy emerged, echoing Williams's predictions of positive selection.
Unveiling the Genetic and Reproductive Balance
Long and Zhang's research introduces another theory regarding aging. Constrained by limited resources, organisms may prioritize investing more in reproduction than DNA repair, leading to mutation accumulation and subsequent aging.
A recent study in Nature strengthens this hypothesis, revealing a negative correlation between somatic mutation rates in various mammal species and their life expectancy. This relationship suggests that species with more offspring might have shorter lifespans due to resource allocation.
However, the study emphasizes that while genetic factors promoting fertility may contribute to decreased life expectancy, social influences exert a more significant impact, especially in recent times.
Environmental and biological factors often intertwine, creating effects that deviate from straightforward genetic predictions. For instance, mutations impairing learning abilities might simultaneously reduce reproductive capacity and life expectancy.
Cultural and Global Impacts
Diverse cultures exhibit distinct outcomes regarding the correlation between fertility, life expectancy, and genetic influences.
In China, women bearing more children tend to have shorter lives, while the reverse is observed among the Amish community.
Long and Zhang's study presents data that underscores this complexity. It reveals that women with two children outlived those with three. Additionally, they surpassed childless individuals in terms of lifespan, adding excitement to the phenomenon.
Societal Trends and Evolutionary Balance
Across the globe, life expectancy has dramatically increased over recent decades, averaging over 72 years—an impressive leap from the mere 46-year average in 1950.
Simultaneously, birth rates have also declined due to multifaceted social and technological advancements.
These juxtaposing trends create a balancing act, countering natural selection's push on genetic variants, as evidenced by the revelations in the Science Advances study.