Higher Categories and Taxa
University of the Punjab
Zainab Saeed Z08-13 7th semester Bsc (Hons.) Zoology University of Punjab
Submitted To:
Mrs Dr. Abida Butt
Table of Contents Higher Categories and Higher Taxa......................................................................................................... 4 The Genus: .......................................................................................................................................... 4 Generic Characters:......................................................................................................................... 5 Meaning of a genus:........................................................................................................................ 5 The Family: .......................................................................................................................................... 6 Orders, Classes and Phyla: .................................................................................................................. 6 The Process of Ranking ........................................................................................................................... 7 Relationship and Similarity ..................................................................................................................... 7 Homology: ........................................................................................................................................... 8 Serial Homology: ............................................................................................................................. 9 Analogy: .......................................................................................................................................... 9 Homoplasy: ..................................................................................................................................... 9 Convergence in characters:................................................................................................................. 9 Parallel Characters: ....................................................................................................................... 10 Reversed characters:.....................................................................................................................11 Difficulties Encountered in Macrotaxonomy ........................................................................................ 11 Mosaic Evolution:..............................................................................................................................11 Fossils: ............................................................................................................................................... 12 Fossils & Converging Evidence ...................................................................................................... 13 Fossils & Scientific Predictions ...................................................................................................... 14 The Improvement of Existing Classification.......................................................................................... 14 Stability: ............................................................................................................................................ 15 The Printed Sequence: .................................................................................................................. 16 Graphical Representation ............................................................................................................. 16 Phylogenetic Trees: ........................................................................................................................... 16 Importance of Sound Classification: ................................................................................................. 17 References: ...........................................................................................................................................18
Higher Categories and Higher Taxa ―A higher category is a class into which all the higher taxa that are ranked at the same level in a hierarchic classification are placed‖ This means that the taxa which are on the same level in the classification are given some rank and the category in which they are placed indicated their rank in the hierarchical classification. When we say that categories are based on concepts and taxa are based on zoological realities, it means that organisms are placed in taxa while the categories are some concept on which the taxa are arranged in categories. In Linnaean hierarchy there is no difference between species category and higher categories but in other respects it is quite different. The species category actually is signifies singularity, distinctness and differences but the other higher categories are based on the comparison among each other. In species categories all the taxa are placed according to their individual characters and their uniqueness but in the other higher categories, the taxa are compared and those which have similarities are placed together. They are arranged according to affinities among group of species. The taxon is given a certain limit and according to its characters it is placed in a higher category, as long as it is consistent with the theory of common decent. The higher taxa are themselves separated by a certain gap from other taxa of same rank. Meaning there are different taxa in a category and these taxa are separated from one another by some limit or boundary in characteristics. Another difference in higher categories and species is that in higher categories differentiation is through comparative studies which delimit the taxa and placed into higher categories but in species we use the concept of interbreeding and producing fertile offspring. Darwin stated the matter of hierarchy as; ―The natural system is genealogical in the arrangement, like a pedigree; but the degree of modification which the different groups have undergone has to be expressed by ranking them under different so-called genera, sub-families, families, sections, orders, and classes.‖ It must be noted that the higher categorical rank evolve from a lower rank not the other way round. Because when we classify, we only have the organism and on that basis, we make all the ranks of classification. Different criteria and operations for ranking are employed by different schools of macrotaxonomy.
The Genus: ―The genus is the lowest obligatory higher category and the lowest of all categories established strictly by comparative data (Cain, 1956).‖
A modern pragmatic definition of genus is as follows. ―The genus is the obligatory taxonomic category directly above that of the species in the Linnaean hierarchy.‖ The generic taxon or genus is a monophyletic group containing one or more species and are separated from other generic taxa by a decided gap. It is recommended that the size of the gap should be the inverse of the side of the taxon. The more the species in a group, easier it is to recognise it as a separate genus i.e. less gap is required. Smaller the species group more the gap required. Delimiting the species group as genera required a lot of experience, good judgement, and common sense. Generic Characters:
It is the genus that gives characters not the characters that makes the genus.
