The Meme Machine

THE LIMITS OF SOCIOBIOLOGY 109
Gene–gene interactions
Gene-gene interactions are the stuff of biology. When white bears manage to
stalk more seals on the arctic ice than brown bears do, genes for producing white
fur spread at the expense of genes for producing brown fur. In this way, rival
versions of genes (alleles) compete with each other. Genes also cooperate,
however – otherwise we would not have organisms at all. In our own bodies,
thousands of genes cooperate to produce muscles and nerves, liver and brain,
and to result in a machine that effectively carries all the genes around inside it.
Gene-gene cooperation means that genes for digesting meat cooperate with
genes for hunting behaviour, while genes for digesting grass cooperate with
genes for grazing and chewing the cud. Of course, they do not cooperate out of
kindness but because it benefits their own replication to do so.
But these are not the only kinds of gene-gene interaction. Genes in one
creature can affect genes in another. Mouse genes for fast running drive cat
genes for pouncing quicker. Butterfly genes for camouflage drive better
eyesight in birds. In this way, ‘arms races’ develop in which each creature tries
to outwit the other. Many of the most beautiful creations of the natural world
are the result of genetic arms races. Organisms exploit each other, as when ivy
uses a tree to get height without building its own trunk, or parasites live inside
the bodies of people and get their food for free. But others cooperate with each
other in symbiotic relationships, such as ants and aphids that provide each other
with protection and nourishment, or the many bacteria that live inside our own
intestines and without which we could not digest certain kinds of food. It is
even thought that the tiny mitochondria that provide the energy inside every
living cell originated from symbiotic bacteria. They have their own genes, and
these mitochondrial genes are passed on from mother to child in addition to all
the more familiar human genes in the cell nucleus.
Another way of looking at the world is to see whole ecosystems as
constructed by the interactions between selfish genes. Genes can have multiple
effects (a single gene for a single effect is a rarity) and may be packaged inside
different organisms. Dawkins (1982) provides many examples of what he calls
the ‘extended phenotype’, by which he means all the effects of a gene on the
world, not just on the organism in which it sits. Beavers build dams and those
dams are as much an effect of genes as are spiders’ webs or snail shells or
human bones. But the genes concerned need not even be part of the organism
that builds the structure. For example, there is a parasitic fluke that lives inside
snails and causes them to grow thicker shells. Dawkins suggests that the