Essay on Taxonomy (For School and College Students) | Organisms | Biology

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Essay # 1. Meaning of Taxonomy:

The science of taxonomy and systematics involves the classification of organisms according to evolutionary relationships; how closely they are related to each other. Before scientists were able to use DNA sequencing to examine evolutionary relationships, organisms were classified based on physical similarities and differences. Modern systematics combines data from many sources, including- the fossil record, comparative homologies (similarity of struc­tures due to shared ancestry), and comparative sequencing of nucleic acids (DNA and RNA) among organisms.

Essay # 2. Types of Taxonomy:

a. Evolutionary Taxonomy:

It is based on the fossil materials collected from the field. In constructing a hierarchy, a tra­ditional and very flexible combination of criteria was used. Firstly, morphological resem­blance and then phylogenetic relationships, the way in which the animals actually related to each other, in terms of the regency of a common ancestor (as far as could be determined).

The order of succession in the rock record (biostratigraphy) and the geographical distribution may play an important part in deciding these relationships. This practical approach, which takes all factors into consideration, has long been the basis of paleontological classification, and is still seen as the best method by many.

b. Numerical Taxonomy:

Evolutionary method has limitations like uncertainties and subjectivity of classification by observation, along with the preservation of the fossil record. To avoid this, numerical taxonomists attempt to use quantified observations of the animal in an attempt to decide on natural groupings. They consider that if enough characteristics are measured, and computed, then represented by the use of ‘cluster scatters’ (a form of graph) followed by the distance between clusters can be used as a measure of their differences.

However, although useful in some instances, the operator still needs to (subjectively) choose how best to analyze the measurements taken, and possibly give greater precedence (weight) to certain more- important characteristics again, a subjective choice. Thus, numerical taxonomy is not as ob­jective as it may first appear.

Essay # 3. Basic Taxonomic Categories:

The seven basic taxonomic categories are:

a. Kingdom,

b. Phylum,

c. Class,

d. Order,

e. Fam­ily,

f. Genus and

g. Species;

a. Kingdom:

Kingdom being the broadest category, while species being the most specific. It can be easily remembered by the mnemonic sentence “King Philip came over for green soup”.

Living organisms are subdivided into 5 major kingdoms:

i. Monera:

Prokaryotes (i.e., without a nucleus) Unicellular and colonial, including the true bacteria (eubacteria) and cyanobacteria (blue-green algae) [~10,000 species].

ii. Protista:

Unicellular protozoans and unicellular and multicellular (macroscopic) algae with 9 + 2 cilia and flagella (called undulipodia[~250,000 species]).

iii. Fungi:

Haploid and dikaryotic (binucleate) cells, multicellular, generally heterotrophic, without cilia and eukaryotic (9 + 2) flagella (undulipodia) [~100,000 species].

iv. Planta:

Haplo-diploid life cycles, mostly autotrophic, retain embryo within female sex organ on parent plant [~250,000 species].

v. Animala:

Multicellular animals, without cell walls and without photosynthetic pigments, form diploid blastula [~1,000,000 species].

b. Phylum:

Classes are grouped into:

i. Phyla (the plural of phylum), and

ii. Phyla into Kingdoms.

Within the kingdom Animalia, the most common phyla are:

(a) Arthropoda (e.g. insects)

(b) Mollusca (e.g. snails)

(c) Chordata (e.g. fishes, amphibians, reptiles, birds, mammals)

(d) Platyhelminthes (e.g. tapeworms)

(e) Nematoda (i.e. unsegmented worms)

(f) Annelida (i.e. segmented worms)

(g) Cnidaria and Ctenophora (e.g. jellyfish)

(h) Echinodermata (e.g. starfish)

(i) Porifera (e.g. sponges )

c. Class:

The class is a major division within the Kingdom, and forms the basis on which most fossil study is based.

