Soft inheritance

Soft inheritance is the term for a largely discredited set of theories. It was coined by Ernst Mayr to include such ideas as Lamarckism, that an organism can pass on characteristics that it acquired during its lifetime to its offspring. It contrasts with modern ideas of inheritance, which Mayr called hard inheritance. Since Mendel, modern genetics has held that the hereditary material is impervious to environmental influences (except, of course, mutagenic effects). In soft inheritance "the genetic basis of characters could be modified either by direct induction by the environment, or by use and disuse, or by an intrinsic failure of constancy, and that this modified genotype was then transmitted to the next generation." Concepts of soft inheritance are usually associated with the ideas of Lamarck and Geoffroy.

One of the first statements in favour of hard inheritance was made by the English surgeon William Lawrence in 1819. His ideas on heredity were many years ahead of their time, as this extract shows: "The offspring inherit only [their parents'] connate peculiarities and not any of the acquired qualities". This is as clear a rejection of soft inheritance as one can find. However, Lawrence qualified it by including the origin of birth defects owing to influences on the mother (an old folk superstition). So Mayr places Wilhelm His, Sr. in 1874 as the first unqualified rejection of soft inheritance. August Weismann, in 1883, gave a comprehensive denial of Lamarckian soft inheritance and with his distinction between germ and soma provided a general ideology of hard inheritance which survives to the present day.

Recent work in plants and mammals on the role of the environment on epigenetic modifications of DNA have led to the argument that inherited epigenetic variation is a kind of soft inheritance. This, however, is not one of the ideas explored in any detail by Mayr. Epigenetics does not claim that newly acquired traits can be directly transmitted to offspring without the mechanism of genetics (which was Lamarck's claim), but instead highlights the notion that the environment can help set up the functioning of existing genes. The notion that such change in the functioning of existing genes can lead to actual changes in the structure of genes remains highly controversial.

"Contrary to the established view, soft inheritance is common. Variations acquired during an individual’s lifetime can be passed on through epigenetic, behavioral and symbolic inheritance. They can affect the rate and direction of evolution by introducing additional foci for selection, by revealing cryptic genetic variation, and by enhancing the generation of local genetic variations. Moreover, under conditions of stress, epigenetic control mechanisms affect genomic re-patterning, which can lead to saltational changes."