No, you've increased the survivability. If the mutants are a new subspecies, then the species as a whole has now gone from 4 to 5 different environments. If they're a new species altogether, then you went from 1 species in 4 environments to 2 species in 5 environments. But in another sense you've actually gone from 4 to 7! You've gone from (1 species in 4 environments) to (1 species in 4 environments plus 1 new species in 3 environments). No matter how you measure it, the end result for the original population's offspring as a whole is increased biodiversity.
However, you've changed the terms of my argument somewhat, from Vertical Evolution (A into B) to Horizontal Evolution (A coexisting with B). That's not actually what I was talking about, but I can dig it.
Let us say that you continually sub-divide the Species into less-survivable sub-groups of the Core Species.
What you end up with is a bunch of Genetic Variants which are relatively more vulnerable to Local Extinction Events (since their overall range of Adaptation is absolutely reduced) and a Genetic Core Species which is as vulnerable as ever to a Mass Extinction Event (leaving, in the Best Case, a Genetic Variant which is still relatively more vulnerable to Local Extinction Events -- since their overall range of Adaptation is absolutely reduced).
Any way you slice it, sub-division into relatively less-survivable Genetic Variants simply cannot be a recipe for Species Survival and Genetic Success... because as long as the Species is subdividing into LESS-Survivable Variants with no Subspecies enjoying an ABSOLUTE expansion of its genetic adaptability in the presence of Local and Mass Extinction Events, you still end up with an Agatha Christie novel... "And Then There Were None."
However, you've changed the terms of my argument somewhat, from Vertical Evolution (A into B) to Horizontal Evolution (A coexisting with B). That's not actually what I was talking about, but I can dig it.
Ah, well that is the standard model for speciation.
Let us say that you continually sub-divide the Species into less-survivable sub-groups of the Core Species.
What you end up with is a bunch of Genetic Variants which are relatively more vulnerable to Local Extinction Events (since their overall range of Adaptation is absolutely reduced) and a Genetic Core Species which is as vulnerable as ever to a Mass Extinction Event...
That's an interesting way of looking at it! But the problem is, we're talking about your hypothetical scenario: One species originally suited to "biomes" A, B, C, & D, splits into the original plus the second suited for C, D, & E. Why not postulate the offshoot being suited for B, C, D, & E? Or indeed why not A, B, C, D, & E? I've never heard of any evidence or biological/ecological principle that says that a breakaway subspecies or species has to be adapted to fewer environments than the original population was. It could just as easily be more.