This is generally valid. The species included in the genus have many characters in common and the recognition of higher presence of correlated complex. This may include some minute and inconspicuous characters but as Darwin said; ―The importance, for classification, of trifling characters, mainly depends on their being correlated with several other characters of more or less importance. The value indeed of an aggregate of characters is very evident in natural history.‖ This principle led to the many generic splitting. Whenever a new character was discovered it often led to the formation of new genera. Many genera can’t be diagnosed on the basis of a single character. Meaning of a genus:
Whenever we assign a generic rank to group of species, we always try to describe the characters of all the species in that genus. Genus is a phylogenetic unit. This means that all the species in genus have been descended from near ances tors. Sometimes the genus is an ecological unit, consisting of species which have been adapted for same kind of environment. Species of the same genus also possess genetic identity. It is also possible that the species of the same genus can produce hybrids. Dubois has gone so far as to demand that all the species that produce hybrids, be placed in the same genus. For the recognition of generic taxon, ―Where alternative are available, we stand by the concept or theory that are more useful-the one that generalizes the most observation and permits the most reliable predictions‖ (Inger 1958:383).
The Family: A non-arbitrary definition of the family category is not possible. We can say that a kind of animal is often a family. To determine how distinctive a group of genera be to be classified as family differ from one group of thought to another. There is no definite criterion which indicated the rules for the family classification. Family can be defined as taxa; ―The family is a taxonomic category for a taxon composed of a sin gle genus or a group of related genera. It is separated from other families by a decided gap.‖ As in the case of genus the size of the gap is inverse ratio to the size of the family. The family is distinguished by certain adaptive character in a much greater extent than genera. The more distinct the character, the greater is the gap between the families. The families are older than genera and have a worldwide distribution. If an entomologist has 422 families of British insects and goes to Africa he will see almost all the families there two. The characters of a family are especially important for a general zoologist as the each family presents some general characters that can be recognized at a glance, so one can easily recognize the members of that family easily. For example in spiders each family has some general characters which separate it from other families. The family Oxyopidae has large front eyes which is its distinguishing character. At a given locality the various families are generally distinct. They have their gaps which separate them. It is sometimes the case that when broadening the spectrum of families, we encounter some difficulties. Families are known to form some distinctive group in each continent. So we have to make a larger group also known as a super family. Some families were based on homoplasy. These members were to be placed in the different families or separate families were to be made. Linnaeus did not recognise the family as a category but many of his genera have been elevated to the family rank. This shoes that there was a little difference between Linnaean genera and our families. With only 312 genera of animals, Linnaeus had no need for an intermediate category between genus and order. Now many new animals have been discovered, so new families have been formed. Nowadays 5600 families of Metazoa and 580 families of protozoa, totalling approximately 6200 families, have been formed.
Orders, Classes and Phyla: These highest categories above the family are, on the whole, very well defined. The taxa ranked in these highest categories represent the main branches of the phylogenetic tree. They are characterized by a basic structural pattern laid down early in evolutionary history. Taxa in higher categories are definable in terms of a basic structural pattern, but except for certain highly specialized groups, the higher taxa are not primarily or even predominantly distinguished by special adaptations. The taxa included in higher categories are widely distributed in space and time.
According to the recent tabulations, there are approximately 29 phyla, 144 classes, and 722 orders of recent animals.
