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Experimental taxonomy; "biosystematics" 1 (13-Mar-03)


The process of observation, hypothesis generation, and experiment lies in between "the evolution of diversity" out there in the woods, and the process of devising and applying "concepts of taxa." This process has been called "experimental taxonomy," "biosystematics," and perhaps more commonly now, "evolutionary biology." It is the process by means of which we can try to understand how evolution works, and make sense of the patterns of variation that we observe at the different hierarchical levels that we can minimally insist are out there in the woods: populations, individuals, organs, cells, and molecules.

At the heart of these investigations is the question of what species are, how they arise, and what happens to them once they are in existence. Prior to the mid-19th c. only the first of these questions was meaningful. In the spirit of Linnaeus' writings, the task of the taxonomist was to correctly discern and catalog the products of divine creation. For botanists this task required a comprehensive understanding of plant structure in all its variety, as well as developing an encyclopedic knowledge of the flora of the entire world. The ideas of Darwin and Wallace, and of their followers, added the further dimension of discovering the geneological relationships that ravel together global diversity into a common evolutionary history.

Historically, this process of observation began in gardens, at first in botanical gardens where exotic plants could be grown and studied, and later in less formal settings in which specific questions were addressed, as by Darwin and Mendel. The 19th c. French botanist Jordan grew out successive generations of plants so as to demonstrate the constancy of patterns of variation within and between what he referred to as "elementary" species of genera in the Brassicaceae. We have subsequently learned that many of these "species" are in fact highly inbred lines within what are now considered to be the "real" species, although a few of Jordan's species correspond to ones that are still recognized. Garden cultivation provided the botanist with evidence of whether unusual variants were heritable, or merely contingent on environmental or other conditions in the place where they were first discovered. Moreover, as the consequences of hybridization became better understood, cultivation of progeny from these variants was used to demonstrate whether the novel combination of features was stable, or the features segregated as might be expected in the offspring of hybrids.

The different approaches to taxonomy include...

Morpho-geographic taxonomy

catalogs variation, seeking morphological and geographic discontinuities on the basis of which species can be recognized.

Underlying biological and historical processes may or may not be implicated as contributing to observed patterns. If they are, this is usually in the form of assumptions or hypotheses. Botanical classification has been criticized for the way in which it was assumed (by some) that classifications based on observed patterns of variation must necessarily reflect evolutionary history. Because the data used may be readily collected, typically from herbarium specimens, the success of this approach is the fact that we have a catalog of some 250,000 species of plants worldwide, many of them from remote areas that it are difficult to access.


Experimental taxonomy

In the 1920s the Swedish botanist Turesson insisted on the importance of non-morphological, physiological adaptations to the environment in detrmining whether a plant could survive in a particular place. This led him to use common garden experiments in order to study the extent to which population samples from different parts of a species' range exhibit long-lasting and heritable differences in habit, size, and time to flowering. Particular genotype-environment specializations were identified as ecotypes. The ecotype concept was refined by Turesson so as to emphasize its genetic component, by basing the distinction between ecotypes on their inability to exchange genes.

In the U.K. similar work was carried out under the rubric of "experimental taxonomy" by Gregor and co-workers. Their experiments tended to document the way in which plants exhibit continuous variation in response to environmental gradients.

In North America an ambitious series of experiments was set up along an altitudinal transect in California that ran from the coast to beyond the crest of the Sierra Nevada. This work represented a collaboration between a cytologist, Clausen, a taxonomist, Keck, and an ecologist and physiologist, Hiesey. Their results documented not only both continuous and discontinuous variation along their transect, but also the interplay between this variation and the occurrence of polyploidy.


Biosystematics

A critical observation that was made repeatedly in many of the species studied by Turesson, by Gregor et al., and by Clausen, Keck, and Hiesey, as well as in other species, was that simple concepts based on the assumption of regular outbreeding could not be applied uniformly to plants. Most plants are hermaphrodites, and are regularly capable of inbreeding. Some species in fact regularly inbreed, although many species have evolved mechanisms by which the frequency of inbreeding can be reduced. Some species are capable of vegetative propagation, unaided by humans, and produce plantlets that can be dispersed much like seed. Still other species produce seeds asexually. As a result of these observations it became obvious that the reproductive biology of plants was integral to an understanding of species limits and of the process of speciation.

The term "Biosystematy" was defined by Camp and Gilly (1943) as the attempt "(1) to delimit the natural biotic units and (2) to apply to these units a system of nomenclature adequate to the task of conveying precise information regarding their defined limits, relationships, variability, and dynamic structure. Biosystematics came to be understood as including the collection of observational and experimental data on the breeding system as part of the the basis for making taxonomic decisions.


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Further reading

Davis, P. H. & V. H. Heywood (1965). Principles of Angiosperm Taxonomy. Edinburgh, Oliver & Boyd.

Judd et al. (2002) - Chapter 6, The evolution of diversity.

Porter, C. L. (1967). Taxonomy of Flowering Plants. San Francisco, W. H. Freeman - Ch. 6, Concepts of taxa.


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