I don't believe there is anything I could identify as "the foundational evidence for the theory of evolution", what makes it compelling is not any individual factor but the overwhelming spread and depth of the available evidence and the startling simplicity yet astonishing explanatory power of its key feature: natural selection.
I wrote this six and a half years ago now, since then I've graduated from my BSc in Life Sciences, completed an MSc in Molecular Genetics, working on the evolution and molecular mechanisms of the double sperm cell production in plants, and carried out almost three years of PhD research on
Campylobacter.
And I came into this thread to post something almost identical to what I wrote back then. Evolution is not evidenced by any one thing. It's the extraordinary weight of evidence from so many independent sources.