The Process of Ranking The process of ranking is not complete when species are placed in genera, because these genera have to be placed in family and these families into higher taxa until Linnaean hierarchy is complete. The reason for the hierarchy was clear to Darwin as he saw that diversity must be originated after speciation and that chance and adaptive processes were responsible for the gradual evolution of higher taxa and still higher taxa separated by the gap of divergent evolution and extinction. The three major schools of macrotaxonomy differ in the matter of classification. Phenetic system of classification is that, that does not try to reflect evolutionary relationships; instead it is based on physical similarities among organisms (phenotype); organisms are placed in the same category because they look alike. The pheneticist, only consider the similarities in characters in classification. Cladistic system of classification is based on the phylogenetic relationships and evolutionary history of groups of organisms. Cladists who follow Henning introduce a new rank at each branching point of cladogram and give sister groups identical categorical rank. For classical taxonomist, ranking results from the degrees of difference found among taxa; much divergence from the ancestral condition requires that a taxon be given a higher rank. Classifications proposed by the Cladists are on the whole rather more elaborate than those of evolutionary taxonomists, because Cladists want their classifications to reflect as minutely as possible the actual branching pattern of the genealogy (Wiley 1981:199-238); gaps are consciously ignored. Evolutionary taxonomist tends to emphasize major groupings and the existence of major gaps.
Relationship and Similarity Relationship is used in different terms. Pheneticists take this relationship in only similarities of characters while Cladists take this relationship as only in genealogy. The evolutionary taxonomist consider both ancestor-descendent relationship and collateral relationship among sister lineages, while Cladists consider only holophyletic lineages. Developed mostly on the basic
Relationship between taxa is generally according to the similarities but sometimes similarities can lead to false grouping. This can be solved by the careful analysis of taxonomic characters. In this case one must distinguish between different potential causes of similarities.
Homology: Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying structure of bones and muscles. Owen reasoned that there must be a common structural plan for all vertebrates, as well as for each class of vertebrates. Homologous traits of organisms are due to sharing a common ancestor, and such traits often have similar embryological origins and development. This is contrasted with analogous traits: similarities between organisms that were not present in the last common ancestor of the taxa being considered but rather evolved separately. An example of analogous traits would be the wings of bats and birds, which evolved separately but both of which evolved from the vertebrate forelimb and therefore have similar early embryology. Whether or not a trait is homologous depends on both the taxonomic and anatomical levels at which the trait is examined. For example, the bird and bat wings are homologous as forearms in tetrapods. However, they are not homologous as wings, because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods. By definition, any homologous trait defines a clade — a monophyletic taxon in which all the members have the trait (or have lost it secondarily); and all non-members lack it. A homologous trait may be homoplasious – that is, it has evolved independently, but from the same ancestral structure – plesiomorphic – that is, present in a common ancestor but secondarily lost in some of its descendants – or (syn)apomorphic – present in an ancestor and all of its descendants. The word homology, coined in about 1656, derives from the Greek homologos, where homo = agreeing, equivalent, same + logos = relation. In biology, two things are homologous if they bear the same relationship to one another, such as a certain bone in various forms of the "hand." Ray Lankester defined the terms "homogeny", meaning homology due to inheritance from a common ancestor, and "homoplasty", meaning homology due to other factors. As most problems in science, obvious hypothesis are accepted provisionally unless they lead to logical contradictions. The establishment of homologies ranges from simple comparison of features of closely related species, where the matter need hardly be given a second thought, to the frustratingly difficult comparison of dissimilar features in higher taxa.
Serial Homology:
Serial homology is the representative or repetitive relation in the segments of the same organism, as in the lobster, where the parts follow each other in a straight line or series. This was coined by Owen (1866). Serial homology is the concept that initially existing structures were gradually modified via discrete intermediary steps until such time as an evolutionary novelty (e.g., jaws) appeared.
