Species as Individuals in the Hierarchical Theory of Selection 691
For this reason, Brosius and Gould (1992) suggested that we use a more
general term—"nuon" for nucleic acid sequence (DNA or RNA)—to recognize any
stretch of nucleic acid, functional or not in organismic terms, that can evolve by
differential origin or replication:
Genomes do not consist only of genes. Sequences located between and also
within gene boundaries, accounting for a large portion of the genomes of
higher Eucarya, are not being addressed in a similar manner, partly due to
the widespread opinion that these sequences are without function... We
propose to name all identifiable structures represented by a nucleic acid
sequence (DNA or RNA) as "nuons." A nuon can be a gene, intergenic
region, exon, intron, promotor, enhancer, terminator, pseudogene, short or
long interspersed element ... or any other retroelement, transposon, or
telomer—in short, any unit from a few nucleotides to thousands of base
pairs in length.Proceeding upwards, aggregates of genes can also function as units of se-
lection—including, as prominent agents in evolution, chromosomes (Nei, 1987),
and organelles and bacterial plasmids within cells (Eberhard, 1980, 1990).
Organismic selection generally works with great effectiveness in suppressing
"revolts" to organismic integrity by differential proliferation of elements from
within (see Buss, 1987; Leigh, 1991). Most of the characteristic properties of
genomic organization and embryological development—from Hamilton's "gavotte
of the chromosomes" in meiosis, to such phenomena as germ line sequestration
and maternal determination in embryogenesis—may have evolved largely to
suppress suborganismic selection, thereby assuring the integrity of multicellular
organisms. Meiosis itself presumably evolved to place one copy of each gene "in
the same [gametic] boat," thus converting organisms, rather than genes, into a
primary unit of selection by the Musketeer's criterion of "all for one." But, once
achieved, meiosis must be actively guarded by organismal selection against
destabilizing drivers and distorters— all to preserve what Leigh (1991, p. 258)
calls "the genome's common interest in honest meiosis."
Nonetheless, the evolutionary literature abounds with cases, both "classic"
and new, of meiotic drivers, chromosomal segregation distorters, and other
phenomena that favor the plurifaction of individual genes or sequences (including
entire chromosomes) within the genome or population of genomes— usually with
negative consequences for organismal selection above. Perhaps such cases must be
relatively rare in nature, and only prominent in our literature for their intriguing
oddity and exceptional status in the light of organismal selection's usual power to
suppress such "outlaws."
Driving genes and chromosomes use a variety of devices to increase their
relative representation by suborganismal selection. Some, including the classic t-
allele of house mice (Lewontin, 1970), cause dysfunction in sperm carrying the
nondriving homologue; others, like the supernumerary chromosomes