The 1st splice requires 2 population, the 2nd 4, 3rd 6 etc. When you access this it will show you how many other species you have in your empire and then once you have enough of a certain type you can add them into your genetics.
The gene splicing screen is at the bottom of the info panel at the top left of your screen. The Horatio can absorb other civilization's population traits into their own.
