Two complementary approaches underlie the study of evolutionary morphology—one a direct result of the Darwinian revolution, the other with roots that can be traced back to pre-Darwinian times. The former focuses on the issue of diversity and the origin of adaptation. Morphological change is depicted as a chronological progression of various combinations of adaptations and preadaptations (e.g., Bock, 1977). To define a morphological adaptation one has to focus on the relationships between form and function, i.e., on the selective pressures that fuel evolutionary change. This approach is dominated by an attempt to characterize the organism-environment interrelationship via the concept of natural selection as the driving force for morphological diversification. Conversely, the second approach, adopted in this work, seeks the structural and morphogenetic unity within diverse forms irrespective of the functional or adaptive differences between them. Essentially typological, its method goes back to the ideas of the German Naturphilosophie and French transcendental schools of morphology (Russell, 1916), and it addresses the ordered diversity of forms from the perspective of internal organization.

A morphogenetic approach to the origin and basic organization of the tetrapod limb

Principles of Animal Taxonomy By Prof. George Gaylord Simpson. (Columbia Biological Series, No. 20.) Pp. xii + 247. (New York: Columbia University Press; London: Oxford University Press, 1961.) 35s. net.

/pdf/principles-of-animal-taxonomy-4gbxvwbmhr.pdf

Principles of Animal Taxonomy

http://aracnologia.macn.gov.ar/st/biblio/Sereno%202007%20(an)%20Logical%20basis%20of%20morphological%20characters.pdf

Logical basis for morphological characters in phylogenetics

Evolutionary developmental biology ("evo-devo") has revolutionized evolutionary biology but has had relatively little impact on systematics. We show that similar large-scale developmental changes in distantly related lineages can dramatically mislead phylogenetic analyses based on morphological data. Salamanders are important model systems in many fields of biology and are of special interest in that many species are paedomorphic and thus never complete metamorphosis. A recent study of higher-level salamander phylogeny placed most paedomorphic families in a single clade based on morphological data. Here, we use new molecular and morphological data to show that this result most likely was caused by the misleading effects of paedomorphosis. We also provide a well-supported estimate of higher-level salamander relationships based on combined molecular and morphological data. Many authors have suggested that paedomorphosis may be problematic in studies of salamander phylogeny, but this hypothesis has never been tested with a rigorous phylogenetic analysis. We find that the misleading effects of paedomorphosis on phylogenetic analysis go beyond the sharing of homoplastic larval traits by paedomorphic adults, and the problem therefore is not solved by simply excluding suspected paedomorphic characters. Instead, two additional factors are critically important in causing paedomorphic species to be phylogenetically "misplaced": (1) the absence of clade-specific synapomorphies that develop during metamorphosis in nonpaedomorphic taxa and allow their "correct" placement and (2) parallel adaptive changes associated with the aquatic habitat of the larval stage. Our results suggest that the effects of paedomorphosis on phylogenetic analyses may be complex, difficult to detect, and can lead to results that are both wrong and statistically well supported by parsimony and Bayesian analyses.

/pdf/ontogeny-discombobulates-phylogeny-paedomorphosis-and-higher-5el6fnh8c7.pdf

Ontogeny discombobulates phylogeny: paedomorphosis and higher-level salamander relationships.

Bolitoglossa occidentalis, a lowland salamander of Mexico and Guatemala, has a highly derived morphology. The features that are derived with respect to the condition in generalized members of the genus include the following: (1) small body size; (2) short tail; (3) fully webbed hands and feet; (4) reduction and loss of certain phalangeal elements; (5) fusion of carpals and tarsals; (6) absence of prefrontal bones; and (7) reduced skull ossification. The ontogeny of this species was analyzed quantitatively and compared with the patterns of growth and differentiation encountered in two morphologically generalized members of the genus, B. rostrata and B. subpalmata. Most of the derived features can be explained by invoking a single heterochronic process: truncation of development at a small size (most likely the product of early maturation). Therefore, B. occidentalis is a paedomorphic species whose morphology has been attained through the process of progenesis. This result supports Alberch's ('80a) prediction, based on functional analysis, that the principle adaptation to arboreality in B. occidentalis is small size; other derived morphological features are associated with the organism's truncated development and may have no adaptive significance. However, patterns of dissociation are found within this overall progenetic process. Some of these include the following: (1) accelerated growth rates of the metatarsals and first phalanges, and retarded growth rates of the second and third phalangeal elements; (2) dissociation between rates of ossification of the skull and the autopodial elements; and (3) dissociation between the timing of termination of the process of shape change during the ontogeny of the foot (the product of differential growth between digital and interdigital areas) and termination of growth in overall foot size (foot surface area). This later result illustrates the independence of morphogenetic phenomena (shape change) from processes of growth (size increase).

Heterochronic mechanisms of morphological diversification and evolutionary change in the neotropical salamander, Bolitoglossa occidentalis (Amphibia: Plethodontidae)

The basis of all comparative biology is the determination of phylogenetic relationships and lineages. The most difficult question in the determination of lineage is whether the similarity between two forms is the result of monophyly, parallel evolution, or convergent evolution. The erection of a new phylogeny or the testing of a previously proposed phylogenetic grouping involves several steps: (1) the relation of characters and their homologous character states-the morphocline; (2) determination within the morphocline of its primitive and derived states and their polarities; (3) evaluation of the character states by means of weighting analysis; and (4) construction of the new phylogeny. In this paper we have concentrated on the third step, and we have formalized a method for character state weighting. The criterion for character state importance is the information contained within the character and its states. Character states may be designated in increasing value according to the following groups: (I) lo...

J. L. Edwards

Papers

The Determination of Parallel or Monophyletic Relationships: The Proteid Salamanders-A Test Case