Unfortunately, speciation is mostly a continuous process, so there's no way to define species by some kind of "degree of similarity/dissimilarity" metric and ha

Unfortunately, speciation is mostly a continuous process, so there's no way to define species by some kind of "degree of similarity/dissimilarity" metric and have it be a transitive relationship. Any species definition that is completely transitive will necessarily have edge cases where two extremely similar organisms fall on different sides of an arbitrary threshold and are declared to belong to two different species. The classic example of non-transitive species relationships from sexually reproducing organisms is a "ring species": https://en.wikipedia.org/wiki/Ring_species https://en.wikipedia.org/wiki/Ring_species