Science Literature

It is well known that rodents "evolve" fast. Archaeologists have used fossil vole remains as a means of dating because the different morphologies and species link to different time zones: this is the "vole clock". So pervasive are these changes, it has become a matter of note when stasis is observed! A case in point is the birch mouse (genus Sicista). A "living fossil" at the generic level has been found in Inner Mongolia by Yuri Kimura, a doctoral student. It has been identified by its fossilised teeth - the only part of the animal still accessible for study. The teeth can be read like a book, with the cusps, valleys and ridges providing a signature by which the animal is known.

"The new fossils of Sicista primus from the Early Miocene age are also now the earliest known record of Sicista, the birch mouse genus that comprises 13 modern and 7 fossil species, said Kimura. As a result, Sicista now boasts the most ancient ancestry of the 326 genera in the largest rodent suborder to which it belongs, Myomorpha. The suborder includes laboratory mice and rats. "The birch mouse is a rare case of a small mammal genus persisting from the Early Miocene without significant morphological changes," Kimura said in reporting the findings."

Paleontologist Yuri Kimura, Southern Methodist University in Dallas, identified a new species of birch mice, Sicista primus, from 17 tiny teeth. A single molar is about the size of half a grain of rice. The teeth, however, are distinctive among the various genera of rodents known as Dipodidae. Cusps, valleys, ridges and other distinguishing characteristics on the surface of the teeth are identifiable through a microscope. (Credit: Yuri Kimura/Southern Methodist University, source here.)

As far as chronology is concerned, the new fossil is said to be 17 million years old. Previous to this, the earliest fossil of Sicista was considered to be about 9 million years old. Whilst these figures are small compared with those obtained for the coelacanth, or the horse shoe crab, or the Wollemi Pine, they are large for rodents.

"[The observed diversity within this] rodent genus is not unusual, but the long record of the genus Sicista, first recognized at ~17 Ma, is unusual. The discovery of Early Miocene S. primus reveals that Sicista is fundamental to understanding how a long-lived genus persisted among substantially fast-evolving rodent groups."

The above comment concludes the paper, and there is no discussion of how the observation of stasis contributes to a better understanding of stasis. The last sentence says little more than "The fact is fundamental to understanding the fact". Darwinians commonly present everything in terms of adaptive evolution and refer to 'stabilising selection' or to a 'lack of change in the environment'. Whilst this can pass as a reasonable hypothesis with the coelacanth and some other cases, it does not sound at all convincing when the stasis is the exception (as is the case with rodents) rather than an isolated case. This is where multiple working hypotheses are particularly valuable, so that we can weigh alternatives rather than trying to fit everything into a Darwinian framework.

One avenue to explore is whether the rodent radiations could be the result of generating different permutations from the same genetic material. This can occur because the genotype can be expressed in different ways, arising from the phenomenon of phenotypic plasticity. Various triggers, such as from the environment, may be invoked to explain the changes in phenotype followed by the persistence of a specific phenotype for a period of time. A recent analysis of this approach is by Hughes (2011) who points out that phenotypic plasticity provides a ready explanation for abrupt changes in morphology (that cannot be attributed to the gradualistic mechanisms of Darwinism).

"Widespread occurrence of the PRM mechanism [plasticity-relaxation-mutation] would easily explain recently reported cases of apparent phenotypic evolution over ecological time. In most such cases, there has been no genetic evidence demonstrating the operation of the classic Neo-Darwinian mechanism of allelic replacement. In some cases, the time frame seems rather short for a Darwinian process to have occurred, and in other cases, the effective population sizes of the species in question are small, suggesting that there is unlikely to have been extensive genetic variation in the population prior to selection. However, none of these factors are problematic if these cases of apparent rapid evolution in fact represent cases of phenotypic plasticity, perhaps in some cases rendered heritable through germline DNA methylation. Thus, rather than the paradoxical observation of Darwinian evolution over ecological time, we may be merely seeing incipient evolution by the PRM mechanism, which is expected to operate over ecological time."

It is worth saying that Hughes considers that the adaptive evolution mechanism to be poorly supported by data, and he is unimpressed by recent statistical analyses claiming to demonstrate positive adaptation. His non-Darwinian mechanism provides a ready explanation for rapid changes in morphology and he uses as examples adaptive radiations of the cichlid fishes in Africa, and the diversification seen in Darwin's finches in the Galapagos.

"The hypothesis proposed here has the advantage of explaining the available data regarding adaptive evolution on the levels of genomics, ecology and paleontology, without invoking any mechanisms other than the commonly observed phenomena of phenotypic plasticity, purifying selection, mutation and genetic drift. Although it may represent a new perspective to biologists schooled in Neo-Darwinism, this view of life in its own way is not without 'a certain grandeur'."

Kimura is correct that Sicista primus can help us understand better the nature of rapid adaptive radiations. But this is only likely to be realised if scholars are prepared to move out of the comfort zone of neo-Darwinism to test their hypotheses with empirical data.

The earliest record of birch mice from the Early Miocene Nei Mongol, China
Yuri Kimura
Naturwissenschaften, (2011) 98:87-95 | DOI 10.1007/s00114-010-0744-1

Abstract: The earliest species of birch mouse, Sicista primus sp. nov., was recovered from the 17-Ma-old (Early Miocene) Gashunyinadege locality, central Nei Mongol, China. It is ~9 Ma older than the previous first appearance datum of Sicista in Eurasia. This study indicates that North American Macrognathomys is a synonym of Eurasian Sicista, having 12 shared dental characters. As a result, the biogeography of dipodids indicates that Asian Sicista dispersed to North America as opposed to the hypothesis that Sicista originated from the North American clade. Sicista is one of the few extant rodent genera that originated as early as the Early Miocene.

See also:

No Positive Selection, No Darwin: A New Non-Darwinian Mechanism for the Origin of Adaptive Phenotypes, by P.J. Levi (Evolution News & Views, November 14, 2011)

Evolution of adaptive phenotypic traits without positive Darwinian selection, by A L Hughes, Heredity advance online publication, 2 November 2011 | doi: 10.1038/hdy.2011.97

Birch Mouse Ancestor Discovered in Inner Mongolia Is New Species of Rare 'Living Fossil', ScienceDaily (May 25, 2011)