The very phrase "survival of the fittest" clearly entails death of "the unfit". Why then is it improper to ask why the allegedly unfit remain alive? It is not improper at all.
VI. One other principle, which may be called the principle of divergence plays, I believe, an important part in the origin of species. The same spot will support more life if occupied by very diverse forms: we see this in the many generic forms in a square yard of turf (I have counted 20 species belonging to 18 genera),â€”or in the plants and insects, on any little uniform islet, belonging almost to as many genera and families as to species.â€” We can understand this with the higher, animals whose habits we best understand. We know that it has been experimentally shown that a plot of land will yield a greater weight, if cropped with several species of grasses than with 2 or 3 species. Now every single organic being, by propagating so rapidly, may be said to be striving its utmost to increase in numbers. So it will be with the offspring of any species after it has broken into varieties or sub-species or true species. And it follows, I think, from the foregoing facts that the varying offspring of each species will try (only few will succeed) to seize on as many and as diverse places in the economy of nature, as possible. Each new variety or species, when formed will generally take the places of and so exterminate its less well-fitted parent.
Darwin discussed this sort of thing more than once. He seems to me to have grown more and more adamant about it over time. In more recent decades, the competition inherent in evolutionism has been downplayed considerably. "Selection" means death. Victory consists not merely of surviving, but the failure of the competition to survive.
It doesn't matter if one imagines big steps or small. The first monkey-girl on the way to becoming human had to reproduce. Her offspring had to compete with and mix with the regular monkey types. Now either they blend in completely, making one big group of monkey-people, or they form a group of their own and compete with the inferior type.
Typically, "geographic isolation" is introduced, when "the unfit" need to be protected from competition. In the case of humans, we're told it happened in Africa - not on some remote island.
This is an opportune time for this topic I think, because the "Oh well, it's just pure dumb luck" response runs contrary to the current fad claims of evolution being a non-random process..