Serial Homology

The current situation in philosophy of science generally, and in philosophy of biology in particular, is most unsatisfactory. There are at least three general problems that many philosophers thought themselves near to solving twenty years ago, only to find that the anticipated solutions have come unglued. These are (1) the problem of characterizing and understanding the dynamics of conceptual change in science; (2) he problem of understanding the interrelationships among theories including particularly the reduction of one theory to another); and (3) he problem of scientific realism (i.e., the problem of how seriously to take the claims of theoretical science or, at least, of some theoretical scientists, to be describing the world literally--in terms of such theoretical entities as genes and protons, DNA molecules, and quarks). This general situation has significant effects on the philosophical study of particular sciences. In philosophy of biology, for example, although one finds a large number of elegant studies of particular topics, the sad fact is that there is no generally satisfactory large-scale synthesis n sight. We have no agreed-on foundation, no generally acceptable starting point from which to delimit and resolve the full range of theoretical problems of interest to scientists and philosophers regarding biology.

Homology in cladistics is reviewed. The definition of important terms is explicated in historical context. Homology is not synonymous with synapomorphy: it includes symplesiomorphy, and Hennig clearly included both plesiomorphy and synapomorphy
as types of homology. Homoplasy is error, in coding, and is analogous to residual error in simple regression. If parallelism and convergence are to be distinguished, homoplasy would be evidence of the former and analogy evidence of the latter. We discuss
whether there is a difference between molecular homology and morphological homology, character state homology, nested homology (additive characters), and serial homology. We conclude by proposing a global definition of homology.

Most geophilomorph centipedes have segmental clusters of exocrine glands whose opening pores are arranged in more or less well-defined sternal pore areas. We describe here the cuticular structures forming and:or accompanying the gland openings on the sternites and the shape of the pore areas along the body axis in representatives of most geophilomorph families. The cuticular ring around the pore may exhibit either of two forms. In Himantariidae (Himantarium) and in Dignathodontidae (Henia) the ring looks like a continuous ribbon with a visible suture, whereas in the representatives of the remaining families no suture is seen. As to the distribution of the pores on ventral surface of the body, we record the presence of pores on the last leg-bearing segment of Clinopodes flavidus, whereas that segment was described as poreless in all geophilomorphs. We also provide a taxonomic survey of shape and distribution of pore areas in the individual families, where the pore areas may take very different shapes that we regard as transformational homologues. As for the segmental distribution of sternal pore areas, there is a considerable amount of complexity along the trunk of geophilomorph centipedes, in contrast to the apparently uniform trunk structure.

Advances in developmental genetics and evo-devo in the last several decades have enabled the growth of novel developmental approaches to the classic theme of homology. These approaches depart from the more standard phylogenetic view by contending that homology between morphological characters depends on developmental-genetic individuation and explanation. This paper provides a systematic re-examination of the relationship between developmental and phylogenetic homology in light of current evidence from developmental and evolutionary genetics and genomics. I present a qualitative model of the processes that cause de-coupling of morphological and molecular evolution by developmental system drift (DSD), and hypothesize that DSD is a widespread and regular stochastic process that is predictable from parameters such as population size, pleiotropy, and regulatory redundancy. These theoretical findings support an integrative approach in which models of DSD aid in determining when developmental-genetic explanations of homology apply and when other explanations such as stabilizing selection are needed. I argue that a phylogenetic monism about the definition and criteria of homology supports the integration of different explanations into a theory of homology better than pluralism does.

Homology is the fundamental determinant of the sameness of biological characters or traits. When two characters stand in a relation of homology, they belong to the same character kind. For example, the eyes of humans and birds are homologous as vertebrate eyes – that is, they are the same kind of character: vertebrate eyes. Although the concept of homology originated in pre-Darwinian comparative anatomy, it was subsequently revealed to be an evolutionary phenomenon caused by common descent. Contemporary investigators work roughly within the following generic evolutionary conception of homology:

Homology: Two characters in distinct organisms or taxa are homologous if they are genealogically connected by continuous descent from a common ancestor that had the same character.

