Vertical gene transfer is the method of passing along DNA that we are familiar with, when two parents produce offspring that have a combination of their genes. But that's not the only way that organisms can gain new genes. Horizontal gene transfer is not as common, but it can lead to a lot of changes in a species. This is when bacteria, fungi, plants, animals, or any species incorporate genes from a different species into their genomes. Scientists are doing this in labs now, but nature has been doing it forever. We've long known that single-celled species incorporated other single-celled species which ended up becoming the more complex parts of the cell. Thinking about fish and insects and other higher animals doing it is a mind blower. Yeah, it's complicated, but nature is weird. If the new genes prove to be beneficial in some way, the species will keep these new alien genes until they are their own. That means that it goes on a lot more than we realize because only a small percentage of these natural gene transfers will be beneficial.