Many examples of serial homology, e.g. the body segments of many animals (vertebrates, arthropods etc.), are examples of gene duplication on regulatory genes such as homeobox genes, followed by evolution differentiating the duplicated genes. Analogy:
In biology, an analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution. The classical example of an analogy is the ability to fly in birds and bats. Both groups can move by powered flight, but flight has evolved independently in the two groups. The ability to fly does not make birds and bats close relatives. The opposite of analogy is homology, where the ability or organ in question has been inherited from a common ancestor. The British anatomist Richard Owen was the first scientist to recognise the fundamental difference between analogies and homologies, and named them. Analogous traits will often arise due to convergence, where different species live in similar ways and/or similar environment, and thus face the same environmental factors. Both herrings and dolphins are streamlined. Both are active predators in a high drag environment, but the herring is a bony fish, the dolphin a mammal. In the Mesozoic, similarly streamlined ichthyosaurs navigated the world’s oceans, yet another example of a group evolving a similar shape due to the same environmental factors. A similar phenomenon is earless seals and eared seals. It was long debated whether the two groups are a single marine group, or represent two separate episodes of carnivores turning to a marine environment. Homoplasy:
Homology means the similarity due to the common ancestor. Homoplasy, on the other hand, means similarity due to convergent evolution, but independent origins. For instance, take the fin and the caudal fin of tuna and of dolphin; they are similar but have independent histories, and their similarity comes from adaptation to similar environments and functions. This is homoplasy. However, the fin of tuna and bonito are similar because of the common ancestor, and that's homology. The attempt to determine whether an observed similarity is a genuine homology or a homoplasy ought to be an indispensable component of every taxonomic analysis. Unfortunately, it is altogether ignored in unweighted phenetic procedures and often insufficiently considered in the construction of shortest trees.
Convergence in characters: Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.
The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are similar in construction, due to the physical constraints imposed upon wing shape. Similarity can also be explained by shared ancestry. Wings were modified from limbs, as evidenced by their bone structure. Traits arising through convergent evolution are termed analogous structures, in contrast to homologous structures, which have a common origin. Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. Similarity in species of different ancestry that is the result of convergent evolution is called homoplasy. The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes. Convergent evolution is similar to, but distinguishable from, the phenomena of evolutionary relay and parallel evolution. Evolutionary relay describes how independent species acquire similar characteristics through their evolution in similar ecosystems at different times — for example the dorsal fins of extinct ichthyosaurs and sharks. Parallel evolution occurs when two independent species evolve together at the same time in the same ecospace and acquire similar characteristics for instance extinct browsing-horses and paleotheres. Similarity can also result if organisms occupy similar ecological niches that is, a distinctive way of life. A classic comparison is between the marsupial fauna of Australia and the placental mammals of the Old World. The two lineages are clades that is, they each share a common ancestor that belongs to their own group, and are more closely related to one another than to any other clade — but very similar forms evolved in each isolated population. Many body plans, for instance sabre-toothed cats and flying squirrels, evolved independently in both populations. In some cases, it is difficult to tell whether a trait has been lost then re-evolved convergently, or whether a gene has simply been 'switched off' and then re-enabled later. Such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused gene has a reasonable probability of remaining in the genome in a functional state for around 6 million years, but after 10 million years it is almost certain that the gene will no longer function. Convergent characters are mostly found when different animals become adapted to similar niches. For example, loons and grebes, which are both diving birds agree in numerous structural characters, particularly of legs, yet are only very distantly related to each other. Many marsupial adaptive types (wolves, mice, moles. badgers etc.) are remarkably similar to analogous placental types; the similarity is due to selection for similar modes of life. Parallel Characters:
Similar characters derived independently by related taxa with a similar genetic background cause systematists the most trouble. These characters range from distinctive to rather simple characters. Characters that evolve from parallelism are not homologous because they are not ―derived from the same phenotypic feature of their nearest common ancestor.‖ This interpretation is most congenial to taxonomists who are simply concerned with the construction of a character matrix.
Some evolutionary taxonomist consider characters due to parallelism to be homologous, the synamorphy is the potential to develop the character. Reversed characters:
Phylogenists have tended to consider morphological change, an inexorably advancing process. Character analysis, however, remarkably often shows that what appears to be primitive character are actually reversals (psuedoprimitiveness). There is much evolutionary reversal owing to the loss of specialization or other derived characters. Recent cladistic analysis shows that these reversals are very much common. They generally affect a single character of a character complex and can be discovered by character analysis. However, Dolo’s rule, according to which a more or less complex structure that has been lost is not reacquired in the same complexity, has few if any exceptions.
