"...whether or not natural selection is sufficient to change one species into another..."
Species aren't necessarily as well defined as you might like. The (complete form of the) usual definition is something like, "A species is a group of critters (that look alike, and) that interbreed among themselves but which do not (normally) interbreed (in the wild) with members of another group of critters (and produce fertile offspring)." (Plus a few caveats that I've probably missed.) There are many reasons why two purportive species do not interbreed: they physically can't, they produce sterile offspring, they don't find each other attractive, or they are simply geographically separated; although in the last case, separated groups tend to quickly become morphologically different, in which case one of the other options applies.
For a brilliant example of the taxonomic follies, check out ring species[1]:
"The Lesser Black-backed Gulls [of Northwestern Europe] and [European] Herring Gulls [mostly from Great Britain and Ireland] are sufficiently different that they do not normally hybridize [, but the E.H.G. can hybridize with the American Herring Gull, (living in North America), which can also hybridize with the Vega or East Siberian Herring Gull, the western subspecies of which, Birula's Gull, can hybridize with Heuglin's gull, which in turn can hybridize with the Siberian Lesser Black-backed Gull]; thus the group of gulls forms a continuum except where the two lineages meet in Europe."
This is not to say that "species" is not a meaningful distinction, is a "judgement call", or is "open to personal opinion" or something. There are good reasons to divide critters up, but unfortunately, it is not the case that Someone sat down and said, "THESE THINGS ARE NORMAN'S HERRING GULLS, NOW AND FOREVERMORE!"
"We've observed finches turning into finches (with different beaks)..."
Finches are a family (as in kingdom-phylum-class-order-family-genus-species) containing something more than 100 species.[2] Please don't make the creationist's error of declaring, "all bats are bats and are all one thing". If a different beak shape makes Rupert's Seed-Eating Finch unattractive to the Red-Breasted Yellow-bellied Fruit-Smacking Finch, so that they don't typically hook up, then they're legitimately different species, as much as any two others.
Species aren't necessarily as well defined as you might like. The (complete form of the) usual definition is something like, "A species is a group of critters (that look alike, and) that interbreed among themselves but which do not (normally) interbreed (in the wild) with members of another group of critters (and produce fertile offspring)." (Plus a few caveats that I've probably missed.) There are many reasons why two purportive species do not interbreed: they physically can't, they produce sterile offspring, they don't find each other attractive, or they are simply geographically separated; although in the last case, separated groups tend to quickly become morphologically different, in which case one of the other options applies.
For a brilliant example of the taxonomic follies, check out ring species[1]:
"The Lesser Black-backed Gulls [of Northwestern Europe] and [European] Herring Gulls [mostly from Great Britain and Ireland] are sufficiently different that they do not normally hybridize [, but the E.H.G. can hybridize with the American Herring Gull, (living in North America), which can also hybridize with the Vega or East Siberian Herring Gull, the western subspecies of which, Birula's Gull, can hybridize with Heuglin's gull, which in turn can hybridize with the Siberian Lesser Black-backed Gull]; thus the group of gulls forms a continuum except where the two lineages meet in Europe."
This is not to say that "species" is not a meaningful distinction, is a "judgement call", or is "open to personal opinion" or something. There are good reasons to divide critters up, but unfortunately, it is not the case that Someone sat down and said, "THESE THINGS ARE NORMAN'S HERRING GULLS, NOW AND FOREVERMORE!"
"We've observed finches turning into finches (with different beaks)..."
Finches are a family (as in kingdom-phylum-class-order-family-genus-species) containing something more than 100 species.[2] Please don't make the creationist's error of declaring, "all bats are bats and are all one thing". If a different beak shape makes Rupert's Seed-Eating Finch unattractive to the Red-Breasted Yellow-bellied Fruit-Smacking Finch, so that they don't typically hook up, then they're legitimately different species, as much as any two others.
[1] http://en.wikipedia.org/wiki/Ring_species
[2] http://www.bbc.co.uk/nature/life/Finch