Most family has more than one species.
For the second calculation, I would like to find the number of unique species present in each family. I've browsed unique command in help page, with the thought that by creating another file containing unique species along with other variables, i.e. family and size. Then use tapply to create the required data output. But the unique command only apply to a vector instead of a data frame. I've also tried using the following command but the results is not what I expected.