Science Literature

Despite the intense interest in transitional forms, nearly all the candidates offered for our attention relate to stages of diversification within an Order or Family assemblage. These provide examples of morphological change, but rarely bridge the discontinuities that distinguish the higher taxa. In a minireview, Rodney Honeycutt introduces the problem in this way:

"The evolutionary biologist George Gaylord Simpson defined major morphological discontinuities among higher taxa, specifically the orders of mammals, as the result of macroevolution or 'quantum evolution'. In many cases, these discontinuities lack fossil evidence of transitions, appearing as what Simpson termed 'breaks in the fossil record', [. . .]
These discontinuities, as well as the short time periods associated with the diversification of many mammalian orders, are still presenting a challenge to paleontologists, geneticists and developmental biologists attempting to reconstruct the 'Mammal Tree of Life', a first step in understanding the geological and biological processes that are responsible for mammalian diversity."

There are legitimate questions that need to be asked about this methodology of "attempting to reconstruct the 'Mammal Tree of Life'". The fossil data speaks 'discontinuity' but the evolutionary biologists are committed to 'continuity' (common ancestry). They do not ask the question whether some or all of these discontinuities are real. The 'common ancestry' theoretical model governs completely the way research proceeds and what outcomes are deemed acceptable.

Evolutionary biology is locked within the common ancestry paradigm, and discontinuities merely provide structure for the classification. (Source: go here.)

Molecular data provides another avenue for developing cladograms and, subsequently, evolutionary trees. The methodology in general use adopts the same evolutionary presuppositions - namely, that all taxa can be fitted into the theoretical model known as the Tree of Life. Sometimes, the molecular data and the morphological data complement each other, but there are numerous examples of conflict. One of these, noted by Honeycutt, concerns the Afrotheria. This group of African animals has been defined using genetic data and links together elephants, manatees and dugongs, hyraxes, elephant shrews, aardvarks, golden moles and otter shrews.

"The superorder Afrotheria is another challenging case of morphological discontinuity in mammalian evolution, containing animals as morphologically distinct as elephants and aardvarks. [. . .] Despite the obvious morphological differences distinguishing the members of this superorder, extensive molecular phylogenetic studies consistently support a monophyletic origin for the Afrotheria (that is, the group all descend from a single common ancestor). But there are few unequivocal morphological synapomorphies (shared-derived characteristics) supporting monophyly of this clade. As indicated by Archibald, the superorder is "not predicted by fossils"."

This tension can be eased, it is proposed, via the evo-devo approach. Honeycutt refers to recent work on the genetics of mammalian tooth formation, with some very tentative conclusions. He then links this research with the general message emerging from evo-devo advocates and declares:

"As with many other examples, changes in gene regulation probably account for morphological similarities and differences among the Afrotheria."

Turning to bats, which appear abruptly in the fossil record, Honeycutt's review leads to the same general explanation:

"it is now clear that small changes in the spatiotemporal pattern of gene expression during development account for the dramatic changes represented by the chiropteran forelimb, and the genes responsible are beginning to be identified. [. . .] Although the processes responsible for the evolution of powered flight in mammals are not yet known in detail, these comparative studies indicate that small changes in the timing and extent of expression in key genes can have large developmental effects."

Reading this minireview has raised some serious concerns about the way evolutionary biology is practised.
1. There is an unhealthy dominance of the Tree of Life presupposition. This is steering and controlling thinking and effectively cuts off other avenues of legitimate enquiry (e.g. are the discontinuities real?).
2. There is a tendency to elevate tentative research findings to the status of being a panacea for solving major research questions. The reported research is said to be in its infancy, with very little known in detail, but this does not prevent drawing grand (and unwarranted) conclusions. Evo-devo enthusiasts should be more open to the possibility that their research is uncovering vast storehouses of complex specified information that explains stasis and diversification within an Order or Family but actually prevents transformation between discontinuities.

Small changes, big results: evolution of morphological discontinuity in mammals
Rodney L Honeycutt
Journal of Biology, March 2008, 7:9, 1-4 | doi:10.1186/jbiol71

Abstract: Comparative morphological and developmental studies, including a recent comparative study of tooth development among the Afrotherian mammals, are indicating the types of genetic mechanisms responsible for the evolution of morphological differences among major mammalian groups.