OK. The first thing to note is that behaviour is the result of neurochemistry, hormones, electrical activity etc.
By biasing the action of these processes, genes can alter behaviour. As all these biological actions are based in genetics (ABE: not to ignore the
effects of environment and experience though, but even these rely on genetically derived systems).
A good one I had to explain a while back, was how rabbits have an instinct to fear poop from carnivores. There is a study that shows how rabbits
raised away from the natural environment show avoidant behaviour of carnivore poop (e.g., fox), but not of herbivore poop (e.g., sheep).
Obviously it couldn't be a learned behaviour, as the rabbits had never seen a fox or sheep. So, what is happening here is that the olfactory system
is sensitized to certain chemicals in carnivore poop. When these are picked up, the FFFS (fight, freeze, and flight system) is activated, and
avoidant/vigilant behaviour results.
Likewise, a human baby is hard-wired to certain emotional behaviours (i.e. disgust responses to certain tastes), and so the rabbit is hard-wired to
avoid/fear the smell of carnivore poop.
As long as these behaviours stay adaptive, the genes underlying the behaviour will be maintained within the population. And I'm sure for rabbits in
natural environments, it will.
More complex behaviours, such as nest building, will be developed and maintained through the same process. These behaviours are essentially mediated
by hormones and chemicals, which are set in place through genetics.
Thus, one study shows how one single gene mediated sexual behaviour in a fly:
Courting behaviour 'in the genes'
A single gene changed the flies' sexual behaviour
Men who are no good at wooing the ladies may be able to blame their genes, after researchers have made a discovery in the humble fruit fly.
Manipulation of a single gene in male fruit flies made them less adept at courting female mates, a US and an Austrian team have both found.
However, it would be remiss to think behaviours are the result of single genes. There are likely multiple interacting genes, regulators of genes, and
developing hormones underlying many behaviours.
For example, take away the gonads of a male rat at birth, and it will show female sexual behaviours when adult (i.e. Lordosis) if the appropriate
hormones are available. Conversely, female rats who have been masculinised will not show lordosis even if hormones are made available, and feminised
male rats won't show normal male behaviour if testosterone is made available. Thus, hormones control sexual behaviour, but are also involved in the
development of the neural basis of sexual behaviour. Much of this is mediated by activity in the hypothalamus (it even shows sexually dimorphic
structure). Similarly, the neocortex is altered by sexual hormones.
Therefore, hormones are also important as motivators of many behaviours (homeostasis). And the systems underlying hormonal activity are basically set
in place by genetics.
[edit on 6-8-2007 by melatonin]