The proximal articular surface angle of orientation (AO) of proximal phalanges of the hand and foot has been used to infer the locomotor profile of extinct Miocene catarrhines and early hominins. Previous work has found that joint orientation distinguishes quadrupedal from suspensory anthropoids. The purpose of this study is to expand on previous research by examining this feature within and across several primate clades, allowing us to investigate the potential influences of locomotion, substrate usage, hand posture, and phylogeny. We also report AO measurements in human proximal hand phalanges, allowing us to examine human skeletal variation within a wide comparative context. The angle of orientation was measured on manual proximal third phalanges of 21 extant anthropoid species using a Microscribe digitizer. Comparisons were made between locomotor groups within hominoids, platyrrhines, and cercopithecoids. Proximal phalanges of quadrupedal species were characterized by greater dorsal orientation than those of suspensory taxa in hominoids and atelids. In addition, arboreal quadrupeds had greater AO values than terrestrial quadrupeds within the Cercopithecoidea. However, within the terrestrial locomotor group, mean AO values did not differ between palmigrade and digitigrade taxa. Thus, while there appears to be a functional signal related to substrate usage, differences in use of hand postures when moving on the ground were not reflected in proximal joint orientation of the proximal phalanx. Finally, we measured relatively low AO values in human phalanges, which might be related to integration with serially homologous pedal phalanges that are under strong selective pressure related to bipedalism.

Canalization and developmental stability refer to the tendency of developmental processes to follow particular trajectories, despite external or internal perturbation. Canalization is the tendency for development of a specific genotype to follow the same trajectory under different conditions (different environments or different genetic backgrounds), while developmental stability is the tendency for the development of a specific genotype to follow the same trajectory under the same conditions. Morphological integration refers to the tendency for structures to show correlated variation because they develop in response to shared developmental processes or function in concert with other structures. All three phenomena are emergent properties of developmental systems that can affect the interaction of development and evolution. In this paper, we review the topics of canalization, developmental stability, and morphological integration and their relevance to primate and human evolution. We then test three developmentally motivated hypotheses about the patterning of variability components in the mammalian limb. We find that environmental variances and fluctuating asymmetries (FA) increase distally along the limb in adult macaques but not in fetal mice. We infer that the greater variability of more distal segments in macaques is due to postnatal mechanical effects. We also find that heritability and FA are significantly correlated when different limb measurements are compared in fetal mice. This supports the idea that the mechanisms underlying canalization and developmental stability are related. Finally, we report that the covariation structure of fore-and hindlimb skeletal elements shows evidence for morphological integration between serially homologous structures between the limbs. This is evidence for the existence of developmental modules that link structures between the limbs. Such modules would produce covariation that would need to be overcome by selection for divergence in hind-and forelimb morphology. Yrbk Phys Anthropol 45: 131-158, 2002.

In this study, new ultrastructural data on the nephridiogenesis in Epiperipatus biolleyi (Onychophora) are provided, and the general distribution of nephridial organs, their vestigia, and derivatives within the onychophoran body is revised. Transient anlagen of nephridial organs proved to be present in the anteriormost segment bearing the antennae. These nephridial anlagen were never found to open to the exterior in any developmental stage studied, but are nevertheless equipped with well-developed cilia. The ciliated nephridial canals are situated at the antennal bases, hence in a more dorsal position than in the remaining body segments. In postantennal segments, the nephridial anlagen constantly arise at the bases of the presumptive legs or their derivatives. These results provide the first evidence that onychophoran antennae are modified legs that retained the original arrangement of the nephridial anlagen at their bases, despite the evolutionary change in position and function of these legs. Current assumptions are accordingly confirmed that the onychophoran antennae and the first antennae in Mandibulata (Euarthropoda) are non-homologous, since they have to be attributed to different head segments. Although the fate of the homologue of the onychophoran antennae in the Euarthropoda remains to be clarified, insights obtained in the present study question previous claims that an ocular segment is absent in extant Mandibulata. q

Mammals exhibit a similar pattern of integration among homologous limb elements, the strength of which is believed to vary in response to selection for functional coordination or similarity. Although integration is hypothesized to primarily reflect the effect of genes intrinsic to limbs, extrinsic genetic or epigenetic factors may also affect the strength of integration through their impact on the magnitude and direction of skeletal variance or covariance. Such factors as neuromuscular coordination or bone-muscle interactions may therefore play a role in both canalization and the structure or magnitude of limb integration. If this were the case, then increased levels of locomotor activity would be predicted to increase canalization and the magnitude of covariation between limbs. To investigate whether postnatal activity levels can have a significant effect on variance within or covariance among homologous limb elements, we compared four groups of male mice from a long-term selective breeding experiment: (1) mice from lines bred for increased voluntary activity on running wheels and allowed free access to a wheel for 8 weeks beginning at weaning (''active''), (2) selected mice that did not have wheel access (''sedentary''), (3) active mice from non-selected control lines, and (4) sedentary control mice. Mice from selected lines that had wheel access ran significantly more than control-line mice. However, when controlled for activity, linetype, and body mass, results indicate few significant differences in means, variance, or covariation structure, and no significant differences in integration between limbs, suggesting that postnatal activity levels do not significantly affect canalization or integration of limb lengths. A possible explanation for this result is that whereas baseline levels of postnatal activity may help to maintain patterns of variance and integration, increased levels of activity do not further increase these measures. Investigations into disrupted epigenetic processes (e.g., via models in which neuromuscular coordination is impaired) are required to further test hypotheses about how canalization or integration of limb variation is affected by epigenetic factors.

