I suggest that you take a look in one of the many books which discusses
this concept, for example the books by Richard Dawkins, "The Selfish Gene"
and "The Blind Watchmaker".
With regard to the genes controlling immune responses there are very many
of them and they are clustered on different chromosomes. The immune
has to deal with many varied infections such that what might be appropriate
in one case may not be in another. The overall health of an individual will
be reflected in physical features such as feather condition and nutritional
state. Thus by selecting for these features there is a good chance that the
individual has a capable immune response. However this may not hold true
over several generations if a different type of infection is encountered
which selects for a different combination of immune response genes.
Mike Clark, mrc7 at cam.ac.ukhttp://www.path.cam.ac.uk/MikeClark/
--
o/ \\ // || ,_ o Mike Clark, C.U. Dept. Pathology
<\__,\\ // __o || / /\, "to pay for my hobbies I teach
"> || _`\<,_ // \\ \> | immunology and engineer antibodies :-)"
` || (_)/ (_) // \\ \_