Difficulties Encountered in Macrotaxonomy No matter what school of macrotaxonomy do the scientists belong to, they encounter many problems while classification and speciation. Some of the difficult situations that are not always considered by taxonomists require careful analysis.
Mosaic Evolution: Mosaic evolution (or modular evolution) is the concept that evolutionary change takes place in some body parts or systems without simultaneous changes in other parts. Another definition is the "evolution of characters at various rates both within and between species". Its place in evolutionary theory comes under long-term trends or macroevolution. In the Neo-Darwinist theory of evolution, as postulated b y Stephen Jay Gould, there is room for differing development, when a life form matures earlier or later, in shape and size. This is due to allomorphism. Organs develop at differing rhythms, as a creature grows and matures. Thus a "heterochronic clock" has three variants: 1) Time, as a straight line; 2) General size, as a curved line; 3) Shape, as another curved line. When a creature is advanced in size, it may develop at a smaller size; alternatively, it may maintain its original size or, if delayed, it may result in a larger sized creature. That is insufficient to understand heterochronic mechanism. Size must be combined with shape, so a creature may retain paedomorphic features if advanced in shape or present recapitulatory appearance when retarded in shape. These names are not very indicative, as past theories of development were very confusing. A creature in its ontogeny may combine heterochronic features in six vectors, although Gould considers that there is some binding with growth and sexual maturation. A creature may, for example, present some neotenic features and retarded development, resulting in new features derived from an original creature only by regulatory genes. Most novel human features (compared to closely related apes) were of this nature, not implying major change in structural genes, as was classically considered.
By its very nature, the evidence for this idea comes mainly from palaeontology. It is not claimed that this pattern is universal, but there are now a wide range of examples from many different taxa. Some examples:
The early evolution of bipedalism in Australopithecines, and its modification of the pelvic girdle took place well before there was any significant change in the skull, or brain size. Archaeopteryx. Nearly 150 years ago Thomas Henry Huxley compared Archaeopteryx with a small theropod dinosaur, Compsognathus. These two fossils came from the Solnhofen limestone in Bavaria. He showed that the two were very similar, except for the front limbs and feathers of Archaeopteryx. Huxley's interest was in the basic affinity of birds and reptiles, which he united as the Sauropsida. The interest here is that the rest of the skeleton had not changed. Meadow voles during the last 500,000 years. The pterosaur Darwinopterus. The type species, D. modularis was the first known pterosaur to display features of both long-tailed (rhamphorhynchoid) and shorttailed (pterodactyloid) pterosaurs. Evolution of the horse, in which the major changes took place at different times, not all simultaneously. Mammalian evolution, especially during the Mesozoic is undoubtedly one of the best examples.
