The International Association for Plant Taxonomy (IAPT) considers cannabis to be a single species. This goes against popular opinion which holds sativa and indica to be different species, or at the very least differing subspecies.
The varieties in cannabis, as with wine grapes (vitis vinifera) all fall within one species. So, that clarifies things a bit. But what should we do about the fact that the cannabis varieties look different and have different psychotropic effects? Sativa, for instance, is considered to be lighter in color with narrower leaves than indica. It was this difference, as well as the fact that the sativa plants grow taller, while sativas are shorter and denser that lead Jean-Baptiste Lamarck to consider them two different species. Until very recently (the 1990s) species were “primarily distinguished by their appearance.” Nonetheless it has long been known that ”looks don't always tell us all we need to know about whether two organisms are different.” Therefore scientists have now turned to DNA analysis to identify and describe species." 1
How does it this apply to cannabis. It’s fairly simple – the qualities that we consider to be indica or sativa have nothing to do with whether or not the leaves alternate or are opposite; whether the bushes are tall or short and bushy; or any other physically identifiable trait.
The plants’ psychotropic affects are due to a particular combination of THC, CBD, terpenes and other cannabinoids and this chemical combination can not be particularly determined by its outward appearance. Hopefully, some day, there will be DNA analysis that will help us pinpoint these particular factors.
But you may be interested to know that a recent study shows “that marijuana strain names often do not reflect a meaningful genetic identity” but the “genetic structure of marijuana is in partial agreement with strain-specific ancestry.” 2
What does all this mean to you? It means that there has been so much cross-cultivation between different plants that until we have perfected DNA analysis the terms indica and sativa will only have meaning in how people experience the plant. Or, in other words, a Weed Sommelier, will first have to try the new strain and label it such.
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1 Why are different breeds of dogs all considered the same species https://scientificamerican.com/article/different-dog-breeds-same-species/
2 The Genetic Structure of Marijuana and Hemp
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0133292