692 THE STRUCTURE OF EVOLUTIONARY THEORY
of rye, segregate preferentially into functional gametes. Werren (1991, p. 393)
attributes the interest generated by these cases to implications for the hierarchical
model of selection: "Driving chromosomes are of general interest in population
genetics as examples of 'selfish' or 'parasitic' genetic elements. Such elements
challenge the concept of the individual genome as a 'cooperative' unit because they
gain a transmission advantage relative to the rest of an individual's genome but are
often detrimental to individual organisms."
Werren (1991; Werren, Nur, and Wu, 1988; Werren and Beukeboom, 1993)
has also discovered and developed one of the most elaborate and interesting cases
of suborganismal selection, a testimony to the complexity of interaction among
levels of selection as well. In the parasitoid wasp Nasonia vitripennis, a
supernumerary chromosome called PSR (paternal sex ratio) has evolved "an
extreme and unusual form of transmission drive" (Werren, 1991, p. 392). This
chromosome, carried in sperm, induces supercondensation of all male
chromosomes (except itself) into a chromatin mass before the fertilized egg's first
mitotic division. These chromosomes are then eliminated, while PSR survives.
Since wasps are haplodiploid, this elimination converts an egg that would have
become a diploid female into a haploid male (with PSR). This procedure obviously
gives PSR a selective advantage in transmission drive because males produced
from fertilized eggs will always transmit this unpaired chromosome.
Just as obviously, organismic selection must oppose PSR, lest the entire
population become both male and extinct. Werren (1991) had modeled conditions
of maximal opposition from organismal selection. Subdivision of populations will
be most effective in producing increased competition among PSR males, with
reduced availability of females. But the story becomes even more complicated
because suborganismal competition against PSR has also evolved by at least two
devices that bias the sex ratio in a female direction (Werren and Beukeboom,
1993): (1) a maternally transmitted bacterium, called son-killer, that prevents the
development of unfertilized (male) eggs; and (2) a cytoplasmically inherited agent
of unknown structure and origin, that induces female wasps to produce nearly 100
percent daughters (called MSR, for maternal sex ratio). The possibilities
introduced by haplodiploidy surely influence this variety and complexity in
competing selection among suborganismic units—so stories this elaborate may not
be common in nature. Still, as students of teratology in anatomy have always
argued, we test and illustrate general rules by studying such cases at the limits.
But the main weight of gene selection in nature—the category that establishes
a high relative frequency for the phenomenon—probably resides in cases that are
synergistic with, or orthogonal to, organismic selection, and therefore not opposed
by this powerful, conventional mode. Any genetic element that can propagate itself
within the genome, either by iteration in tandem or by duplication and
transposition to other chromosomes, works thereby as a vehicle of its own relative
increase—and therefore as an agent of positive Darwinian selection at the genie
level. If this propagation encounters no resistance at some other level (particularly
by the watchful organism)—