For example, the phylum Molluscas contains 4 classes:

i. Gastropoda,

ii. Cephalopoda,

iii. Pelecypoda and

iv. Scaphopoda.

d. Order:

Families are grouped into orders, whose individuals may vary in many ways; such as the order of Carnivora – which includes cats, dogs and weasels. Orders begin with a capital and usually end in “a” – but not always.

e. Family:

Genera are grouped into families, which are major groups of generally similar or­ganisms; such as Felidae, which includes all cat-like animals from domestic cat to wild lynx to tiger to cheetah to jaguar to snow leopard. Family names always end in the letters “ae”, but are not printed in any special way.

f. Genus:

The generic name refers to the genus, which is a group of species that are fairly closely related – such as the genus Equus which includes several species, such as the Equus caballus, Equus asinus and Equus zebra (domestic horse, wild ass and zebra respectively). ‘Genus’ is the taxonomic classification lower than ‘family’ and higher than ‘species’. In other words, genus is a more general taxonomic category than the species.

g. Species:

It is the fundamental unit of taxonomy. This is a group of very similar individuals that typically have similar anatomical characteristics and have the potential to interbreed freely, to produce fertile offspring – but cannot interbreed successfully with individuals from other species.

A mule, for example, is not a distinct species. It is an infertile hybrid of a male donkey (Equus asinus) and a female horse (Equus caballus). There are, as ever, exceptions where the rule breaks down, especially in the plant Kingdom. However, in the majority of cases, interbreeding of species does not produce fertile offspring.

Essay # 4. Domains and Kingdoms of Life:

In 1990 American molecular biologist Carl Woese proposed a new category, called a Do­main, further highest level- Archaea, Bacteria, and Eukaryota. Archaea are a group of organ­isms that are adapted to live in extreme habitats like thermal volcanic vents, saline pools, and hot springs (Fig. 2.1). Though, they are quite similar in appearance to bacteria, molecular studies have shown that they are biochemically and genetically very different.

Bacteria are simple single-celled organisms that generally lack chlorophyll (an exception is cyanobacte­ria). Bacteria generally obtain energy for survival by breakdown organic matter through fer­mentation and respiration. They are generally heterotrophic. Bacteria such as cyanobacteria and those belonging to the genus Rhizobium play an important role in the fixing of atmos­pheric nitrogen.

Eukaryota are organisms that have a eukaryote type of cell.

This group of life includes the four primary kingdoms:

i. Protista,

ii. Fungi,

iii. Plantae (Plants) and

iv. Animalia (Animals).

King­dom Protista is made up of single celled organisms and some of their simple multi-cellular close relatives. Some examples of unicellular protists include dinoflagellates, amoebas, Para­mecium, diatoms, and volvox. Slime molds, brown, red and green algae like Ulva are typical examples of multi-cellular forms of protests (Fig. 2.1).

According to recent estimation there are about 1.5 million different species of fungi exist in our planet. Most of these life-forms are multi-cellular. The biologists at large have grouped the fungi with plants. However, investigations of this life-form indicate that fungi are quite different from other eukaryotes in terms of feeding strategies, physiological organi­zation, reproduction and growth.

Many species of fungi are heterotrophic decomposers or they live in symbiosis with another species. Lichens are good examples of this type of biotic relationship (Fig. 2.2). Lichens involve the symbiotic relationship between a fungus and a photosynthetic alga.

The kingdom Plantae is composed of multi-cellular photosynthetic organisms that can convert inorganic elements, with the help of the sun’s energy into organic compounds. Plan­tae includes all land plants, including mosses, ferns, conifers, and flowering plants. Diversity in this kingdom is quite large with more than 250,000 species. Two other important traits associated with plants are cell walls made of cellulose and a large central cellular vacuole.

Animals are multi-cellular heterotrophic eukaryotes. Species in the kingdom Animalia must ingest produced organic molecules for food. Animals also differ from other forms of life by having two unique tissue types- nervous tissue and muscle tissue. Most animals pro­duce their offspring through sexual reproduction.