What is variation in Speciation
You are asking explanations about a creationist argument. One could provide explanations about some hypothesis that you may found in a science paper but not explanations for a creationist argument just because, by definition, a creationist argument is not supported by any evidence and logical thoughts. However, you may want to link to a creationist website making a specific claim of interest and we may eventually talk about whether the claim is wrong/misleading. It would be very handy to have the claim in quotes rather than an explanation of what the claim is.

You say


  [..] after the evolution and speciation [..]


This piece of sentence sounds weird. You could say after the evolution of something (meaning the appearance of this something through evolutionary processes that are mutations, natural selection, drift and eventually others) but just "after the evolution" sounds as if evolution may not exist during some periods of time.

Talking about speciation in fruit flies, you will likely be interested in reading this post.

UPDATE

In general, when talking about fruit flies we talk about the Tephriditae. Tephriditae is a family of flies which contain about 5000 described species (including the model organism Drosophila melanogaster).

So, there is definitely different species of fruit flies. Of course, within any population (to the exception of populations of exact clones by definition), you have variation. This variation is a key concept in evolutionary biology. If this is unclear to you you may want to learn a bit more about evolutionary biology, including the concept of heritability ($h_N=\frac{V_A}{V_P}$).

Speciation took place in Tephriditae (otherwise we wouldn't have the species we have today) and is still taking place. The post I linked above gives an example of speciation between lineages of Drosophila melanogaster that has been directly observed in the labs.

Your issue seems to likely be a semantic one. As there is variation within a lineage and this variation is associated with groups that are in reproductive isolation, then the two lineages you consider do not belong to the same species anymore. Therefore, there was a speciation event.