No, I'd imagine the finches evolved long before Darwin ever came upon them, as they had already well-adapted to their respective new environments.
Evolution tends to take place on timescales far longer than the average human's lifetime, although there have been notable exceptions.
Consider the lizards of Pod Mrcaru:
Or if you want a more practical example, look no further than the bacterium Escherichia coli (E. coli) of which bacteriologists use as a "model organism" in the lab, precisely because of its very rapid regeneration time.
There have been experiments on E. coli, all taken from one ancestral generation, separated into different flasks and fed the same exact "broth" (consisting of glucose and other nutrients), repeated day after day over tens of thousands of generations, where in one flask the bacteria evolved to be able to digest not just the usual glucose in the broth, but citrate as well. In that flask, and that flask only, their population levels exploded -- in that generation, and every generation following.
Again: all taken from the same ancestral strain, separated and allowed to just "do their thing", evolution happening before our very eyes.
The biggest tool biologists now have in their arsenal is the molecular clock -- analyzing different species' genomes and comparing them over time.Or do you mean based upon the interpretation of fossil evidence? Or by simply comparing the habitats and relative distance of living birds?
That's how they've been able to piece together much of the evolutionary pathways, especially in areas where the fossil evidence is lacking.
That's also how they've been able to confirm that humans evolved from the same ancestral primate as our closest cousins the chimpanzees, which share 98% of our DNA.
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