The rate of evolution is driven mostly by how fast the turnover is. Bacteria can have a new generation every few minutes. Many insects and the like can have several generations a year, most mammals a year upwards. Fecundity comes into it too- many fish produce tens of thousands of offspring at once, people generally one at a time. Obviously, the more offspring produced at a go, and the more often, the bigger chance selection has to work on them, and the faster this can affect our view of them as "variants" or "species".

As for whether evolution will "fill in" for lost species, that depends on whether the ecological niche has been vacated (by hunting to extinction say), or totally destroyed as in the killing of a coral reef. If some members of another species occupying a (slightly?) different niche can take advantage of the opportunity, their offspring may have taken the first steps towards the spawning of a new species.

A lot depends on what has been killing the Yangtse dolphins. If it is loss of habitat or food source, we can't look forward to their replacement. In any case, they and their ancestor species have lived there for 20 million years apparently; we humans are likely to be extinct before their replacement evolves.

By the way, I don't subscribe to the idea that bacteria are "lower down" the evolutionary tree than we are- we and they have had exactly the same time to evolve since the beginning of life on Earth. I'd contend that we are (from the evolutionary point of view only of course) merely failed bacteria, that had to find another way of surviving that involved us in endless complications, Heath Robinson kludges and Machiavellian cellular politics.