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ral selection cannot “see” and suppress it. 
But if middle-repetitive DNA is self- 
centered, why does it only exist in hun- 
dreds of copies within genomes? If it can 
spread by transposition while other 
genes cannot, why does it not generate 
millions and billions of copies, eventu- 
ally crowding everything else out? What 
stops it? Why is it behaving as an “intel- 
ligent” parasite (enough copies to be 
comfortable and powerful, but not 
enough to destroy the host and itself), 
rather than as a voracious cancer? 
The potential answer to this question, 
proposed by both sets of authors, illus- 
trates another interesting point about 
the hierarchical mode of thinking that 
underlies the theory of self-centered 
DNA. In hierarchical models, levels are 
not independent, walled off by impene- 
trable boundaries from those above and 
below. Levels leak and interact. Arthur 
Koestler, whom I do not usually praise 
but whose commitment to hierarchy I 
find admirable, chose as his metaphor 
for hierarchy the double-faced god 
Janus, standing at one level but looking 
for connections in both directions. 
Now consider different forms of se- 
lection working at levels of gene, body, 
and species. A transposon enters a ge- 
netic system and begins to amplify itself 
by replication and movement. In the 
process of selection among genes, it is 
increasing by an analog of what we 
would call “differential birth” in natural 
selection among bodies. Its increase ini- 
tially produces no interaction with the 
level of natural selection upon bodies, 
and nothing suppresses its intrinsic drive 
to manufacture more copies. 
But eventually, if its increase contin- 
ues unabated, bodies must begin to no- 
tice. There is an energetic cost attached 
to the replication, generation after gen- 
eration, of hundreds or thousands of 
copies of DNA sequences that do noth- 
ing for the bodies investing that energy. 
Bodies may not notice a few copies, but 
vast numbers must eventually produce a 
disadvantage at the good old Darwinian 
level of natural selection among bodies. 
At this point, further increases in copies 
of self-centered DNA will be suppressed 
because bodies carrying too many 
copies will suffer in natural selection, 
taking all their copies with them when 
they die or fail to reproduce. The usual 
level of tens to hundreds of copies may 
well represent a balance between inexo- 
rable increase at the level of selection 
among genes and eventual suppression 
at the next level of selection among 
bodies. Levels are connected by com- 
plex ties of feedback. My plea for a 
recognition of levels other than natural 
selection acting upon bodies is not a 
negation of Darwinian theory but an 
attempt to enrich it. 
The arguments will continue for a 
long time. One group of scientists notes 
the similarity in arrangement within 
chromosomes of repetitive sequences in 
two creatures as evolutionarily distant 
as the toad Xenopus laevis and the sea 
urchin Strongylocentrotus purpuratus. 
This similarity refutes self-centered 
DNA and points to common function, 
since wandering transposons, beholden 
only to their own level, should disperse 
more randomly among chromosomes. 
Others point out that an important 
transposable element in yeast and an- 
other in the fruit fly Drosophila me- 
lanogaster are represented in different 
strains of the same species by about the 
same number of copies, but in very 
different positions among chromosomes. 
Do the different positions represent self- 
centered amplification and the similar- 
ity in numbers reflect suppression at the 
higher level of selection upon bodies? 
As with all interesting questions in 
natural history, the solution requires an 
inquiry about relative frequency, not an 
absolute yes or no. The logic of self- 
centered DNA seems sound. The ques- 
tion remains: how important is it? How 
much repetitive DNA is self-centered 
DNA? If the answer is “way less than 
one percent” because conventional se- 
lection on bodies almost always over- 
whelms selection among genes, then 
self-centered DNA is one more good 
and plausible idea scorned by nature. If 
the answer is “lots of it,” then we will 
need a fully articulated hierarchical the- 
ory of evolution. My own inclinations 
are, obviously, for hierarchy. Cartesian 
reductionism has been the source of 
science’s triumph for 300 years; but I 
suspect that we have reached its limits 
in several areas. 
We have legitimate, idiosyncratic rea- 
sons for continuing our linguistic habit 
of identifying “individuals” with bodies, 
and for granting a primacy to bodies 
among the objects of nature. I can’t, for 
example, imagine any acceptable poli- 
tics that does not focus upon the pri- 
macy of individual bodies — and we 
weep for the inhumanity of those that 
did not, but flourished for a time none- 
theless. Nature, however, acknowledges 
many kinds of individuals, both great 
and small. 
Stephen Jay Gould teaches biology, geol- 
ogy, and the history of science at Har- 
vard University. 
14 
