So, the number of finches diminished during a drought, and the birds which survived had, on average, larger beaks. This is hardly surprising, as I assume the birds with smaller beaks starved. This is far from suggesting that a new sub-species had evolved. These birds would have carried with them genes allowing smaller beaks, so that this anomaly would not necessarily have continued when conditions improved - hence the other change in beak size in 1983, when the 'blip' in 1977 had ironed itself out and finches with larger beaks found it more difficult to survive in wet conditions.
A main tenet of Darwinist theory is genetic mutations brought on by blind chance. If Mr Weiner is suggesting that blind chance brought about the right kind of beak in 1977 and then in 1983, a trip to the roulette table would be a surprising experience for him. Besides, a true genetic mutation would occur in one individual only, and take years to affect the population at large - if at all. Beak shape and size is presumably somewhat variable in finches, as lip and mouth size is in the human species.
Admittedly, I do not have an answer to how we have such a wide variety of animals on this planet. But nor, I suspect, did Darwin.
11 JulyReuse content