Like in the picture below the bottlenecking effect works the same way in nature. When we shake the bottle some kinds of candies may fall out and some may not. This doesn't represent the whole bottle. Some type may not even fall out because the bottle wasn't wide enough for it to fall out. This can happen in nature too. In a forest fire where some animals survive they didn't survive because they were adapted to fire. They just weren't in the part of the forest that was on fire. So with the new allele frequency in the new population, this is a bottleneck effect.
