A new understanding as to why mutation rates vary among species

MUTATIONS (i.e. alterations in DNA nucleotides in the haploid genome, base-pairs in the diploid genome) occur when cells copy their DNA incorrectly, or fail to repair damage from chemicals (endogenous or exogenous) or radiation (environmental effects). Some mistakes are beneficial, providing variation that enables organisms to adapt; this is one of the fundamental reasons why EVOLUTION has been occurring for ~3.8 billion years on Earth. However, some of these genetic mistakes can cause the mutation rate to rise –– thus fostering more mutations.

For many years, evolutionary biologists had assumed that mutation rates were identical among all species, and that these rates were SO consistent and predictable they could be used as “molecular clocks” to estimate divergence of one species from another –– as a function of time. Therefore, by counting the number of differences between the genomes of two species, evolutionary geneticists could date when they diverged. And it is well established that Africans, for example, as an older population on Earth, exhibit more mutations in their DNA than Caucasians. However, today, now that geneticists can compare whole genomes of parents and their offspring (by means of whole-genome sequencing [WGS] methodology), we can count the actual number of new mutations per generation.

WGS methodology has thus enabled researchers to measure mutation rates in about 40 species –– including newly reported numbers for orangutans, gorillas, and African green monkeys. All primates so far studied have mutation rates similar to that of humans. As recently learned from an evolution meeting, several labs have reported that bacteria, paramecia (single-celled freshwater protist animals having a slipper-like shape and covered with cilia), yeasts, and nematodes (round worms), all of which have much larger populations than humans, have mutation rates orders of magnitude lower than that in primates [see the 1-page editorial attached].

In the early years of Homo sapiens (modern human), humans originated in very small numbers (of dozens or hundreds). In large populations of any species (e.g. the bacterium, paramecium, yeast, nematode), natural selection can efficiently eradicate the harmful genes. In contrast, among smaller groups –– such as the earliest of humans –– undesirable genes can arise (including those that foster mutations). Support of this hypothesis now comes from data on a range of organisms that show an inverse relationship between mutation rate and ancient population size. This new knowledge offers insights into how cancers and other deleterious disases might develop. These fascinating new findings also have important implications for efforts of evolutionary biologists to use DNA to date branches on the Tree of Life.

Science 13 Apr 2o18; 360: 143 [single page]

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