Ok so evolutionists like to apply the phrase "less evolved" to extinct species, and "primitive" to extant species? If anything it would be more intuitive to do it the other way around. Terminology however is important for communication and the English language isn't always logical, is this really the common usage among well qualified evolutionists?
If memory serves correct, the term primitive isn't restricted to extant species. It's just used to refer to more ancient characteristics (as opposed to more recently evolved characteristics). It is probably the reason why some species appear to have "evolved less" or stopped evolving; because they've retained many of these primitive traits.
I'm going to try to clarify this a bit further by providing a hypothetical scenario:
Picture a single species (species A) existing at a specific time in history. Lets say that at some point, some members of this population begin to migrate away such that the original population breaks off into two populations (population 1 and 2) that never interact again. Each population is subject to mutations at identical rates (x
mutations per base pair per generation). The mutations remain, disappear or propagate depending on natural selection and chance. Since the two populations don't interact, different mutations are likely to accumulate in the two populations, and these mutations will not be exchanged between the two populations. So the gene pools of these two populations continues to diverge at a rate that is twice the mutation rate.
Now, imagine that after a long period of time has passed, there could be overwhelming pressure in environment 1 for the many of the characteristics of population 1 to remain similar to their ancestral forms. Thus, due to natural selection in this environment, the mutations that have accumulated (x mutations per base pair per generation) may be mutations that have not altered many of the characteristics passed down from the ancestral population (Species A). Thus, population 1 retains many primitive characteristics.
In contrast, population 2 might find itself in a different environment. Also, there may be a founder effect, where the gene pool of this population, being a subset of the orginial population, may contain alleles in different frequencies. In any case, the environmental pressures on this population and their gene pool is different to begin with. There may not be as strong of a pressure to retain some of these ancestral characteristics. In this case, the mutations that accumulate (at the same rate of x mutations per base pair per generation) may very well cause alterations in these traits, as these traits are no longer advantageous in their present form. In some cases, the original characteistic may be completely supplanted by new traits.
After a very long time, Species B, which has descended from population 1, may very much resemble the original Species A because it has retained many ancestral, primitive characteristics. Species C, which has descended from population 2 however, may look completely different than the original Species A.
We know that both populations have changed over time, and we know that these changes have been accumulating in each population at the same rate. The only difference is with respect to how drastically the changes that are retained cause alterations in the ancestral traits. So, given this, would it be more accurate to say that Species B
has "evolved less" than Species C
? Or would it make more sense to say that Species A
is less evolved than both Species B
, while Species B
is just as evolved as Species C
, but has retained more of the primitive, ancestral traits?