The following points highlight the eight current approaches in taxonomy. The approaches are: 1. Morphological Approach 2. Embryological Approach 3. Ecological Approach 4. Behavioural Approach 5. Genetical Approach 6. Biochemical Approach 7. Numerical Taxonomy 8. Differential Systematics.
1. Morphological Approach:
Morphological characters such as wings, antennae, mouth parts, genitalia etc. mainly among arthropods and insects in particular are still of immense taxonomic importance. New techniques have, however, been developed to understand the time structures of some morphological characters which would be more reliable.
The use of scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet rays etc. have helped the cause of new systematics. The use of SEM has provided excellent minute information (three-dimensional pictures) in insects, mites, ticks and other smaller arthropods, which could not be adequately studied under stereo-microscopes. These highly magnified three dimensional figures helped in the discovery of new characters.
These characters would lead to the discovery of new species and also prepare dichotomous keys. TEM is also of great value in groups like Protozoa where features appear to be few. The pattern of ultraviolet reflection on the wings of butterflies has been used as taxonomic characters in some butterflies. It can be of great help in the recognition of sibling and closely related species.
2. Embryological Approach:
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During embryonic development individuals pass through quite different morphological stages. Thus, the taxonomic identification is based not only on the morphological characters of the adult but rather it is based on the sum total of all characters of all stages.
A very good example, where the character of the immature stages is useful in classification, is Anopheles maculipennis. This species has a number of sibling species and their identification is based on the basis of their egg structure.
Another example is Dacus oleae and Ceratitis capitata, the two economically important fruit flies that show superficial similarity in the shape and size of their eggs. However, the fine structural analysis of their egg shells reveals distinct structural differences of anterior pole, studied under SEM.
The egg shell of C. capitata has very thick vitelline membrane and the endo-chorionic complex is composed of two trabecular layers and is inverted in respect to each other. In case of D. oleae the vitelline membrane is thin and the endo-chorionic layer is compact with only one trabecular layer.
3. Ecological Approach:
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It is an established fact that each species has its own niche in nature, differing from one another on its closest relative in preferences of food, breeding season, tolerance to various physical factors etc. If two species coexist in the same habitat, they avoid inter-specific competition by their species-specific niche characteristics, with each species subsiding on different types of food.
For example, although the larvae of Drosophila mulleri and D. aldrichi, both live on the decaying pulp of cactus (Opuntia lind-heimeri) fruits, yet both have specialities in their preferences for certain bacteria and yeast. Another example is the sibling species of Anopheles maculipennis, which are broken into six different species (Table 3.1) based on their ecological differences.
4. Behavioural Approach:
The use of behavioural characteristics is one of the most important sources in animal systematics. Comparative ethology has proved very useful in improving the classification of insects (particularly bees, wasps, some beetles and cricket), fishes, frogs, birds etc.
These behavioural characteristics play a vital role in isolating mechanisms and initiating new adaptations. The characteristics are genetically determined and are passed on from generation to generation like morphological and physiological characteristics.
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Various sound recording devices are used. Ultrasonic sounds are produced by a number of animals in both intra- and interspecific communications. These sounds can be exploited (through sound spectrograms) in the discovery of not only sibling species but also in the separation of closely related species and simplification of classification.
A few examples are:
1. Alexander (1962) discovered about 40 species of crickets in North America on the basis of sound analysis.
2. Barber (1951) distinguished 18 sibling species in the genus Photuris (fire flies) in North America on the basis of the height and length of the marks indicating intensity and pattern of flashes.
3. van der Kloot and Williams (1953) classified spiders on the characteristics of their web construction.
4. Depending on the materials used in the construction of nests, the bee genera Anthidium and Dianthidium can easily be separated. The former genus uses cottony plant fibres, while the latter of resinous plant exudations and sand or small pebbles.