A unique set of morphological characters based on the hemipteran (sensu lato) thorax are used to define the pleuron of Psylloidea. New external and internal topographical descriptions of pleurites of all three thoracic segments are provided based on observations of specimens from various genera representing the current taxonomic arrangement of Psylloidea. Variations in propleurite morphology and the anteroventral angle of the episternum among taxa are clarified. The mesothoracic pleural sulcus is found to be a distinct groove formed by the deep fossa of the pleural apophysis and is not a secondary structure, as assigned by previous authors. A newly discovered internal apodeme of the metathoracic trochantin and a serially homologous structure in the mesothorax isolate the trochantinal territory of the second segment. The metathoracic pleural sulcus appears to be pressed against the ventral edge of the metepimeron, as previously described for the mesothorax of certain species of other insects having a strongly developed meron. Use of morphological interpretations and newly discovered apodemes to assess primary homology of structures for phylogenetic and taxonomic studies is discussed. A glossary of standardized morphological terms for thoracic structures of non-heteropteran Hemiptera based on this study and other recent and former dissertations on the insect thorax is provided.

Mammals exhibit a similar pattern of integration among homologous limb elements, the strength of which is believed to vary in response to selection for functional coordination or similarity. Although integration is hypothesized to primarily reflect the effect of genes intrinsic to limbs, extrinsic genetic or epigenetic factors may also affect the strength of integration through their impact on the magnitude and direction of skeletal variance or covariance. Such factors as neuromuscular coordination or bone-muscle interactions may therefore play a role in both canalization and the structure or magnitude of limb integration. If this were the case, then increased levels of locomotor activity would be predicted to increase canalization and the magnitude of covariation between limbs. To investigate whether postnatal activity levels can have a significant effect on variance within or covariance among homologous limb elements, we compared four groups of male mice from a long-term selective breeding experiment: (1) mice from lines bred for increased voluntary activity on running wheels and allowed free access to a wheel for 8 weeks beginning at weaning (''active''), (2) selected mice that did not have wheel access (''sedentary''), (3) active mice from non-selected control lines, and (4) sedentary control mice. Mice from selected lines that had wheel access ran significantly more than control-line mice. However, when controlled for activity, linetype, and body mass, results indicate few significant differences in means, variance, or covariation structure, and no significant differences in integration between limbs, suggesting that postnatal activity levels do not significantly affect canalization or integration of limb lengths. A possible explanation for this result is that whereas baseline levels of postnatal activity may help to maintain patterns of variance and integration, increased levels of activity do not further increase these measures. Investigations into disrupted epigenetic processes (e.g., via models in which neuromuscular coordination is impaired) are required to further test hypotheses about how canalization or integration of limb variation is affected by epigenetic factors.

ABSTRACT In segmented animals it is possible to define morphological complexity as the degree of morphological differentiation of segments. A quantitative method for measuring morphological complexity of segmental patterns was devised by McShea in 1992, who introduced three geometrical indices. Here, we introduce a new index of morphological complexity and emphasize the possible decoupling between segmentation and segment differentiation and illustrate different patterns of variation within segmental series and how these could affect morphological evolution and evolvability. Concepts are illustrated by contrasting the segmental models of two groups of centipedes (Chilopoda): the elongate Geophilomorpha and the short-bodied Lithobiomorpha. A preliminary application of the new metric provides no evidence of macroevolutionary increase in morphological complexity of centipede segmental organization.

A unique set of morphological characters based on the hemipteran (sensu lato) thorax are used to define the pleuron of Psylloidea. New external and internal topographical descriptions of pleurites of all three thoracic segments are provided based on observations of specimens from various genera representing the current taxonomic arrangement of Psylloidea. Variations in propleurite morphology and the anteroventral angle of the episternum among taxa are clarified. The mesothoracic pleural sulcus is found to be a distinct groove formed by the deep fossa of the pleural apophysis and is not a secondary structure, as assigned by previous authors. A newly discovered internal apodeme of the metathoracic trochantin and a serially homologous structure in the mesothorax isolate the trochantinal territory of the second segment. The metathoracic pleural sulcus appears to be pressed against the ventral edge of the metepimeron, as previously described for the mesothorax of certain species of other insects having a strongly developed meron. Use of morphological interpretations and newly discovered apodemes to assess primary homology of structures for phylogenetic and taxonomic studies is discussed. A glossary of standardized morphological terms for thoracic structures of non-heteropteran Hemiptera based on this study and other recent and former dissertations on the insect thorax is provided.