dhw: I don’t think anyone would claim that every instance of speciation was triggered by a global change in the environment. Speciation has to take place in individuals or groups, and individuals and groups live in local environments. It makes perfect sense to me that a change in local conditions might trigger changes in certain organisms, and if they are successful they will flourish and spread. And I still don’t see how conditions and life can “co-evolve” and yet there is no link between speciation and the environment.

There is a link in adaptations. We don't know how they might lead to speciation, since there is no Darwin-desired step by step fossil pattern.