>Art, I ran into this several years ago and it seems to fit the bill (no pun
>intended) of the requirement of observation.
>
>"The i'iwi (Vestiaria coccinea) is a Hawaiian honeyeater which, as records
>from the last century show, used to feed largely by extracting nectar from
>lobelioid flowers. The i'iwi's long, downcurved bill is well adapted for
>extracting the nectar from the base of the deep corollas of these flowers,
>which they pollinate in the process. But lobelioids are no longer
>abundant, and in a paper in Conservation Biology Smith and colleagues
>describe evidence that the i'iwi's bill is evolving in response to the
>bird's enforced change in feeding habits.
Lobeliads are still very abundant in the habitat of the iiwi, at least in
my experience (I only spend three weeks looking per year). I don't know of
any present iiwi habitat that lacks lobeliads, but then I haven't been
everywhere.
>
Smit et al. thus predicted that the i'iwi
>should evolve a shorter bill and they compared measurements of museum
>specimens collected before 1902 with measurements of live specimens. This
>analysis showed that i'iwis have undergone statistically significant
>declines in upper mandible length by about 2-3 percent, whereas characters
>such as wing or tarsus length are the same. No such change in mandible
>length was recorded in a related honeyeater species, the aparine Himatione
>sanguinea, that had not altered its diet.
I would hate to use that as an argument in a textbook for beginning biology
students...I just don't think 2-3 percent is going to cut it with most
beginning students. Maybe the larger billed specimens were the ones kept.
After all that is the prominent feature of iiwis. It would be like trying
to ascertain the average lengths of elephant tusks by looking in the
parlors of african big game hunters. I think you could easily prove the
length of tusks had changed since the early 20th century.
> "The classic example of a microevolutionary change in bill morphology is
>that in one of Darwin's finches, Geospiza fortis, on Daphne Major Island in
>the Galapagos. There the population shows rapid declines in bill depth and
>width after severe El Nino events, as a result of a short-term fall in the
>abundance of large seeds and an increase in small ones.
This example probably would work better if we know the mechanism well.
Art
http://biology.swau.edu