Nematode Taxonomy & Systematics 1 st lecture

Transcription

1 Nematode Taxonomy & Systematics 1 st lecture Systematics is the branch of biology that deals with classifying living beings: the diversity and interrelationships of living beings, both current organisms ("neontology") and prehistoric ones ("palaeontology"). It can be divided into three parts. Taxonomy - the describing and naming new taxa (a taxon is any specifically defined group of organisms). Taxonomic groups are used to categorize similar taxa for identification-like field guides. These do not necessarily represent evolutionary trends. Smaller taxonomic groups used to relate organisms at greater levels of similarities. Classification - the organization of information about diversity arranging them into a convenient, formal classification into a hierarchical system, and providing means of identifying them (e.g., diagnostic keys). Phylogeny - determination of the ancestral relationships of organisms, and the group's evolutionary history through time. Phylogenetics is the field of biology concerned with identifying and understanding the evolutionary relationships between the many different kinds of life on earth. Systematics, classification, taxonomy used interchangeably - not correct. Simpson (1961): Systematics is the study of the diversity of organisms and all their comparative and evolutionary relationships, including such topics as comparative anatomy, comparative ecology, comparative phsiology, comparative biochemistry. As a subtopic of systematics, classification is defined by Simpson as the orderiing of organisms into groups and taxonomoy is considered to be the study fo the principles and procedures of classification. Early Systematists: Correspondences by Association In shamanistic cultures, the world (including living organisms and inanimate objects) was classified according to the archetypal mandala - the four cardinal points, each associated with an element, a season, a deity, an animal, and so on, which constituted the underlying structure of the world (much as the elementary particles and forces in quantum physics is considered nowadays). This system of correspondence by association, which is metaphysical rather than scientific, reached great sophistication in the Chinese system of Five States of Change, the Indian doctrine of the 25 tattwas, and more recently (late 19th century) the 10 essences (sefiroth) of the Hermetic Qabalah. *Whereas Western thought developed the idea of elements as subtsances, and Indian thought as emenations, Chinese philosophy conceived of the five elements, or Wu Xing, as dynamic states of change. *The concept of Wu Xing is central to all elements of Chinese thought, including science, philosophy, medicine, astrology, andfengshui. Although the term is generally translated as "five elements", this is incorrect. The word Wu does indeed mean "five".

2 But there is no simple translation for Xing. Translations such as "five elements", "five agents", "five qualities", "five properties" "five states of change", "five courses", "five phases" and "five elementals", are all used. (Earth, Metal, Wood, Fire & Water) The Scala Natura A different slant on things was given by Plato and his student Aristotle, who introduced the idea of the scala natura - the scale of nature - according to which the natural world is interpreted in terms of the principle of plenitude, the overflowing abundance of the first principle or Godhead which creates successive beings. The further the beings are from the source the more ontologically impoverished they are. So you have formless matter right at the bottom, then rocks, plants, lower animals, higher animals, man, and finally spiritual and divine beings. This hierarchical view of the world - the Great Chain of Being, persisted through the middle ages and up until the scientific revolution of the seventeenth and eighteenth centuries, when it was replaced by a sort of monotheistic dualism - there is the material world or creation, and there is God in his heaven. Aristotle and the dawn of science Though Aristotle's work in zoology was not without errors, it was the greatest biological synthesis of the time, and remained the ultimate authority for many centuries after his death. Animals could be classified by their way of life, their actions, or by their parts. His observations on the anatomy of octopus, cuttlefish, crustaceans, and many other marine invertebrates are remarkably accurate, and could only have been made from first-hand experience with dissection. Aristotle described the embryological development of a chick; he distinguished whales and dolphins from fish; he described the chambered stomachs of ruminants and the social organization of bees; he noticed that some sharks give birth to live young -- his books on animals are filled with such observations, some of which were not confirmed until many centuries later. Aristotle's classification of animals grouped together animals with similar characters into genera (used in a much broader sense than the current Linnean definition) and then distinguished the species within the genera. He divided the animals into two types: those with blood, and those without blood (or at least without red blood). These distinctions correspond closely to our distinction between vertebrates and invertebrates. The blooded animals, corresponding to the vertebrates, included five genera: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes, and whales (which Aristotle did not realize were mammals). This basic division (except for the whales) was to be adopted by Linneus (for whom the "genera" became classes). The bloodless animals were classified as cephalopods (such as the octopus); crustaceans; insects (which included the spiders, scorpions, and centipedes, in addition to what we now define as insects); shelled animals (such as most molluscs and echinoderms); and "zoophytes," or "plant-animals," (e.g. corals) which supposedly resembled plants in their form.

