What I'm noticing with my base species is that they
always get cybernetics. I'm late game at this point. I modified one newly uplifted species (manually using the method I
linked above) and added brain slugs and when one of them come up in the leader pool they always have brain slugs. I think it's only random when not all of that species have the trait, but when they have both it seems to always prefer cybernetics and excludes brain slugs.
well i can tell you from my personal experience in 2.2.3 experimental.
Early on in game I got brain slug event. After which, i randomly got 3 pop with brainslug while vast majority of my species did not. My leaders seemed to have a random chance to show up with slug or with out it. Some existing leaders before event got it, some didn´t.
Then trading carvan came along and sold me some psionc tomb worlders with a boatload of negative traits, they were rat people of some sort.
The broken game population growth formula made it so over time, all species reach same total population, so eventually half my population were psionic rat tomb worlders.
I took genetic ascention perk, so fixed the tomb worlders to be rather decent with extremely adaptable, later gave them the advanced traits of erudite and the +30% habitiblity one. Also edited my base species to all be brain slugs.
Result was that my leaders show up randomly with brain slug, but every single one of the rat people show up as psionic. Strangely though, occasionally leaders of any race will show up with brain slug, even though none of their pop has the trait. So my best science leaders are psionic slug brainers with Erudite bonus and spark of genious, for a total +40% to research.
No mods were used in this playthrough.