All of the factors above explain how the population of a species can dramatically decrease and become very small. It is worth thinking about why small populations are so much more vulnerable to extinction.Several things are happening at the genetic level. A smaller population has increased levels of inbreeding, which reduces the genetic variation in the population. Genetic drift has a larger impact on a small population leading to an even further decrease in genetic variation. As we know from natural selection genetic variation is important as it allows a species to adapt and survive environmental change; it improves its fitness. So a small population has a lower fitness meaning increased mortality and decreased reproduction.