Like English sparrows in the US, they are here about 150 years or something and spread all across North America, and are showing significant regional variations

Like English sparrows in the US, they are here about 150 years or something and spread all across North America, and are showing significant regional variations in plumage and song. No doubt they would give rise to a whole clade of sparrow species if the humans all disappeared. Rapid speciation on and island is like the finches on Galapagos; there were just one kind of bird on the island so all the ecological notches were available (bug eating, grass eating, seed eating on ground, eating stuff on trees, etc.), so those finches evolved relatively quickly into different species with different beaks and behaviors to get different food sources. Evolution itself is more or less constant process, genes reproduce in populations whose size is constrained by the conditions, with heritable variations. How much evolution pushes the phenotypes (actual bodies of the organisms) to change over time has a lot to do with the conditions and probably a lot to do with “how does the genetic and developmental system allow things to change easily” nature of the creature. So getting larger or smaller is real simple. Flying is possible but takes more time. For vetebrates, getting four eyes seems to be right out. Getting smarter seems to be hard. Etc.