Hypothetical Ancestors and Rooting in Cladistic Analysis

Most hypothetical ancestors that are used to root trees in cladistic analyses summarize character-state information in one or more outgroup taxa. Nonetheless, hypothetical ancestors also provide a means of rooting trees using the ontogenetic and paleontological methods of polarizing character transformations, and for incorporating the inferences of more than one of these methods into a single analysis. However, the use of one hypothetical ancestor that combines inferences based on outgroup comparison with those based on other methods of polarizing character transformations to root a cladogram is invalid. Inferences regarding plesiomorphic character states based on outgroup comparison apply to the outgroup node, whereas inferences based on either the ontogenetic or paleontological method apply to the ingroup node. These inferences cannot be combined into a single hypothetical construct. A hypothetical ancestor based on outgroup information is included in the data matrix and used to root the resulting network; however, because this ancestor places potentially problematic constraints on the analysis, the use of actual outgroup taxa is preferable in most instances. Correct use of a hypothetical ancestor inferred with the ontogenetic and paleontological methods involves the Lundberg method in which the shortest ingroup network is rooted at the internode to which the hypothetical ancestor attaches most parsimoniously. Because inferences of polarity based on outgroup comparison cannot be combined directly with those based on other polarization methods, the synthesis of information from all three methods in a single tree must involve taxonomic congruence.     

Ancestors and variants: tales from the cryptic

SUMMARY Those who work at the interface of development and evolution are united by the conviction that developmental comparisons can shed light on both the evolution of specific morphologies and the macroevolutionary process itself. In practice, however, the field comprises a diversity of approaches. As the field grows and practitioners attempt to digest a growing mountain of comparative data, the various approaches of "Evo Devo" have themselves evolved. A meeting organized by the authors and held at the University of Chicago in the Spring of 1999 illustrated some of these changes. This review will draw on its content to discuss recent developments in two areas: the reconstruction of common ancestors and the developmental basis of evolutionary change.     

When is homology not homology?

Although genes have specific phenotypic consequences in a given species, this functional relationship can clearly change during the course of evolution. Many cases of evolutionary dissociations between homologous genes and homologous morphological features are now known. These dissociations have interesting and important implications for understanding the genetic basis for evolutionary change in morphology.     

Functional homology and homology of function: biological concepts and philosophical consequences

“Functional homology” appears regularly in different areas of biological research and yet it is apparently a contradiction in terms—homology concerns identity of structure regardless of form and function. I argue that despite this conceptual tension there is a legitimate conception of ‘homology of function’, which can be recovered by utilizing a distinction from pre-Darwinian physiology (use versus activity) to identify an appropriate meaning of ‘function’. This account is directly applicable to molecular developmental biology and shares a connection to the theme of hierarchy in homology. I situate ‘homology of function’ within existing definitions and criteria for structural assessments of homology, and introduce a criterion of ‘organization’ for judging function homologues, which focuses on hierarchically interconnected interdependencies (similar to relative position and connection for skeletal elements in structural homology). This analysis of biological concepts has at least three broad philosophical consequences: (1) it provides the grounds for the study of behavior and psychological categories as homologues; (2) it demonstrates that philosophers who take selected effect function as primary effectively ignore large portions of comparative, structural, and experimental research, thereby misconstruing biological reasoning and knowledge; and, (3) it underwrites causal generalizations, which illuminates inferences made from model organisms in experimental biology.

Good characters and homology

This paper is a critical comment on a recent article by Lieberman. 1 We question his opinion that DNA is a better data source for phylogenetic reconstructions than bone and discuss his “problems and potential solutions” regarding the homology concept. We conclude that phylogenetic systematics requires a phylogenetic homology concept, and that Lieberman's “solutions,” though useful terms, should not be designated as homology.     

On homology

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. ©The Will Henning Society 2011.