Even if natural selection does not fully operate in modern human societies, a reassuring study suggests that the loss of evolutionary fitness should not be a concern for survival of the species.
The research indicates that improvements in healthcare and living conditions would not sufficiently result in the accumulation of potentially damaging spontaneous mutations that impact the viability of human populations for the foreseeable future.
The findings, examining the loss of evolutionary fitness from the accumulation of mutations in mammals, appear in the journal PLOS Biology.
Researchers did notice average reductions in fitness traits among mice that were bred specifically to accumulate mutations compared with their control counterparts.
But they add: “The magnitude of the reduction in fitness suggests that it is doubtful whether a reduction in mean fitness due to mutation accumulation will be observable in human populations in the near future.
“Undoubtedly, of far greater concern is the unsustainable overexploitation of finite global resources against a background of an expanding human population.”
Spontaneous mutations in each generation introduce heritable changes that tend to reduce the fitness of highly adapted living creatures. But this is counteracted by natural selection, which keeps these mutations rare and removes most of them from populations.
Nonetheless, concerns have been raised over the potential for new mutations to reduce human fitness over a measurable timescale and affect the viability of populations.
Humans have long generation times, and their mutation rate is higher than many other species, with each baby carrying an average of 70 new mutations in their genome.
Furthermore, improvements in living conditions and healthcare have reduced the strength of natural selection in some human populations, which could lead to a higher frequency of potentially damaging mutations in a human population.
If natural selection were to be completely removed, a loss of evolutionary fitness would be realized were natural selection to be restored in future.
To investigate further, Jobran Chebib, PhD, a lecturer in genetics education at the University of Edinburgh, and co-workers conducted a mutation accumulation experiment in mice, as a model of mammalian species.
Specifically, they examined the impact of mutation accumulation on traits among inbred mice, which studied over 21 generations and compared with a cryopreserved control to account for environmental influences.
The team showed that variation for morphological and life history traits accumulated at a sufficiently high rate to maintain genetic variation and selection response.
Weight and tail length measures decreased significantly by between 0.04% and 0.3% per generation, with narrow confidence intervals.
The fitness proxies of litter size and surviving offspring decreased on average by about 0.2% per generation, but these confidence intervals overlapped zero.
“We therefore predict a reduction in mean fitness from mutation accumulation of about 0.38% per generation in humans,” the authors calculate.
“Over a period of 200 years, or about eight generations, the predicted reduction in fitness in humans would therefore be approximately 3%.
“However, for a number of reasons the impact of mutations in humans is likely to be substantially smaller than this value implies.”
Some of these reasons could include the deliberate inbreeding of mice in the experiment, compared with outbreeding in humans that results in partially recessive deleterious mutations existing in a heterozygous state.
Overall, the team concludes: “When extrapolated to humans, our results imply that the rate of fitness loss should not be of concern in the foreseeable future.”
