Imagine if we could compute across phenotype data as easily as genomic data; this article calls for efforts to realize this vision and discusses the potential benefits.

We demonstrate that by formulating guidelines for evolutionary morphology the transparency, reproducibility, and intersubject testability of evolutionary hypotheses based on morphological data can be enhanced. The five main steps in our concept of evolutionary morphology are (i) taxon sampling, (ii) structural analysis, (iii) character conceptualization, (iv) phylogenetic analysis, and (v) evolutionary interpretation. We illustrate this concept on the example of the morphology of the circulatory organs in peracarid Malacostraca. The analysis is based on recently published accounts in which detailed structural analyses were carried out, and on the older literature. Detailed conceptualizations of 22 characters of the circulatory system are given for 28 terminals. In a further step these characters are included in a recently revised matrix, resulting in 110 characters. The resulting parsimony analysis yielded a single most parsimonious tree with a length of 309 steps. The most significant results are that Peracarida is monophyletic, Amphipoda is the sister taxon to the Mancoida sensu stricto, the relict cave-dwelling taxa Thermosbaenacea, Spelaeogriphacea, and Mictocarididae form a monophylum and Tanaidacea is the sister group to a monophylum comprising Cumacea and Isopoda. The evolutionary analysis shows that the ground pattern features of the circulatory organs in Peracarida are a tubular heart extending through the whole thorax, a posterior aorta with lateral arteries, and a ventral vessel system. Important features within the Peracarida are the backward shift of the anterior border of the heart, the reduction of the ventral vessel system, and two patterns of cardiac arteries, one common to the amphipod and tanaidacean terminals, and one to the cumacean and isopod terminals.

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Throughout the history of biology since the time of Goethe, morphology as a discipline has been not only descriptive but explanatory too. Both, morphology itself and its central concept, homology, are pre-Darwinian in origin, and as a result dubious in the eyes of many. Although morphology has taken evolution into account since Darwin, its contribution as a scientific discipline to evolutionary biology is a matter of dispute. This paper can be regarded as the conceptualization of a research program for Evolutionary Morphology, a term we use to characterize the field in evolutionary biology which includes the description, comparison and explanation of all the predicates (i.e. form and function) of phenotypic objects. Evolutionary Morphology deserves a central place in an extended evolutionary synthesis. The descriptive aspect of Evolutionary Morphology describes and documents the form of parts of an organism (referred to here as morphemes). In comparative morphology, evolutionary units are identified and their homology between species is tested. Then, within the framework of phylogenetic analyses, putatively homologous evolutionary units (character states) are arranged in transformation series (characters in cladistic terminology), tested against each other and, ultimately, ordered chronologically. Establishing the relative chronological order of evolutionary units is the main goal of the phylogenetic analyses conducted in Evolutionary Morphology. Phylogenetic analyses form the basis of the explanatory aspect of Evolutionary Morphology, the area of 'causal morphology'. Evolutionary units are the result of adaptation and need to be studied in terms of potential selective forces. Their evolvability, however, is limited by material constraints. In addition, coherence may exist both between morphemes (e.g. by architectural constraints) and between evolutionary units, and this too is important for our understanding of evolutionary transformations. In this context, it is crucial to note that to reach a causal understanding of the predicates of morphemes, it is important to remember that they all undergo a process of development, and that ultimately, changes in developmental pathways are responsible for changes in the predicates in adults. Understanding these developmental pathways and the genetics behind them is indispensable if we are to understand morpheme form and the transformation of evolutionary units. The discipline of Evolutionary Developmental Biology (evo-devo) focuses on precisely these questions. Its results, yet, do not replace those of Evolutionary Morphology but supplement them. None of the aspects of the area of 'causal morphology' listed is exclusive; they are complementary in their contribution to our understanding of phenotypes. They all embody different approaches which are indispensable to our understanding of form. Evolutionary Morphological investigations take place within research cycles, which implies that findings in the area of causal morphology might influence the descriptive and comparative ...

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