A question about evolution

This is a question from my ecology course about evolution:"To an original population of 900 birds who are all homozygous aa, migrated 100 birds who are all homozygous bb. After a rough storm only 90 aa birds survived and only 10 bb birds survived. Relative to the original population, did evolution occur?"Answer: Yes. "Before migration the frequency of allele a was 100%, after migration it was 90%, therefore evolution happened. The storm was not an evolutionary process and therefore did not change the allele frequency."I don't understand how the mere movement of different alleles is an evolutionary process.

General reply:
1) It is reasonably to assume that they are the same species (they can interbreed).2) It is reasonable to assume that a and b are different alleles of the same gene (If you were wondering).*As this is a hypothetical question there is no reason to argue over these points (they are arbitrary).*As stated in the OP this is a question in ecology, and when studying ecology you cannot take into account the whole population of the species - you must restrict yourself to researching a certain population (meaning "population" is also arbitrary).
Population (as my professor defines it): All of the creatures that can potentially interbreed and give fertile offspring, but...
1) Not all of the individuals breed in practice ( this also describes me).
2) They must be together for awhile (geographic isolation --> different populations).
BTW: How would you define a population that "breeds" asexually.Reply to RAZD: The definition of population resolves this - so by this definition evolution did occur. Forget the storm - It was just an obvious diversion for those students who didn't do their homework. I always do my homework . I have a lecture Sunday, I'll ask him.(BTW: thank you for your responses Crash, RAZD, NJ, Jar, CS, Phalanx, Modulous)

I had a class on Tuesday were I though I would ask the tutor about thisquestion, but... when I got to class I realized that I couldn't think ofany specific question to ask, on Sunday I will ask the prof. (Dr.), soif you could give me ideas as to how to approach this... I'll post onSunday (promise).

I still don't have a question to ask him.
I have a class on Sunday again, maybe you can suggest a specific question?

This tread is too long. I need a specific question to ask.

I'll definitely ask that one, thanx. It's a bit of a stretch, but I'll try. This is a hypothetical question in ecology, this kind of assumption is a bit irrelevant.BTW: I have been watching the videos Modulous posted ("Beyond Belief"), highly recommended!!!