"How does one transition from, say, the Pacific Northwest with rain forests, to the semi-arid California coast, to the deserts of Mexico, to the rainforests of the Central American mountains, to the Andes, and on to Tierra del Fuego."
I'm not sure those conditions existed like that at that time.
Good point. However, I WILL not let possible facts get in the way of my predispositions. I will continue to argue that the ecosystems were sufficiently varied that diffusion from proto-Alaska to proto-Tierra del Fuego and from the mountainous western half of North America across the proto-Great Plains would take longer than the 1000 or so years allotted by the Clovis model.
But your point is certainly well-taken. I don't enough about the ecosystems of North America ca 10,000 BC and 20,000 BC to be more specific.
Also if you are being displaced by a stronger tribe to the north of you, you should head south.