Phylogeny

History of Phylogenetics

Phylogeny was discussed in detail by the 19th-century German morphologist Ernst Haeckel, who proposed a biogenetic law (or the law of recapitulation). The biogenetic law states that phylogeny, or the evolutionary history of an organism, is recapitulated through its ontogeny, or the development of an individual organism in embryo. The subsequent rejection of Haeckel's law was a significant move away from using mechanical explanations or causes, such as embryonic development, to explain the relationship between organisms. Haeckel's most significant contribution was that of the phylogenetic tree (Pbylogenetiscbes Stambaum), the now universally accepted way to depict genealogical relationships.

A phylogenetic tree may depict hypothetical ancestor-descendant relationships, sometimes called a transformation series, between groups of organisms (species, genera, and families) or their characteristics, through time. Such phylogenetic trees have been popular tools of paleontologists who use them to establish so-called ghost lineages between similar-looking fossils throughout the stratigraphie [Page 992]record. Phylogenetic trees were challenged in the early 20th century by the German-speaking systematic morphologists, led by Adolf Naef. The evolutionary relationships that phylogenetic trees were claimed to depict were based on linking similar-looking organisms that overlapped through time, rather than considering relationships of form.

The systematic morphologists considered homologues (different manifestations of the same morphological structure) to be a sounder basis for the discovery of relationship than the assembly of ghost lineages. If organisms are related, their characters are homologous, that is, the same; as opposed to analogous, that is, similar but not the same. Naef's trees related organisms only at the terminal branches, rather than depicting hypothetical lineages, with organisms (hypothetical or real) at both the nodes and tips. Homologous organisms belonged to “natural groups or classifications” that share a greater relationship among themselves than they do to any other group.

The rejection of phylogenetic trees and the concomitant support for natural groups was criticized by Anglo American phylogeneticists such as George Gaylord Simpson and Ernst Mayr, who defended the depiction of lineages in phylogenetic trees rather than the discovery of natural groups, which challenged some traditional taxonomie groups. Anglo American phylogenetics, however, changed considerably in the latter half of the 20th century when the work of Willi Hennig, a German entomologist, was translated into English.

Phylogenetic Systematics

Hennig's Phylogenetic Systematics attempted to resurrect Haeckel's systematic phylogenetics by reintroducing the causal mechanisms that had been rejected by Adolf Naef. Hennig's phylogenetic systematics combined Haeckel's transformational viewpoint—but at the level of character rather than taxon—with Naef's trees of relationships to form ancestor-descendant schemes of relationship with organisms only at the tips, and character transformations leading from the nodes to the tips. The resulting trees attempted to group homologous organisms into “natural” or mono-phyletic classifications based on a causal mechanism, thus combining Haeckel's phylogenetic tree with Naef's systematic morphology.

Phylogenetic systematics developed into a numerical method by incorporating the principal notion of phenetics, that is, similarity concepts, with a causal mechanism to find optimal trees.

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