system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some plants have poisonous berries if it's in the interest of the plant that the berries are eaten by animals and thus seeds are spread?
gpt
Most plants aren’t poisonous to all species so a plant that is poisonous to humans will still reproduce. Plant poisons are also many times effective on organisms such as microbes or fungi or insects, types of organisms that may eat the fruit but not do anything to spread the seeds. It could also be a defensive mechanism to protect the berries as they will ultimately drop to the ground if not eaten thus ensuring some seeds reach the ground. With evolution, it only matters that enough survive to reproduce. As long as the poison is not entirely detrimental it can/will exist in a plant and may exist because of entirely random chemical reasons.