Darwin-L Message Log 1:125 (September 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

<1:125>From HOLSINGE@UCONNVM.BITNET  Wed Sep 15 06:50:24 1993

Date: Wed, 15 Sep 1993 07:37:24 -0500 (EST)
Subject: Re: Lamarkianism in linguistic change
To: darwin-l@ukanaix.cc.ukans.edu

Re: How does extranuclear transmission of DNA affect thinking about the
   transmission of acquired characteristics?

Not at all, I should say.  The fundamental distinction between what biologists
sometimes refer to as "soft" inheritance (inheritance of acquired characters)
and "hard" inheritance (transmission of hereditary material) is easiest to
explain in terms of the distinction Weismann made between germline and soma
(even though the distinction doesn't apply to plants).

Weismann noted that in animal development the cells that form the germ line
are differentiated from those that form the body (soma) early in development.
Using this observation he argued that transformations that affect only the
soma, the girth of a blacksmith's arm for example, will have no effect on the
germ line.  Therefore, these acquired characters will not be transmitted to
offspring.  Only mutations that alter the characteristics of the germ line
will be passed to offspring (regardless of whether they affect the soma of
the animal carrying the germline mutations).

Thus, the fundamental distinction is between inheritance of *environmentally
induced* somatic changes and inheritance of germ line changes.  The
transmission dynamics of extranuclear DNA, e.g., mitochondria and chloroplasts,
is quite different from that of nuclear DNA, but any changes in mitochondria
and chloroplasts are inherited only if they occur in the germ line.  In short,
transmission of extranuclear DNA is *non-Mendelian*, but it is not an example
of "soft" inheritance.

-- Kent E. Holsinger

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