Assembling the Tree of Life

Assembling the Tree of Life

Assembling the Tree of Life

Assembling the Tree of Life

Synopsis

This edited volume is provides an authoritative synthesis of knowledge about the history of life. All the major groups of organisms are treated, by the leading workers in their fields. With sections on: The Importance of Knowing the Tree of Life; The Origin and Radiation of Life on Earth; The Relationships of Green Plants; The Relationships of Fungi; and The Relationships of Animals. This book should prove indispensable for evolutionary biologists, taxonomists, ecologists interested inbiodiversity, and as a baseline sourcebook for organismic biologists, botanists, and microbiologists. An essential reference in this fundamental area.

Excerpt

Michael J. Donoghue

Joel Cracraft

Many, perhaps even most, people today are comfortable with the image of a tree as a representation of how species are related to one another. The Tree of Life has become, we think, one of the central images associated with life and with science in general, alongside the complementary metaphor of the ecological Web of Life. But this was not always the case. Before Darwin, the reigning view was perhaps that life was organized like a ladder or “chain of being,” with slimy “primitive” creatures at the bottom and people (what else!) at the very top. Darwin (1859) solidified in our minds the radically new image of a tree (fig. I.1), within which humans are but one of many (as we now know, millions) of other species situated at the tips of the branches. The tree, it turns out, is the natural image to convey ancestry and the splitting of lineages through time, and therefore is the natural framework for “telling” the genealogical history of life on Earth.

Very soon after Darwin, interest in piecing together the entire Tree of Life began to flourish. Ernest Haeckel’s (1866) trees beautifully symbolize this very active period and also, through their artistry, highlight the comparison between real botanical trees and branching diagrams representing phylogenetic relationships (fig. I.2).

However, during this period, and indeed until the 1930s, rather little attention was paid to the logic of inferring how species (or the major branches of the Tree of Life) are related to one another. In part, the lack of a rigorous methodology (especially compared with the newly developing fields of genetics and experimental embryology) was responsible for a noticeable lull in activity in this area during the first several decades of the 1900s. But, beginning in the 1930s, with such pioneers as the German botanist Walter Zimmermann (1931), we begin to see the emergence of the basic concepts that underlie current phylogenetic research. For example, the central notion of “phylogenetic relationship” was clearly defined in terms of recency of common ancestry—we say that two species are more closely related to one another than either is to a third species if and only if they share a more recent common ancestor (fig. I.3).

This period in the development of phylogenetic theory culminated in the foundational work of the German entomologist Willi Hennig. Many of his central ideas were put forward in German in the 1950s (Hennig 1950), but worldwide attention was drawn to his work after the publication of Phylogenetic Systematics in English (Hennig 1966). Hennig emphasized, among many other things, the desirability of recognizing only monophyletic groups (or clades—single branches of the Tree of Life) in classification systems, and the idea that shared derived characteristics (what he called synapomorphies) provided critical evidence for the existence of clades (fig. I.4).

Around this same time, in other circles, algorithms were being developed to try to compute the relatedness of species. Soon, a variety of computational methods were implemented and were applied to real data sets. Invariably, given the tools available in those early days, these were what would now be viewed as extremely small problems.

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