Fossils: The fossil record has one important, unique characteristic: it is our only actual glimpse into the past where common descent is proposed to have taken place. As such it provides invaluable evidence for common descent. The fossil record is not "complete" (fossilization is a rare event, so this is to be expected), but there is still a wealth of fossil information. If you look at the fossil record, you find a succession of organisms that suggest a history of incremental development from one species to another. You see very simple organisms at first and then new, more complex organisms appearing over time. The characteristics of newer organisms frequently appear to be modified forms of characteristics of older organisms. This succession of life forms, from simpler to more complex, showing relationships between new life forms and those that preceded them is strong inferential evidence of evolution. There are gaps in the fossil record and some unusual occurrences, such as what is commonly called the Cambrian explosion, but the overall picture created by the fossil record is one of consistent, incremental development. At the same time, the fossil record is not in any way, shape, or form suggestive of the idea of sudden generation of all life as it appears now, nor does it support transformationism. There is no way to look at the fossil record and interpret the evidence as pointing towards anything other than evolution — despite all the gaps in record and in our understanding, evolution and common descent are the only conclusions that are supported by the full spectrum of evidence. This is very important when considering inferential evidence because inferential evidence can always, in theory, be challenged on its interpretation: why interpret the evidence as inferring one thing rather than another? Such a challenge is only reasonable, though, when one has stronger alternative — an alternative that not only explains the evidence better than
what's being challenged, but which preferably also explains other evidence that the first explanation does not. If you look at the fossil record, you find a succession of organisms that suggest a history of incremental development from one species to another. You see very simple organisms at first and then new, more complex organisms appearing over time. The characteristics of newer organisms frequently appear to be modified forms of characteristics of older organisms. This succession of life forms, from simpler to more complex, showing relationships between new life forms and those that preceded them is strong inferential evidence of evolution. There are gaps in the fossil record and some unusual occurrences, such as what is commonly called the Cambrian explosion, but the overall picture created by the fossil record is one of consistent, incremental development. At the same time, the fossil record is not in any way, shape, or form suggestive of the idea of sudden generation of all life as it appears now, nor does it support transformationism. There is no way to look at the fossil record and interpret the evidence as pointing towards anything other than evolution despite all the gaps in record and in our understanding, evolution and common descent are the only conclusions that are supported by the full spectrum of evidence. This is very important when considering inferential evidence because inferential evidence can always, in theory, be challenged on its interpretation: why interpret the evidence as inferring one thing rather than another? Such a challenge is only reasonable, though, when one has stronger alternative an alternative that not only explains the evidence better than what's being challenged, but which preferably also explains other evidence that the first explanation does not. We don't have this when with any form of creationism. For all their insistence that evolution is only a "faith" because so much evidence is "merely" inferential, they are unable to present an alternative that explains all that inferential evidence better than evolution — or even anywhere close to evolution. Inferential evidence isn't as strong as direct evidence, but it's treated as sufficient in most cases when enough evidence exists and especially when there are no reasonable alternatives. Fossils & Converging Evidence
That the fossil record in general suggests evolution is certainly an important piece of evidence, but it becomes even more telling when it is combined with other evidence for evolution. For example, the fossil record is consistent in terms of biogeography — and if evolution is true, we would expect that the fossil record would be in harmony with current biogeography, the phylogenetic tree, and the knowledge of ancient geography suggested by plate tectonics. In fact, some finds, such as fossil remains of marsupials in Antarctica are strongly supportive of evolution, given that Antarctica, South America and Australia were once part of the same continent. If evolution did happen, then you would expect not just that the fossil record would show a succession of organisms as described above, but that the succession seen in the record would be compatible with that derived by looking at currently living creatures. For example, when examining the anatomy and biochemistry of living species, it appears that the general order of development for the major types of vertebrate animals was fish to amphibians to reptiles to mammals. If current species developed as a result of common descent then the fossil record should show the same order of development.
In fact, the fossil record does show the same order of development. In general, the fossil record is consistent with the developmental order suggested by looking at the characteristics of living species. As such it represents another independent piece of evidence for common descent, and a very significant one since the fossil record is a window to the past. Fossils & Scientific Predictions
We should also be able to make some predictions and retrodictions as to what we would expect to see in the fossil record. If common descent occurred, then the organisms found in the fossil record should generally conform to the phylogenetic tree — the nodes on the tree at which a split occurs represent common ancestors of the organisms on the new branches of the tree.
We would predict that we could find organisms in the fossil record showing characteristics that are intermediate in nature between the different organisms that evolved from it and from the organisms from which it evolved. For example, the standard tree suggests that birds are most closely related to reptiles, so we would predict that we could find fossils which show a mix of bird and reptile characteristics. Fossilized organisms that possess intermediate characteristics are called transitional fossils. Exactly these sorts of fossils have been found. We would also expect that we would not find fossils showing intermediate characteristics between organisms that are not closely related. For example, we would not expect to see fossils that appear to be intermediates between birds and mammals or between fish and mammals. Again, the record is consistent.
