The true split off of a separate species (higher plants and animals) is something that a) isnt generally 'visible' since it's at a genetic level and not always expressed in the outward physical form of an organism, b) is often taking place in front of us and we see 'transitional' forms of populations...which then causes confusion over sub-species, etc. THere is no GOAL to evolution....it only occurs as a response to environmental changes....so we dont 'know' how those forms will continue to develop, or if they even will, and c) the obvious....it generally takes place over thousands and thousands of years.
However species are genetically adapting to their environments ALL THE TIME. The genetic differences in different populations of species are visible and internal. For humans, geographic populations have loads of identifiable genetic differences, most of which are not visible, but are traced and tracked all the time.
This required a change in genetic coding for the lizards...one which became more and more prevalent in the northern race as those were more suited to reproducing successfully in the colder area.
The overall development of that separate species would have taken hundreds, if not thousands of years...and it would not have been 'visible' to anyone around to observe.