5. Schmidt (1955) separated the various species of the termite genus Apicotermes, on the basis of their nest structure.
6. In molluscs, the way in which the materials are attached to the shells provide useful taxonomic characters in classifying the species, particularly in the genus Xenophora.
5. Genetical Approach:
a. The DNA complement:
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Deoxyribonucleic acid is the essential material for heredity, is a known fact. It is possible that if the DNA complement of all species is known, then their evolutionary course would become quite apparent. We know that the amount of DNA per chromosome set is constant for each species and this can be used in the identification of species.
b. DNA Hybridization:
‘Hybridization’ between single stranded DNA components from different origins can provide a physicochemical means for assessing genetic relatedness among species. The DNA from one organism is extracted and made to hybridize in vitro with the cell-lines of other organisms. Such DNA matching techniques hold much promise in solving complex taxonomic problems.
c. Karyological studies:
By using various staining techniques, the number, shape and banding of chromosomes can be determined. Such karyotype is a definite and constant character of each species. Chromosomal taxonomy can be quite useful both in determining the phylogenetic relationships among taxa as well as in the segregation of sibling or cryptic species. Such reliable karyotypes are now available for about 1,000 species of mammals, several hundred species of birds, reptiles, amphibians and fishes.
For example, on the basis of the shape and number of chromosomes, Grewal (1982) separated some important fruit fly species (Fig. 3.1). Similarly, Patterson and Stone (1952) were able to differentiate 16 species of the genus Drosophila on the basis of number and shape of chromosomes.
Closely related species may show considerable rearrangement of chromosomes and sometimes the reverse may also occur. Reproductively isolated species may also have similar chromosomal structure and differ only in their gene content.
Geographical races of many insect species may differ in their banding patterns of their polytene chromosomes. Therefore, to solve systematic problems, karyomorphology cannot be treated as the only answer. It can only be used in selective cases.
6. Biochemical Approach:
Biochemical approach has been extensively studied in plants than in animals. Animals contain a large number of complex compounds like hormones, enzymes and protein molecules comprising of peptides, nucleic acids, amino acids etc.
The primary work of a biochemical taxonomist is concerned with the comparison and contrasting of compounds of the same class and performance of similar functions in different animal species, in respect to their properties as well as their distribution in different body organs.
Based on the above, taxonomy can be:
1. Protein Taxonomy:
Protein taxonomy was coined by Crick (1958), as species can be differentiated based on the sequence of amino acids in the proteins of an organisms. Thus, species differ in the differences of their amino acid sequences.
2. Molecular Taxonomy:
Molecular taxonomy, the term coined by Lahni (1964), was primarily based on the nucleotide sequences of polynucleotides. When trying to measure degrees of genetic relationship it is very important to look for the genetic material they are composed of and this is when molecular taxonomy comes into play. It is also believed that the changes in the enzyme structure can help in the discovery of new species.
Turner in 1966 divided molecular taxonomy into –
(i) Micro-molecular taxonomy:
It lays stress upon the distribution and biosynthetic interrelationships of small molecular weight compounds like free amino acids, alkacids, terpenes, flavonoids etc. These are commonly referred to as secondary compounds. This type of approach is particularly useful in resolving systematic problems where hybridization has been a factor.
(ii) Macromolecular taxonomy:
Macro molecular taxonomy is concerned with the polymeric molecules. It is more or less close to the core of hereditary information that is the DNA sequence, RNA, polysaccharides and proteins. This approach is useful in solving some of the more intractable systematic problems, especially those involving relationships among higher categories. Biochemical characters have been found to be extremely useful in solving various taxonomic problems.
A few examples are:
1. Phylogenetic relationship among various orders of birds have been demonstrated by Basu Chaudhary and Chatterjee (1969), based on the quantative analysis of ascorbic acid. Ascorbic acid is produced by some birds (Anseriformes, Columbiformes etc.) in the kidney; in some (piciformes) it is produced in the liver; while in some in both liver and kidney, and in some of the more evolved passerine birds, it is completely lost.
