Just an assumption. Extrapolating evolution from variation within species is just due to the philosophical leanings of the evos. Assuming the conclusion is not science, it's a cop out.
Evolution = change in the genes or frequency of a gene within a population.
How could variation arise within a species without it being evolution by definition?
How does this “rapid” variation within a species arise?
How is this variation somehow not evolution?
‘Because I say so!’ is not a reasonable answer, but somehow I suspect it is all you are capable of coming up with.