This is something I will return to after some more research, but I find it very intriguing how population bottlenecks have affected the human race. A population bottleneck occurs when either a significant percentage of a population or species is reduced by 50% or moreor when a small group gets isolated from the main population (called the "founder effect"). This results in increased gene frequencies (genetic drift), as well as resulting in increased inbreeding. This has profound effects on the future of the species affected by the bottleneck.
These bottlenecks usually occur due to some large disaster, whether cosmic, natural, or man-made such as meteor impacts, super volcano eruptions, climate change, disease, or war, just to name a few.
So when thinking about alien realities, we need to keep in mind that unexpected evolutionary pathways can occur because of bottlenecks. Think of this hypothetical situation: a small group of a sentient species is exiled or chased away by a larger, stronger group (of the same species). Left alone, Darwinian forces here would predict that eventually the stronger group's genes would win out over the weaker group. But if a disaster comes along that wipes out the stronger group but, by the luck of the draw, the small, weaker group manages to survive - guess who will repopulate the species?
On Earth we have isolated groups where color-blindness is much higher than the average human population, or where six-fingers is the norm, or people who walk on all fours. If a major disaster were to happen and by fluke stroke of luck one of these isolated groups survived, the future of the human race would be greatly changed (where, for instance, those with five fingers would be the odd ones). Of course, some of these attributes are recessive, and eventually the dominant genes would reassert themselves as the norm.