Tumors are automatically spawned all around the map and lings can also drop a few (ONLY if they pick the ability).
With the abilities you can make it every round each ling can unlock an extra ability it just depends on what you start of with and how you build your ling towards the end of the rounds/game.
Experiment with the terrain by creating choke points.
Note: When a zergling dies it restarts at the beggining of the map, but if you had 2 life ability then you could continue from where you died last.
I hope this helps.