Hi,

No straightforward answer.

A combination of the enumerated, based on the studied species.

A good discussion on the issue (regarding bacteria):

https://www.ncbi.nlm.nih.gov/books/NBK8406/

Some interesting approaches on plants:

Article Distinguishing species

https://phytokeys.pensoft.net/article/30330/download/pdf/

Article Infrared Spectroscopy of Pollen Identifies Plant Species and...

morphological

I agree with Valeria Tananska , the reasoning being that morphological differentiation has been utilized exclusively, until recently. However, these days, molecular methods can provide exact quantification of the relatedness between two organisms. The issue here is that the molecular methods are not as well developed and as common to practice as morphology. As molecular methods become more commonplace within organism differentiation we will see a different, and hopefully more exact, classification of species. This does also mean that different classification methods might showcase alternative features, and perhaps even the opposite of what you would expect, so the methods might diverge in interpretation.

Thanks,

Valeria Tananska &

Carl Alexander Frisk