Deconstructing the long‐standing a priori assumption that serial homology generally involves ancestral similarity followed by anatomical divergence

Abstract

It has long been assumed that serial homologues are ancestrally similarpolysomerism resulting from a "duplication" or "repetition" of forms-and then often diverge-anisomerism, for example, as they become adapted to perform different tasks as is the case with the forelimb and hind limbs of humans. However, such an assumption, with crucial implications for comparative, evolutionary, and developmental biology, and for evolutionary developmental biology, has in general not really been tested by a broad analysis of the available empirical data. Perhaps not surprisingly, more recent anatomical comparisons, as well as molecular knowledge of how, for example, serial appendicular structures are patterned along with different anteroposterior regions of the body axis of bilateral animals, and how "homologous" patterning domains do not necessarily mark "homologous" morphological domains, are putting in question this paradigm. In fact, apart from showing that many so-called "serial homologues" might not be similar at all, recent works have shown that in at least some cases some "serial" structures are indeed more similar to each other in derived taxa than in phylogenetically more ancestral ones, as pointed out by authors such as Owen. In this article, we are taking a step back to question whether such assumptions are actually correct at all, in the first place. In particular, we review other cases of so-called "serial homologues" such as insect wings, arthropod walking appendages, Dipteran thoracic bristles, and the vertebrae, ribs, teeth, myomeres, feathers, and hairs of chordate animals. We show that: (a) there are almost never cases of true ancestral similarity; (b) in evolution, such structures-for example, vertebra-and/or their subparts-for example, "transverse processes"-many times display trends toward less similarity while in many others display trends toward more similarity, that is, one cannot say that there is a clear, overall trend to anisomerism.

Reconstructing serial homology with a special emphasis on the nature of limbs in vertebrates and with references to Cruveilhier, Wyman, Belogolowy, and Balinsky

“…In fact, Belogolowy (1911) deconstructs the special homology like Siomava et al (2020) deconstruct the serial one. More than that, Belogolowy's evidence for discontinuity in the special homologues is structural, being based on the segment specification by fine details of the cranial nerves.…”

“…Indeed, the reference of Siomava et al (2020) to the nonquantitative approach developed by Owen is important. It was not quantitative but F I G U R E 2 Dorsal view scheme of the wrong-side limbs which developed in urodelan larvae after implantation of a nonspecific inductor on the body flank between the normal limbs in experiments of Balinsky.…”

“…The next concern is about the other kind of a quantitative approach to morphology which is represented on Figure 2 of Siomava et al (2020).…”

“…This is the only mention of the serial homology between the bilateral organs by Siomava et al (2020). According to Owen, any symmetrical parts are serial homologues.…”

“…This looks inconsistent to me. Siomava et al (2020) claim that the serial homologues are false, because they are ancestrally anisomeric (dissimilar) while the special homologues are true because, traced to the common ancestor, they come to the same ancestral structure and thus form a continuum in bifurcating phyla. However, in every generation the continuum of the morphological structures is briefly interrupted by a genetic transfer through gametes (and spores in plants).…”