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What is the difference between that is to say and which is to say? Can that is to say replace which is to say in the passage?

In species with seasonal natality and mortality, which is to say nearly all animal species, the age distribution will undergo annual fluctuation. But even then the age distribution can be said to approach stability, in the sense that the fluctuation is periodic and predictable when corrected for season.

Sociobiology

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Hmm very nice questions. They don't mean the same thing. Looking at that is to say, Cambridge english dictionary gives us:

or more exactly

for That is to say. So you are specifying extra conditions on what you said before, that is to say you're further restricting the original meaning.

Looking at which is to say, Merriam-Webster gives us

which means that

for "Which is to say". So here you are just clarifying the meaning the way I read it.

This also corresponds to the restrictive non restrictive quasi dichotomy between which and that. That can only be used for starting restrictive relative clauses whereas which can be used to start both restrictive and nonrestrictive relative clauses but some varieties of English (and some prescriptive grammarians which are long dead) insist that it should only be used to start non restrictive relative clauses.

In the which is to say meaning from M-W you are just adding some extra information which is non essential but possibly clarifying or interesting. Whereas the That is to say, gives a restriction or qualification of the original statement.

This becomes even more pronounced, in my opinion, with a variation of that is to say that is presented in the CED, where you drop the "to say" part. Example being:

I'll meet you in the city, that is, I will if the trains are running.

Oh to answer the second question. I don't think you can replace "which is to say" with "that is to say" in that example since "nearly all ..." doesn't really work as a reasonable restriction. It doesn't actually sound ungrammatical to me, but I've learned to be careful with these kinds of things. The distinction is quite narrow and the meaning shift is subtle.

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