Pull out your map. You probably have a good sense now of where your climates are, and you already know that there are people living in a few key places. Now ask yourself, what do the people who evolved in this place look like? Don't worry about conqueror races yet-- just those who natively evolved there. If there's a lot of sun, you might have darker-skinned inhabitants. In higher-temperature areas, a wider, flatter nose is an evolutionary advantage. In colder climates, a tendency to gain fat is an advantage as well.
Based on the climates you've set out in your map, what do the people look like who live there? Write down some typical characteristics, and then perhaps a couple of features that are considered especially attractive or unattractive.
Part of the purpose of this exercise is to move away from the very typical white Anglo-Saxon fantasy and science fiction heroes, and to get a bit closer to representing the kind of genetic and racial diversity that's actually available to the speculative fiction writer. It's not that you can't write a white person into spec fiction, but it's important that you don't *have* to.
Wherever there's a boundary, such as an ocean or mountain, you'll have more racial diversity. Wherever two groups might meet over land, you'll have more blending. In your cataclysmic events, you might have a boundary that abruptly appeared or disappeared. A sudden appearance of a boundary means that one racial group may now diverge, and when they re-connect later, they will find themselves culturally and perhaps genetically differing from the original base group. If a boundary disappeared, suddenly you will have different racial groups encountering each other, often for the first time, with varying effects on your storyline and civilization.
Technology, too, can remove a barrier. Sea-worthy vessels brought Western Europeans to the New World, forever altering the genetic makeup of those from South America; it is now very difficult to find an example of a "pure" Maya bloodline; "blending" with white settlers has changed the racial makeup of that particular group of people.
Spend 10 minutes figuring out what people who evolved in each major area of your world would look like. Then spend another 5 minutes asking "what if this group encountered that group?" Would they fight? Trade? Both? Inter-marry and blend their genetic types? Would they remain largely separate, with pure strains of both racial groups co-existing (not necessarily peacefully)? How would that encounter be brought about in the first place?