Anthony Edgar wrote:Change, yes. But is this evolution? Evolution within the species, yes. But evolution that leads to speciation? That's an extrapolation that's not necessarily true.
Antibiotic resistance can be likened to this scenario: Some aliens come to earth and want to wipe out the human race. So they spray the planet with a drug designed to kill humans. But for some quirk in their genetic make-up, people with red hair aren't affected by the drug. So, the human population is wiped out except for the redheads. Over the centuries the redheads breed up and eventually fill the earth. So the aliens use the drug again, but it doesn't work this time because it doesn't affect redheads. The aliens realize that the humans "have become resistant" to their drug (which is a misleading term, since the redheads didn't "become" resistant, they were always resistant).
The demographic of the humans has changed, but no speciation has occurred. The redheads were present before the application of the drug and they are there after; they haven't changed one iota. This process is natural selection and has nothing to do with speciation. So to relate antibiotic resistance to speciation is nonsense. Simply culling a population will not lead to speciation.
The same principle applies to your Greenish Warblers. If part of the ring becomes extinct, the rest of the ring are still Greenish Warblers. You claim they're on their way to becoming a different species - pelicans, maybe ... or eagles, but that's just speculation.
Your example is actually an excellent analogy for how natural selection works! Good job! Now all you need to do is accept descent with modification and you'll have accepted Darwin's basic theory! However, let me extend the analogy further. Let's say the aliens realize that their human-killing drug isn't working any more, so they come out with an even stronger human-killing drug and return to earth. This drug kills some redheads, but not all of them. Let's say that the amount of freckles on their skin determines how strongly they react to this new drug. This means that the redheads with the most freckles are most likely to survive. So after this application of the drug, only freckled redheads remain. Repeated applications of this drug would of course cause the human population to become more and more dominated by heavily freckled redheads. Now, of course, if the aliens figure out why heavily freckled redheads are immune to their drug quickly enough, they can come back and wipe out all the remaining humans.
However, here is where descent with modification comes in. In this planet populated by freckled redheads, there will still be variations between them, and the longer the redheads are left alone by the aliens, the more diversity there will be among them. Some will be taller, some will have curlier hair, some will have hairier bodies, some will have different-colored eyes, and some might even have more interesting variations, like extra fingers and toes. These traits will be passed down to their children, and the more diverse the population is when the aliens return to spray them again, the greater the chance that some of them will be immune to the newest drug. If the aliens keep coming back and wiping out all but a few humans with a few distinctive traits, and then those humans repopulate the earth and diversify, only to have the aliens return to almost wipe them out again, eventually humans will start to look very different from their ancestors. Maybe the delivery method of the aliens would give an advantage to some humans over others. Let's say that the aliens, instead of spraying their toxin, put the drug into the water supply, but that the humans quickly discover that alcohol destroys the alien toxin. Now, alcohol is itself a toxin, and it has a number of negative effects on the human body. But, since none of these effects are instantly fatal like the drug is, humans who drank alcohol would survive, while those who didn't would die out. Now, as I'm sure you know, some humans can function better under the influence of alcohol than others, and these humans would have a distinct advantage over their lightweight fellows. Over time, alcohol resistance would build up in the human population, and of course new variations within the spectrum of alcohol tolerance would be constantly cropping up. With such strong evolutionary pressures toward being able to metabolize alcohol, it wouldn't be long before an immunity to alcohol developed within the population. Now we would have a population of humans that was VERY different from the original stock, and over time, enough differences could accumulate such that, if an alien with a time machine were to come to earth and take one of these new freckled, redhead, alcohol-immune humans back in time to before the first spraying, they couldn't successfully interbreed with any of their ancestors.
We observe something similar to the alcohol tolerance thing occurring all the time, such as when scientists were able to evolve e. coli to metabolize citrate, or when flavobacteria evolved to digest nylon. Of course, it takes longer to see such traits evolve in creatures with longer lifespans than bacteria, because the generations are longer. But the same principles are at work in both cases.
As for the Greenish Warblers, of course they're never going to evolve into anything other than Greenish Warblers. Nothing ever evolves out of its evolutionary lineage. Even if one strain of the Greenish Warbler evolved to be 13 feet high, breathe fire, and survive in outer space, it would still be a Greenish Warbler (This is a hyperbolic example which is probably physiologically impossible. It is used for illustrative purposes only.). However, at some point, assuming the lineages on the ends continue to diverge, there will be different species of Greenish Warbler, and if those populations diverge, then each species of Greenish Warbler could become two new species of Greenish Warbler, and so on.
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