The concept that traits acquired during an individual's
lifetime can be passed on to offspring and therefore affect evolution—a theory
most associated with the French scientist Jean-Baptiste Lamarck (1744-1829)—was
relegated to the dust bin by the advent of Darwinism. After Darwin, a new orthodoxy prevailed for more than a
century. In present-day
terminology, it held that only traits already embedded in genes are passed on,
and evolution occurs exclusively by natural selection of mutations in the
genome. Such mutations are purely
random events, caused by errors in DNA transcription or by environmental
factors. Any suggestion of
"directed" mutations—beneficial, nonrandom changes in the genome
provoked by an organism in response to specific environmental challenges—was
decried. Further, the notion that
changes in and inheritance of traits could occur through other than genetic
means was anathema.
That orthodox Darwinian viewpoint has been challenged in the
past 30 years or so. Modern
studies have shown that directed, nonrandom genomic mutations can occur; that
acquired non-genomic attributes can be inherited; and that both are subject to
natural selection in the same way as are random genomic mutations. These new insights and much else are discussed in a remarkable, but lengthy and hard-to-read book by Eva Jablonka
and Marion J. Lamb, Evolution
in Four Dimensions: Genetic, Epigenetic, Behavioral and Symbolic Variation in
the History of Life.
I will skip over Jablonka-Lamb's first dimension, genetics,
for it is the Darwinian dimension, with which I assume you are
familiar. The second dimension,
epigenetics, is the most astounding to me. It turns out that molecular "markers" can attach
themselves to the genome as a result of environmental impact or stress. They do not change the genome, but can
turn genes on and off, thus changing the expression of an individual's genetic
traits. What is surprising is that
these epigenetic factors can be passed on to offspring.
The earliest-discovered and most common epigenetic marker is
methylation of DNA. Small methyl
(CH3) groups can become attached to bases in the DNA sequence
because of environmental influences and stress, but they do not change the DNA
other than by turning genes on or off.
Remarkably, the methylation remains in daughter cells after cell
division. Even more remarkably,
when methylation affects sperm or egg cells, it is inherited by an embryo and
thus passed on to the next generation.
To quote from Jablonka-Lamb:
"[B]ecause it
provides an additional source of variation, evolution can occur through the
epigenetic dimension of heredity even if nothing is happening in the genetic
dimension. But it means more than
this. Epigenetic variations are
generated at a higher rate than genetic ones, especially in changed environmental
conditions, and several epigenetic variations may occur at the same time. Furthermore, they may not be blind to
function, because changes in epigenetic marks probably occur preferentially on
genes that are induced to be active by new conditions. This does not mean that all induced
changes are adaptive, but it does increase the chances that a variation will be
beneficial. The combination of
these two properties—a high rate of generation and a good chance of being
appropriate—means that adaptation through the selection of epigenetic variants
may be quite rapid compared with adaptation through genetic change."
This one astonishing paragraph
was to me worth the price of the book.
Less compelling to me, but still impressive, are Jablonka
and Lamb's asserted third and fourth dimensions of evolution—behavioral and
symbolic. If I understand these
dimensions correctly, they overlap with and extend what the British
evolutionary biologist Richard Dawkins called "memes"—an analog of
genes. Just as a gene carries
information about biological traits, a meme carries information about socio-cultural ideas, symbols, and practices that are
passed from generation to generation but not necessarily to direct offspring. That many memes are acquired under the
influence of the environment and unrelated individuals, and propagated by
social interaction rather than genetics, doesn't sound extraordinary—we know
that species, especially Homo sapiens,
pass on a vast repertoire of behaviors derived from their environment and
social groupings. More startling,
however, is evidence Jablonka and Lamb cite indicating that acquired memes may also
be inherited through bio-chemical means; for example, a mother's particular
food preferences have been shown to be reproduced in her offspring through
placental affects on the fetus.
The important point Jablonka and Lamb make is that memes as well as
genes are subject to evolutionary selection: those that are advantageous for
survival will persist, those that aren't will vanish.
The focus of the Jablonka-Lamb book is therefore on
information transmission as the basis of evolution, not only the biological
information of the genome and its epigenetic markers, but also the behavioral
and symbolic information of cultures.
"All four ways
of transmitting information introduce, to different degrees and in different
ways, instructive mechanisms into evolution. All shape evolutionary change. … As molecular biology uncovers more and more about epigenetic
and genetic inheritance, and as behavioral studies show how much information is
passed on to others by nongenetic means, evolutionary biologists will have to
abandon their present concept of heredity, which was fashioned in the early
days of genetics."
For those of us who have been
raised in the pure-genetics tradition, this is a big gob to swallow. We were taught to ignore Lamarck and to
disdain the USSR's ideologically motivated Lysenko; now, it seems, some of their ideas may turn out to have
merit.