Biology Reference
In-Depth Information
but this does not mean that the one word is responsible for the entire cake (Dawkins,
1978). Whenever we talk about 'genes for' certain traits, this is shorthand for gene
differences bringing about differences in behaviour.
Three other important points should be borne in mind when reading these examples.
Firstly, the molecular path linking genes and behaviour is complicated (transcription,
translation, influence on sensory systems, neural activity, brain metabolism and so on).
Secondly, the arrow linking genes and behaviour goes in both directions (Robinson
et al
., 2008). Not only do genes influence behaviour, through effects on brain
development and physiology, but behaviour can also influence gene expression. Thirdly,
just because it can be shown that genes influence behaviour does not imply that genes
alone produce the behaviour. Behavioural development is an outcome of a complex
interaction between genes and environment. The examples now discussed help to make
these general points clearer.
Drosophila and honeybees: foraging,
learning and singing
Larvae of the fruit fly
Drosophila melanogaster
feed in one of two distinct ways. 'Rovers'
wander around in search of food while 'sitters' tend to remain in one small area to feed.
These differences persist into the adult stage, with rover flies also searching more widely
when foraging. In the absence of food, rovers and sitters (larvae or adults) do not differ
in general activity. This difference in foraging strategies is caused by a difference in just
one gene (the
foraging
gene,
for
) which codes for an enzyme which is rather snappily
called cyclic guanosine monophosphate (cGMP) dependent protein kinase (PKG). This
enzyme is produced in the brain and influences behaviour. Flies with the 'rover' allele
(
for
R
) show higher PKG activity than those homozygous for the 'sitter' allele (
for
s
). When
the
for
R
allele is inserted into the genome of sitter larvae, they become rovers (Osborne
et al
., 1997).
Individuals with the
for
R
allele also have better short-term memory for olfactory
stimuli, while those with the
for
s
allele perform better at long-term memory tasks
involving odour cues. These differences may be coadapted with the differences in
foraging behaviour: rovers may benefit from fast learning as they move between food
patches, while sitters, with a sedentary feeding style, may benefit from long-term
memory (Mery
et al
., 2007).
In one orchard population in Toronto, 70% of larvae was rovers while 30% was
sitters. Why do the two feeding types persist? Laboratory experiments reveal that rovers
do best under patchy food and high larval densities (rovers are better at finding new
food patches) while sitters do best with more uniformly distributed food and at low
larval density (when roving is unnecessary as local food is abundant; Sokolowski
et al
.,
1997). Therefore, each morph does best under different ecological conditions. However,
a further factor is involved in maintaining the polymorphism. When food is scarce,
competition is most intense between individuals of the same morph: sitters compete
most with sitters within local food patches, while rovers compete most with other rovers
over the discovery of new food patches. This leads to the situation where the rarer type
has an advantage, which is termed negative frequency-dependent selection; in a
population of rovers a sitter does especially well, while in a population of sitters a rover
Rovers and sitters
in
Drosophila