The Improvement of Existing Classification The complete reclassification of higher taxa may be the greatest achievement of a taxonomist, but the taxonomist’s daily routine consists of minor additions to or modification of existing classifications. The following are the most frequent activities of taxonomists. 1. The assignment of the newly discovered species into the proper genus by answering these questions, a) Can it be included in an established genus? b) Does it require a new genus and possibly a new higher taxon? 2. The transfer of an incorrectly placed taxon to its proper position. 3. The splitting of a taxon into several taxa of the same rank either by cleaving a heterogeneous assemblage of species into several smaller and more homogenous ones or by removing an alien element from an otherwise homogenous taxon. When one breaks up too large a taxon, certain rules must be observed in the naming and rankling of the resulting new taxa. a) The rank of the original taxon is to be maintained if all possible. Finer discrimination can be achieved by means of the elaboration of subtaxa. For instance, it is usually less desirable to raise a heterogeneous family to the rank of superfamily and then to raise the previously recognized subfamilies to the rank of the families than it is to develop a finer subdivision of the subfamilies into tribes and genus groups.
b) In ranking no taxon should fall out of step with its sister groups. The classification of fossil humans by certain anthropologists who recognizes more than 30 genera of fossil hominids is an illustration of an unbalanced classification. c) A minimal number of names are desirable. If one adopts informal groupings such as species group (instead of a new gens or subgenus) and genus group (instead of a new family, subfamily, or tribe), the same information can be conveyed without burdening the memory and disturbing the balance of the hierarchy of categories. d) An inconveniently large taxon should be subdivided only if it can be ―cleaved‖ that is, if it can be divided into taxa of approximately equal size. Splitting off a number of monotype genera from a genus with 500 species would only impede information retrieval. 4. The raising in rank of an existing taxon, e.g., a genus to a subfamily or a subfamily to a family. 5. The fusion of a several taxa of the same rank and the synonymizing of the taxa with junior names. 6. The reduction in rank of taxon, for instance that of genus to a sub genus or that of a family to a subfamily. Such a reduction in rank may lead to a considerable simplification of a classification. Such a reduction is necessary in many groups of animals. For instance, there is little doubt that both birds and fishes are badly oversplit and that natural taxa in these groups are ranked in categories higher than necessary. Even the specialists concerned admit that there is little justification for having 412 families of fishes and 171 families of birds. What which of these families could be reduced to subfamilies? There is no easy answer. 7. The creation of new higher taxon not by raising the rank of taxon but making an entirely new grouping of taxa of the next lower rank. The proposal of a new super family for a number of existing families or a new order for a series of families illustrates this procedure. 8. This search for the nearest relative of an isolated taxon and, if this is successful, the study the question whether a new taxon of higher rank should be created for the newly established group of relatives.
Stability: During such minor improvement activities a determined effort must be made to disturbed the stability of the currently prevailing classification as little as possible and to maintain, if nor improve its information retrieval qualities. The successfulness of a classification as communication system stands in direct relation to its stability, which is one of the basic prerequisites of any such systems. The names for the higher taxa serve as convenient labels for the purpose of information retrieval. Terms such as Coleoptera and Papilionidae must mean the same thing to zoologist all over the world to have maximum usefulness. This is even truer for the genus, which is included in the scientific name. The overriding need for stability, dictates that accepted taxa and their names is maintained in all cases except when they are strongly contradicted by the evidence. In publishing the classification that has resulted from one’s taxonomic studies, one must present it either as a printed list, a diagram, or both. Both methods of presentation raise problems.
