I don’t mean to upset those of you with easily shattered views of reality (actually, yes, I do; if it’s that fragile to begin with, you shouldn’t be near me), but some of you (that’s being conservative) have probably heard of the term “living fossil” in one of the many media outlets available and even in some scientific literature. Here is the short list of “living fossils” which are living, but aren’t fossils:
Crocodilians, Sequoia, Coelacanth, Horsetails, Nautilus, Ginko, Horseshoe Crab, Lycophyta, Vampire Squid, Whisk Ferns, Tuatara, Cycads, Ratites ….
I’m sure you get the idea. Anyway, the term “living fossil” is a falsehood. The reason for this isn’t due to the definitions used in scientific literature, in fact, that is usually used to describe “species with little or no morphological change over a significant portion of geological time with relatively little radiation of the clade after a significant genetic bottleneck.” Additional colloquial definitions include “a living organism with ancient traits” and “a living species which has remains relatively similar to the fossil ancestors.”
A problem with the idea of a “living fossil” is the idea that these organisms are truly unchanged. While they may appear similar, this morphology may, in fact, be the result of stabilizing selection for a specific morphology. A perfect example is the “living fossil” story I am, and you probably are, most familiar with: the coelacanth. There are, in fact, two living species of coelacanth and these two species, while very similar morphologically, are actually quite distinct (color is the main thing that jumps at me). This is an example of an unpreserved trait. There are many of these and without a genetic analysis, it is impossible to make the claim that the modern populations are truly “identical” to the fossil organisms. It would be difficult to distinguish the two living species from each other given only fossil morphological evidence, but genetic analysis indicates they diverged >20 mya compared to the human/chimpanzee lines diverging <10 mya. In other words, these two species are very genetically distinct from one another and there is no reason to suspect that the ancestral species were no less different from modern coelacanths than the two living species are at present.
In case you don’t remember one of the key aspects of evolution I talked about here:
…evolution is not something which is measured within an individual organism, or even within a population at a given time; it is measured between populations separated through time OR space or both
The “time” component is very important. A species will continue to change (on the genetic level) with or without a selective pressure. A population does not maintain identical allelic frequencies in the absence of selection. This is termed genetic drift, and this is even more pronounced after a genetic bottleneck of some kind. In other words, the exact thing that results in a “living fossil” (a species which does not radiate after a genetic bottleneck) is in the exact situation which results in a high level of genetic drift!
Ancestral populations will never (I know, this is a strong statement) be identical to modern populations. These “living fossils,” like all other organisms, are not static in terms of the genetics of their populations. The allelic frequencies (at the very least, mutations within unselected regions of the genome) will continue to change. We must keep in mind, however, that as similar morphologically as the living Latimeria species are to the fossil forms, they would be very genetically distinct from one another.