Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 165
Part III
REAL SELFISH (AND COOPERATIVE) GENES
I
t is remarkable that a field founded on the concept of selfish genes
(Dawkins, 1976b) got so far for so long without paying much atten-
tion to specifiable genes. That is probably because we learned how
phenotypic strategies of cooperation and conflict could be understood as
the results of genes maximizing inclusive fitness. However, studies at the
genic level are now becoming common and should shed light both on the
mechanisms and the manner in which social selection operates.
In Chapter 8, Brielle Fischman and colleagues review and extend what
is known about the molecular genetic mechanisms of eusociality. Some
of the information comes from studies of particular genes and pathways
but much is now coming from evolutionary analyses of genome-scale
data. To the seven sequenced genomes of social insects, the authors add
their own transcriptome-based protein-coding sequences for 10 social
and nonsocial bee species, representing three origins of sociality. Some
of the patterns are idiosyncratic. For example, early results from the
honeybee genome pointed to the importance of odorant receptors and
immunity genes, but these do not hold up in the broader analyses. New
findings include increased rates of evolution of brain-related genes in the
primitively eusocial bees, conceivably because of the increased cognitive
demands of their competitive social environment. Juvenile hormone and
insulin are often important in caste. This is not surprising if caste is nutri -
tionally based, although the effects of juvenile hormone are quite different
than in nonsocial insects. There is also a rapid evolutionary change in
proteins involved in fundamental carbohydrate metabolism. Again, this
165
OCR for page 166
166 / Part III
fits with a nutritional basis for caste, but it seems surprising that changes
are common in such basic pathways. These issues should be clarified with
additional genome sequences and functional studies of individual species.
In Chapter 9, Joan Strassmann and David Queller explore a micro -
bial social system where it is possible to manipulate genes. In the social
amoeba Dictyostelium discoideum, starved cells come together in large
groups in which 20% of the cells sacrifice themselves to make a stalk
that aids in dispersal of the others as spores (Kessin, 2001). Besides this
impressive altruism, this species has been shown to have cheating, kin
recognition, and even primitive farming of their bacterial food. Numerous
genes of many functional types can be mutated to cheaters. Some cheaters
could destroy cooperation, yet cooperation is maintained for a variety of
reasons, one being the rather high genetic relatedness in the field, part of
which is due to kin recognition mediated by highly polymorphic adhesion
genes. Other controls on cheating that have been demonstrated include the
evolution of resistor genes, power asymmetries, and lottery-like mecha -
nisms. Studies of the dimA and csaA genes have shown that cheating can
also be controlled by idiosyncratic pleiotropies of particular genes. The
cheating allele would be favored by selection but other deleterious effects
of the same allele keep it from spreading, suggesting that cheat-proof
cooperation often may be built using elements that are essential for other
reasons. Consistent with ongoing social conflicts and arms races, social
genes evolve rapidly.
Dawkins (1976b) argued that all genes are selfish, but the ones that
show the trait most distinctively are selfish genetic elements. These are the
renegades of the genome, chunks of DNA that replicate in part at least via
different pathways than most genes and thus can be selected to conflict
with other loci. Transposons, for example, increase their representation by
jumping from one place to another, often at some cost to the organism.
Other examples include meiotic drive elements, various modification-
rescue systems, imprinted genes, B chromosomes, and organellar genes.
In Chapter 10, John Werren tackles the issues of the function and adapta -
tion of these elements. He surveys the evidence, sometimes strong and
sometimes suggestive, that such elements have had important functional
consequences for their genomes. For example, parts of transposons some -
times evolve into regulatory regions, and defenses against selfish elements
may have led to the eukaryotic intron-splicing apparatus. But contrary to
some recent suggestions, Werren argues that there is as yet little evidence
that these are the adaptive reasons for the maintenance of these elements.
Instead, selfish genetic elements are maintained by their selfish behavior,
but the new chunks of DNA that they sprinkle throughout genomes some-
times get co-opted, domesticated, or otherwise modified to cause some
beneficial effect to the organism.