The Printed Sequence:
The technology of printing requires a linear one-dimensional sequence for any printed classification. One species will have to come first and another species last, while all others will have to be listed sequentially between the first and the last. An alphabetical sequence is often most useful for information retrieval. The multidimensional phylogenetic tree with the dimensions of time, space and adaptational divergence must be converted into a single linear sequence. To do this, the taxonomists must make some inevitable compromises between various considerations. Most important among these considerations are the following three; 1. Continuity: Each species is to be listed as near as possible to its closest relatives. 2. Progression: Each series of species or higher taxa should begin with the one closest to the ancestral condition (―the most primitive one‖), to follower by derived taxa deviate increasingly from the ancestral state. 3. Stability: one should not change previously accepted sequences unless they are proved unequivocally wrong. A classification is a reference system and adopting undocumented ―experimental‖ changes can drastically reduce its usefulness, particularly in a comparison of faunal lists.
Graphical Representation
The deficiencies in printed sequence have led to scientist to represent the information in diagram form. They are mostly in tree like form with emphasis on the age and prevalence of each taxon. Each three schools of macrotaxonomy use different diagrams. Pheneticists use phenogram which is the representation of degree of phenetic differences. The cladogram of the Cladists is a branching diagram of taxa as inferred from synapomophies. It reflects the cladogenesis. The taxa are delimited by holophyly. The phylogram of evolutionary taxonomists is a phylogenetic dendrogram in which an attempt is made to represent the taxa by the totality of their characters, not only their diagnostic ones, and by changing the lengths and angles of internodes to reflect differing rates of evolution.
Phylogenetic Trees: A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics. The taxa joined together in the tree are implied to have descended from a common ancestor. In a rooted phylogenetic tree, each node with descendants represents the inferred most recent common ancestor of the descendants and the edge lengths in some trees may be interpreted as time estimates. Each node is called a taxonomic unit. Internal nodes are generally called hypothetical taxonomic units (HTUs) as they cannot be directly observed. Trees are useful in fields of biology such as bioinformatics, systematics and comparative phylogenetics. The idea of a "tree of life" arose from ancient notions of a ladder-like progression from lower to higher forms of life (such as in the Great Chain of Being). Early representations of branching phylogenetic trees include a "Paleontological chart" showing the geological
relationships among plants and animals in the book Elementary Geology, by Edward Hitchcock (first edition: 1840). Charles Darwin (1859) also produced one of the first illustrations and crucially popularized the notion of an evolutionary "tree" in his seminal book The Origin of Species. Over a century later, evolutionary biologists still use tree diagrams to depict evolution because such diagrams effectively convey the concept that speciation occurs through the adaptive and random splitting of lineages. Over time, species classification has become less static and more dynamic.
Importance of Sound Classification: A sound classification is the indispensable basis of much biological research. It is a prerequisite for the ap plication of the comparative methods. Consistent with Simpson’s (1961:7) definition of systematics as ―the scientific study of the kinds and diversity of organisms and of any and all relationship among them‖, the systematist studies all aspects of living organism. Such studies are often meaningless without a sound classification. Studies of species formation, the factors of evolution, and the history of faunas are unthinkable unless they are based on sound classifications. Classifications are particularly important in applied biology. The recognition of this importance explains why even today so many biologists are dedicated to the task of improving the classification of animals.
References:
http://www.nature.com/scitable/topicpage/reading-a-phylogenetic-tree-the-meaningof-41956 http://en.wikipedia.org/wiki/Phylogenetic_tree http://en.wikipedia.org/wiki/Homology_(biology) http://en.wikipedia.org/wiki/Convergent_evolution Ridley, M. 1986. Evolution and Classification: The Reformation of Cladism. London and New York: Longman. Salemi, M. and A-M. Vandamme. 2003. The Phylogenetic Handbook, A Practical Approach to DNA and Protein Phylogeny. Cambridge: Cambridge University Press. Schuh, R.T. 2000. Biological Systematics: Principles and Applications. New York: Cornell University Press.
