www.lesswrong.com/posts/oMWXqoCCXYxbGtcWW/wildlife-biology-forgot-how-definition...

Why this is incorrect

The post claims that under the Biological Species Concept (BSC), populations can be treated as different species even if they could interbreed, so long as they are currently prevented from interbreeding by geography (or similar external barriers).

However, in Mayr’s framing of the BSC (and standard explanations of “isolating mechanisms” used with it), geographic isolation is explicitly treated as an extrinsic factor, not an intrinsic reproductive isolating mechanism.

Contradicting evidence

1. Ernst Mayr (1996) explicitly distinguishes isolating mechanisms (intrinsic biological properties) from geographic separation and states plainly:

   • “Geographic isolation therefore does not qualify as an isolating mechanism.”
     In the same passage, Mayr emphasizes that isolating mechanisms are properties of individuals (i.e., intrinsic biological barriers), not mere spatial separation.

2. A recent reference summary (EBSCO Research Starters) makes the same point in standard terminology:

   • “Geographic isolation can prevent interbreeding between populations, but it is an extrinsic factor and, therefore, does not qualify as an isolating mechanism.”

What this means for the post’s claim

Because the claim hinges on geography itself being sufficient under the BSC to make populations “different species,” it conflicts with sources explaining the BSC’s core idea: species boundaries are grounded in reproductive isolating mechanisms (intrinsic barriers), not simply current geographic separation.

(Geographic separation can matter in speciation as a cause or context in which isolating mechanisms evolve, but that is different from saying geographic separation itself is the criterion that makes populations different species under the BSC.)