Wow that is a whole lot of grey area that you just stated.
No, it is just correcting the strawman version of evolution that we see so often on these threads.
What is evolution? It is mutation and natural selection acting on hundreds (thousands) of traits at the same time. Given that, why should evolution proceed in any fixed direction for long? Most mutations are benign, some are beneficial and some are deleterious. Overall, most are benign.
Given this, changes in environmental conditions will "select" for those favorable traits and "select" against those deleterious traits. The climate gets colder? Then a narrow nasal form, shorter, squat body shape, and larger body are good adaptations.
Your tribe migrates to warmer climates, then dark skin, more linear body form, and broad nasal form are better adapted.
Try checking out a few books on human races. There is a lot of fascinating information about our species and how different groups have adapted!
But wait! There's more!
Those adaptations, modified by natural selection, can add up over time. That is where speciation comes in!
Without using a whole lot of grey matter. Very very grey indeed. :)