Common Creationist Complaint: How do ‘living fossils’ remain unchanged over hundreds of millions of years?

Common Complaint: How do ‘living fossils’ remain unchanged over supposed hundreds of millions of years, if evolution has changed worms into humans in the same time frame? Professor Gould wrote, “the maintenance of stability within species must be considered as a major evolutionary problem.” [1]

Logical Fallacies: 

• Straw Man: The comment "evolution has changed worms into humans" is distortion of what evolution actually says, and dismissing it because the author distorts it is illogical.

Answer: "Living fossils" are so named because they are organisms that appear to have undergone very few changes over millions of years and are more similar to fossils than to other living species. Many scientists dislike the term because it's misleading: undoubtedly these organisms have changed, just perhaps not as much or as easily identifiable through the fossil record as others. The fossil record can only tell us so much about an organism; the soft tissue and internal structure of an organism could change, and change dramatically, but we wouldn’t know that just from their fossils. There's also a lot of ambiguity with the term as it's not a scientific term; it's largely used in the media as a catch-phrase and is often associated with inaccurate connotations.

Since the question relies on a misleading term, it would be best to rephrase the question. A legitimate version of this question is, "Why do some organisms change more rapidly than others in the course of the same time frame?"

The answer to this question lies in one of the driving forces of evolution: selective pressure. Selective pressure is anything that alters an organism. Let’s say you’ve got a population of rabbits hunted by a population of wolves. There is selective pressure on the rabbits to be faster, better at hiding, perhaps letting off less scent – in addition to other ways of avoiding being eaten – and there would be pressure on the wolves to be faster, sneakier, more silent, and to have a greater sense of sight and smell. Therefore, there is a “negative” selective pressure on smelly, slow rabbits who are brightly colored (meaning they are less adaptive and thus not likely to survive) and a “positive” selective pressure on non-scented, fast rabbits who blend into their surroundings (meaning they are more adaptive and thus more likely to survive).

Organisms that are already well-adapted to their environment will experience relatively little selective pressure. If they don't have any major competitors for resources, few predators, and are in a stable environment, then we wouldn't predict a great deal of changes through time. If they do, however, have these selective pressures on them, then there will likely be a great deal of changes through time, culminating in the type of evolutionary arms race common between predator and prey. If the population is already “fit,” then “survival of the fittest” wouldn't change the population. And that's how you get "living fossils." 

[1] 15 Questions for Evolutionists. Evolution: the naturalistic origin of life and its diversity (The General Theory of Evolution, as defined by the prominent past evolutionist Kerkut; see introduction to Origin of life.) by Don Batten