Fossil Friday: Chronospecies, a Sinking Ship
In my public presentations and articles on the problems for neo-Darwinism raised by the ubiquitous discontinuities of the fossil record, I usually do not just present a series of abrupt appearances of new body plans in the history of life. Rather, I also describe how gradualism fails to be supported even on lower taxonomic levels. One example is my Evolution News article from September 2019, where I showed how all the three major textbook examples for alleged gradual species-to-species transitions have been debunked by more modern mainstream research (Bechly 2019).
The Concept of Chronospecies
A potential response by Darwinists could be to refer to the concept of chronospecies in paleontology (sometimes called paleospecies or morphospecies), which was introduced by George (1956) for the naming of successive species in a single evolving lineage. Putative examples are known from marine protozoans (e.g., foraminiferans, see Wei 1987), marine invertebrates (e.g., ammonites, see Dzik 1990), and a few cases in vertebrates, such as fossil water rats (see Escudé et al. 2008) and the extinct endemic bovid Myotragus from the Mediterranean Balearic islands of Mallorca and Menorca (Moya Sola & Moya 1982, Köhler & Moya-Sola 2004, Bover & Alcover 2005, Moya Sola et al 2007, Bover et al. 2010).
Of course, such chronospecies are not at all uncontroversial. Some experts deny that these represent macroevolutionary speciation, but instead simply represent microevolutionary changes within a single species (e.g., Willmann 1985, Allmon 2016), and thus are completely arbitrary delimitations (Cain 1954, Thomas 1956, Simpson 1961: 165, Mayr 1963: 24, Mayr & Ashlock 1991: 106) of chunks of a genealogical nexus (Kitts & Kitts 1979, Kitts 1983, Lyman & O’Brien 2002). But in all fairness, such fuzzy chunks arguably are what we should expect to find if there were indeed gradual species-to-species transitions, especially in cases of anagenetic change within a single evolving species lineage.
A Common Pattern
But, even in the few known cases, it has become a common pattern that new research tends to challenge and refute the status of chronospecies. One example are the marine sloths of the extinct genus Thalassocnus, which lived with five successive species in the Late Miocene and Pliocene along the Pacific coast in South America. “They were regarded by McDonald and Muizon (2002) as segments of a single lineage representing the initial and progressively more aquatic adaptations” (Muizon et al. 2003). Nevertheless, the study by Muizon et al. (2003) concluded that:
Parsimony analysis does not resolve the relative positions of T. antiquus and T. natans, and, therefore, does not fully confirm the possibility of a single Thalassocnus lineage, which spans over 4 Ma. However, Thalassocnus is an endemic genus and the stratigraphic distribution of its four species is well known. Furthermore, some characters indicate a continuous evolution from the oldest (T. antiquus) to the youngest species (T. carolomartini). Therefore, we prefer the hypothesis of a single Thalassocnus lineage, although a more complex evolutionary scenario is not discarded.
The authors elaborated that:
The new parsimony analysis presented here indicates that the four species of Thalassocnus may not represent a single evolving lineage. … Although parsimony analysis indicates that the absence and existence of a single time-successive lineage including all four species of Thalassocnus are equally parsimonious, we are reluctant to accept the first interpretation. … To conclude, a definitive decision is not easy to establish because reversals could explain the morphology … In spite of the result of the parsimony analysis, we believe that the exclusion of T. natans from a ‘‘Thalassocnus lineage’’ would be a surprising coincidence and that only a single Thalassocnus lineage is likely to have existed in the southeastern Pacific. … However, we do not discard the possibility of a more complex evolutionary scenario for Thalassocnus.
Slowly Sinking
In short: There is by no means unequivocal evidence for gradual anagenetic speciation in the case of the aquatic sloth Thalassocnus. Given the other refuted examples (see Bechly 2019), this raises further doubts about the validity of the few remaining cases of alleged gradual species-to-species transitions. Stanley (1978) found in his seminal analysis of chronospecies that “most net evolutionary change must have been associated with saltational speciation.” Also the metastudy of Hunt (2010), who looked at 150 years of research on the fossil evidence for speciation since the time of Darwin, found no evidence for the directional change predicted by anagenetic (or even cladogenetic) speciation (see Bechly 2019). The “ship” of chronospecies seems to be slowly sinking, which suggests that non-gradual processes dominated the history of life even on the lower taxonomic levels. That is consistent with intelligent design theory but inconsistent with neo-Darwinian evolution.
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