Re: During evolution why do animals lose bodyparts they dont need ?

Let's take the specific case of the Dodo. Either one of two things
happened:

1) The gene for wings mutated in such a way that the wings became too small
for flight.

2) The gene for size (or combination of such things) mutated in such a way
that the bird was then too heavy for flight.

In wither case, the same effects resulted:

1) The Dodo could no longer fly (duh!)

At the time, this did not have any effect on the survivability of the bird
as it had no preditors. If this had had an effect, it would have quickly
been weeded out and the mutation would have been lost from the gene pool.

Not being able to fly is also a huge saving in energy. Which is a good
thing.

2) The Dodo needed to grow less wings.

Either due to the fact they they were smaller, or the fact they did not
need to grow proportionately to the new body size. This is an energy saving
as growing can be quite expensive. This could have advantagously 
influenced the size of the Dodo egg.

3) The Dodo has less maintenence.

With smaller (or proportionately smaller) wings, the Dodo would have had
less day to day maintenance. Another energy saving. Better for survival as
less food is required.

Admitedly, it may have been the case that flight would have given a broader
hunting range. However, this could have had all sorts of social
implications. If the Dodo's loss of flight change it's ecosystem in such a
way that it's food source was more abundant, then that, again, would have
been a good thing.

So, the Dodo losing flight was not such a bad thing, in fact it had many
positive points.

The golden rules of evolution is that things only get selected if they give
an advantage. That advantage could be a new thing, or the loss of a
redundant thing. 

Both are equally valid and just as strong. In times of crisis, ie. food
shortage, the Dodo that used less energy, would have come out on top.