Muller's genetic load/species extinction hypothesis

Abstract

The genetic load hypothesis of Hermann Muller raised the profound question of possible species extinction, even for humans, following a prolonged accumulation of recessive genes due to ionizing radiation exposure within the population. Two major mouse radiation research teams in the United States provided the most extensive tests of Muller's hypothesis. One group continued its study for more than two decades, over 82 consecutive generations, approximating 2500 human years. Even though Muller had stressed for decades his fear of species-threatening effects, no significant effects were observed for related factors such as reproductive fitness and longevity. Yet, the paper presenting the data of the 82-generation negative study has only been cited five times in 45 years. Altogether numerous laboratories worldwide collected vast amounts of data on mice, rats, and swine in an unsuccessful attempt to see if there was convincing evidence to support the genetic load theory and claims that species might deteriorate or be rendered extinct. This paper re-examines Muller's genetic load hypothesis with a new evaluation of how that hypothesis was tested and the significance of the findings, with most of those studies being completed before the BEIR I Committee Report in 1972. That committee briefly discussed the available evidence, mostly ignoring those results as they proceeded to make hereditary risk estimates both for (1) the first generation after a radiation exposure and (2) for the time, in the distant future, when a hypothetical genetic equilibrium would be reached. Their estimates assumed accumulation of harmful mutations and a linear no-threshold dose response extending all of the way down to a single ionization. More recent data on induction by ionizing radiation of dominant mutations that affect the skeletons of mice provide further robust supporting evidence that the generationally cumulative and LNT-based assumptions underpinning Muller's genetic load hypothesis are not correct.

Keywords: Genetic load; Hermann Muller; Ionizing radiation; Linear no-threshold dose response; Recessive mutations; Species extinction.