taxonomy - Binomial nomenclature: Why am I seeing different genera with the same species name?

I have looked online but still do not understand how two organisms can have the the same species names but be in different genera?

Do all genera share common species names?

Also which would be more closely related, two organisms with the same genus name but different species name or two organisms with different genus names but the same species name?

According to a textbook, the two with the same genus are more closely related, but I do not understand why?

Answer

Two different species can have the same species epithet if they belong to different genera ('species name' is referring to the full binomial name). Consider for example Pinus glabra and Ilex glabra

P. glabra

I. glabra

Two species can be have the same genus name (meaning they belong to the same genus) and will therefore necessarily have different species epithets (otherwise we would name them exactly the same way which would be very confusing). Consider for example Pinus glabra (again) and Pinus resionosa

P.glabra

P. resinosa

As a consequence, if two species share the same species epithet then it tells you nothing about how closely related they are. If they share the same genus name, then they belong to the same genus (except exceptions probably!).

For the above examples, the term glabra comes from "glaber" (latin) which means "Hairless". So the only thing you can tell from P. glabra and I. glabra is that they are probably both hairless when you compare them with their respective closely related species.