2. Using the biochemical characters of the unique venoms of fire ants, Brand (1972) were able to establish the phylogeny of a group of fire ants. Although the biochemical approach is helpful in solving many taxonomical problems, yet in many cases they are not useful.
Moreover, such studies are not possible in extinct organisms and, therefore, it is difficult to trace the course of evolutionary history through this process. However, proteins and nucleic acids provide a much reliable estimate of the degree of genetic homology among animals.
The distribution of free amino acids in different organs of insects is of greater taxonomic value. Similarly, in mammals, the classification of species, based on the amino acid sequences, are in accordance to the accepted one based on the morphological data. Such biochemical studies are conducted in five ways — immunological, chromatographic, electrophoresis, infra-red spectrophotometry and histochemical studies.
7. Numerical Taxonomy:
Instead of Numerical Taxonomy, some workers prefer to use the terms ‘Taximetrics’, ‘Taxonometrics’ and ‘Taximetry’. Degree of similarity was one of the basic criteria on which the recognition of taxa had been based. The first comprehensive effort to develop a new theoretical and practical approach to biological systematics was put forward by Robert R. Sokal and P.H.A. Sneath (1963), illustrated in their book Principles of Numerical Taxonomy.
It is based on an operational attitude where objects are compared ‘at face value’. The numerical concept refuses to incorporate into taxonomy dubiously retrievable phylogenetic informations and the philosophy has been defined as phenetic or directly dependent on the overall similarity of the characters (the phenotypes).
Numerical phenetics is the methodology of assembling individuals into taxa on the basis of an estimate of un-weighted overall similarity. This concept is, thus, based on the use of maximum number of characters and all the characters are given equal weightage. The larger the number of taxonomic characters, the better is the result.
The characters not necessarily be derived only from external or internal morphology, but it may include any attributes of the operational taxonomic units or OTUs (biochemical, behavioural, cytological, ecological, developmental etc.).
However, there are differences in opinion of the number of characters used in this approach. Sokal and Sneath (1963), prefer the use of at least 60 characters, Moss (1967) prefers 135 to 146 characters, while Steyskal (1968) prefers at least 1000 characters (particularly in insects).
However, in phenetic analysis there is no place for homology or for history-dependent concepts, such as ancestry or evolutionary changes. That taxonomy should be freed of all theoretical implications, led pheneticists to reject in their work any reference of species. They were replaced by the concept of the OTU. However, OTUs are very heterogeneous class of entities and may be individuals, populations and some may be historical entities.
The branching diagrams found in the works of phenetics are not phylogenies of species, but simply dendrograms made due to the clustering together of OTUs. This work with matrices and branching diagrams cannot be developed by hand.
It requires the use of several algorithms and computer programs, developed to carry out the calculations. The technical advantages of numerical techniques, thus, have proved to be a more lasting contribution to systematics. However, Blackwelder (1967) and many other taxonomists doubt the usefulness of numerical methods, due to:
1. The use of large number of characters probably tends to reduce the effect of homoplasy on the result.
2. The approach is exposed to a great risk of reaching unsound classification, as in giving equal weightage to all characters it does not allow for mosaic evolution, special adaptations, convergence, parallelism, development and genetic homeostasis and also evolutionary, genetic and developmental phenomena.
3. The use of complex mathematical and statistical methods by numerical taxonomists has led to great difficulties to follow them, by the biological taxonomists.
In spite of the above difficulties it is believed that today all systematics is to some extent numerical. Computers and numerical taxonomic programmes are now standard resources in every museum and systematics laboratory. One of the very useful software is NTSYS.
8. Differential Systematics:
Womble in 1951, proposed differential systematics as a method for summing up the rates of change with distance (differential) for several characters to show zones of differentiation within a taxon. However such laborious procedures to obtain the differential set of characters have been out-rightly rejected by a number of biologists.
However, with the advent of modern technologies, like the use of computer, such drawbacks can be eliminated.