3 The Linnean System In the 18th century the Swedish botanist Carl von Linne, better known under the Latinised form of his name, Linnaeus, developed what's known as the binomial system of classification, in order to simplify the chaotic state of affairs around at his time. (Some plants were given names ten words long for example!). He used Latin because that was the academic language of the time. The modern system of classification in current widespread use is the binomial hierarchical system introduced by Linnaeus. The Linnaean taxonomy is a formal system for classifying and naming living things based on a simple hierarchical structure, from most general to most similar The basic hierarchy as formulated by Linneus, is as follows: * Imperium ("Empire") - the phenomenal world * Regnum ("Kingdom") - the three great divisions of nature at the time - animal, vegetable, and mineral * Classis ("Class") - subdivisions of the above, in the animal kingdom six were recognised (mammals, birds, amphibians, fish, insects, and worms) * Ordo ("Order") - further subdivision of the above - the class Mammalia has eight * Genus - further subdivisions of the order - in the mammalian order Primates there are four. e.g. Homo * Species - subdivisions of genus, e.g. Homo sapiens. * Varietas ("Variety") - species variant, e.g. Homo sapiens europaeus. As can be seen, Linneus wrote in Latin, the standard intellectual language of the time. His hierarchical system still reflected the old medieval feudalistic worldview ("Order" for example referred to an order of monks). And concepts like evolution were alien to him. For Linneus and his contemporaries, the world and all it's creatures was created once and for all, by the Judaeo-Christian God. Nevertheless this basic formula, as set out in the 10th edition of his Systema Naturae, published in 1758, was and still is considered the foundation of all modern taxonomy (at least until the cladists came along! ;-) As time progressed changes were made. The rank of Empire is obviously superfluous, while Variety came to be used only by gardeners, insect collectors, etc. The use of Latin was replaced by the vernacular, although it is still retained in the actual generic and specific names. And two new ranks were erected - Phylum (or Division in the case of Plants) was added between Kingdom and Class, and Family between Order and Genus, giving seven hierarchical ranks in all. So, in this nested system of rankings, kingdoms are made up of phyla, phyla of classes, classes of orders, and so on; each higher rank including at least one and usually more subordinate members. This sevenlayered hierarchy is the version still used today.

4 Kingdom Phylum Class Order Family Genus Species The Darwinian Revolution: While Linneus founded taxonomy and classification, it was left to Charles Darwin in the 19th century to introduce the theory of evolution and hence make possible phylogenetic reconstruction; that is, the evolutionary relationships and history of the various groups of organisms through geological time (millions of years). That's where things really get interesting, because life as a dynamic process is much more fascinating than life as a static series of unchanging types. Darwin: "Our classification will come to be, as far as they can be so made, genealogies we have to discover and trace the many diverging lines of descent in our natural genalogies, by characters of any kind which have long been inherited." Evolutionary Systematics: Evolutionary classification: uses traditional taxonomic categories but gives special consideration to evolutionary relationships and biological attributes rather than to strict morphological relationships. That is, incorporated into group taxonomy are factors such as functional and morphological innovartions, adaptive range, and numbers of species - it tries to provide both the genealobical relationships between groups and the amount of evolutionary change or distance between them. Only Maggenti specifies that this is the scheme used to build his classifications. All others, including Siddiqi and Andrassy say they hope to include phylogeny as part of the scheme, but make no effort to use actual phylogeny per se. Unfortunately, attempts to provide a classification based on evolutionary distances can distort phylogenetic relationships and vice versa. Example: birds, dinosaurs, reptiles - Mayr says they are separate classes, but this clouds phylogeny.

5 The Cladistic revolution: Over the last few decades there has been a major paradigm shift in biology and systematics, with the Linnean system being pretty much superseded by the cladistic one. Cladistics is based not on morphological similarity (as in the Linnean system) but on ancestor and descent relationship (phylogeny). Cladistics - also called Phylogentic Systematics or Phylogenetic Taxonomy - is a method of classifying organisms by common ancestry, based on the branching of the evolutionary family tree. Cladistics is currently the most popular paradigm of phylogenetic classification in biological taxonomy. Based strictly on determining branching points in the ancestry of organisms, it establishes groups based on their shared, derived features (synapomorphies), while ignoring primitive features (plesiomorphies) inherited from ancestors. Organisms that share common ancestors (and therefore have similar features) are grouped into taxonomic groups called clades. Like Evolutionary Systematics (which it has currently supplanted) Cladism is a method of classification based on the evolutionary history of organisms, dividing organisms into meaningful groups and subgroups. It was developed by Willi Hennig, an entomologist, in 1950, but was not really accepted until the 1980s. 3-taxon examples: exercises with Ethan & Helen s plastic animals: Clades can be represented in terms of a cladogram. A cladogram is a branching diagram that depicts species divergence from common ancestors. They show the distribution and origins of shared characteristics. Cladistics is based on three principles (assumptions): 1. Groups of organisms are descended from a common ancestor. 2. At each node (divergence of a population), there are two branching lines of descendants. 3. Evolution results in modifications of characteristics over time. Cladistic Phylogenetic Systematics acknowledges only Monophyletic groupings as valid. Paraphyletic groups (accepted in the Linnean hierarchy and in Evolutionary Systematics) and Polyphyletic groups are rejected as invalid, as is the whole Linenan hierarchy above genus rank (although sometimes taxa such as family etc are used in a more limited context)

6 Examples of the difference between Traditional/Evolutionary and Cladistic Classification Schemes: Traditional Classification: Phenetic, Linnean systsem, grouped according to overall similarity Phylogeny: note the common ancestors to each lineage.

7 Phylogenetic relationships : Note the synapomorphies Cladistic Classification: Note congruence with evolutionary relationsihps There are numerous proposals to move to a cladistic classification scheme but two are probably the most high-profile: Phylocode: Biocode: NodePointing System: de Quieroz and Gauthier (1990, 1992, 1994) But see the backlash: Nixon & Carpenter 2000 We will take this up in greater detail when we discuss the Species Problem (Next time!)