What is the difference between "equal" and "equitable"?

"Equitably" means "fairly" ["with equity" in the sense of fairness] rather than "equally". Often of course the result is the same thing but it is not necessarily so.

In this scientific-sounding context I would venture to suggest that "equally" is probably what the presenter meant and "equitably" is a bit vague with just a hint of ambiguity.

I'll throw in my two cents. I agree that "equitably" means fairly divided and "equally" means divided into portions of the same size.

But "equitably" does not always mean that the portions are the same size, and I would think usually implies some sort of arbitration, or some authority that would determine what was "fair."

This isn't something that would happen with cell nuclei, unless you consider a divine intervention. I think "equally" is the more appropriate word here.

"Equitable" has more of a legal or moral connotation. An equitable settlement among heirs might be that one gets a house because she needs it while the other gets the vacation home because he wants it. The settlement would not necessarily be equal in monetary value, but it might be considered fair to all parties involved.

Concerning your speaker's case of cell nuclei. I think "equal" would be correct if all the nuclei had the about the same spacing. I think "equitable" would be correct if the cells had, say, differing functions, where those cells with critical functions receive more space for their nuclei.

Although my cell biology understanding is limited, here's an example that might enlighten: eukaryotic cells host a true nucleus, in contrast to a prokaryotic nucleoid. Eukaryotic cells get a lot more space between their nuclei. The spacing between the eukaryotic nuclei are fairly equal in the same organism (meaning the distances are about the same). If the speaker were comparing eukaryotic to eukaryotic, then "equal" might have been the better choice.

But the eukaryotic nuclei get much more space than the prokaryotic nuclei -- because they are much more complex. If the speaker were comparing eukaryotic to prokaryotic, then "equitable" might have been the better choice.

(And my apologies to cell biologists for any misinterpretation of the web page; these errors stem from my lack of domain expertise.)

Equally suggests that the space is divided into portions of the same size, while equitably suggests a fair division. In this case, both words seem to be appropriate.