Fish Taxonomy and Systematics

Lecture 2:

Taxonomic systems

Introduction to taxa and speciesWhile grouping or arranging the organisms, a biologist faces three scientific ideas, namely

taxonomy, systematics and classification.

The term taxonomy is a Greek word. Its components are taxis and nomos. While taxis means arrangement, nomos means

law. Hence, taxonomy is defined as the “theory and

practice of classifying organisms” (E. Mayr, 1966).

The term systematics originates from the Greek word systema (meaning ‘placing together’).

Thus, systematics means classification of living organisms in accordance with their natural relationships.

The term classification in meaning partly overlaps with taxonomy.

However, it simply means the activity of classifying.

Thus, according to Simpson “Zoological classification is the ordering of animals into groups on the basis of their relationships”.

Taxon.Based on specific characteristics, organisms are grouped in various categories.

These categories are otherwise called taxa (singular: taxon).

“A taxon is a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category”.

There are several taxa in animal taxonomy are Phylum,

ClassOrder

FamilyGenus

Species.

This arrangement from Phylum to Species is designated as the hierarchic system of classification.

In this system each taxon is based on specific characters of a group of organisms.

However, human error in identification and grouping may happen.

In this hierarchy, the Species is considered as the most important taxon.

A Species represents a natural unit. All other taxa remain arbitrary and are subjected to revision.

A Species is considered a reality. It is the fundamental unit in taxonomy.

Evolution basically operates at the Species level only.

Hence the concept of Species has received much attention.

Concept of SpeciesInitially the Species was considered as a group of organisms showing similar or specific characters. However modern workers have identified three main concepts regarding Species.

1. Typological Species conceptThis concept has its beginning from the essentialism concept of Aristotle.

According to this concept a Species is recognized by its essential characters expressed in morphology.

2. Nominalistic Species conceptAccording to this concept Species are man made ideas. Nature produces individuals and not Species. Thus a Species is considered as a mental concept.

3. Biological Species conceptAccording to this concept, “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”. This concept is mostly accepted by present day taxonomists.

NomenclatureNomenclature forms the basis by which scientists can name and cross refer to organisms.

It is an integral part of taxonomy.

In fact, modern taxonomy started in 1753 with the publication of first part of Systema by Linnaeus.

According to Linnaeus a Species is specified by the combination of both its specific and generic names.

Since it requires two names, it is referred to as the binomial system.

In modern times International Commissions are responsible for naming each major group of organisms.

There are several such commissions.

Naming of animals is monitored by International Code of Zoological Nomenclature (ICZN) (International Commission of Zoological Nomenclature, 1985).

The rules are set out in the ‘codes’.

The codes are modified by occasional science congresses.

Basic principles of nomenclature

1. Providing stability in the naming and classification of organisms is emphasized. Any taxon must have only one correct name.

2. If two or more names are already in use the correct name will be the one that was published earlier. This system is referred to as the law of priority.

3. If two or more workers at one particular time describe the same organism using different names, it results in synonyms. However only one name will be held as a valid name. The validity is provided to the senior synonym.(law of priority)

4. When names referring to two separate taxa of the same nomenclatural level are spelt the same, the two names are called homonyms.

This situation arises when two separate authors used the same name to refer to two different taxa.

This condition is called homonymy. In this situation the junior name is invalid and a

new replacement has to be proposed.

5. A material on which an original description is based, gets a special status. It will form the basis for any future identity of a taxon.

This idea is called the type concept. Thus the concept of a genus and species are fixed

by their type genus or type species.

6. Names that were used prior to those included by Linnaeus in the “Systema Naturae”, tenth edition, 1758 are not recognized.

7. Scientific names must be either Latin or latinized. The name should be mentioned in italics.

8. The genus name should be a single word beginning with a capital letter.

9. The species name should be a single or compound word beginning with a small letter.

Phenetic method or Numerical taxonomy

This method involves clustering or grouping of individuals of a taxon or several taxa.

Based on overall similarity, identifications are being made.

The desired size of the clusters or groupings is called the operational taxonomic unit (OTU).

The identification method involves measurement of taxon to taxon similarity or dissimilarity.

It is measured using a scale of 0 to 1. ‘1’represents perfect identity.

‘-1’designates dissimilarity between taxa.

In this method enormous amount of data are collected for related groups.

Analyses are made, using statistical tools and computers.

It might seem to be a straightforward task to measure a large number of traits of organisms and then assess the degree of similarity among them.

In practice, however, this is not so simple, because decisions must be made about whether some traits are more important than others, and about whether a group of traits that are all direct responses to a single selective pressure should be given the same weight as traits influenced by different selective pressures.

Another difficulty is that, because morphological traits often vary in response to environmental conditions, environmentally induced variation may be confused with genetically based differences.

Classification

The arrangement of organisms into groups and subgroups on the basis of similar characters.

These characters may be about internal morphology (anatomy), external morphology, physiology, cell structure, especially the number of chromosomes and chemical composition (especially of proteins) and embryology of the organisms.

These characters help in study of intra specific (within the same species) and intra specific (between different) species differences.

The presence of similar characters in different organisms indicates their common ancestry.

This similarity because of common ancestral origin is called Homology e.g. arm of a monkey, flipper of a whale and wing of a bat show homology.

They are dissimilar apparently but their internal structure (arrangement of bones and muscles) is same.

Difference betweenHomologous and Analogous Organisms

The fruit of all plants, whether sweet, or sour, small and dry or large and fleshy, all are the homologous structures because they develop from ovary of flower.

Their origin is common.

On the other hand, wings of an insect, and a bird, despite having same function but are different in basic structure and origins are called analogous organs.

HomologySimilarity resulting from common ancestry.E.g., the forelimb bones of a bird, bat, and cat.

Homoplasy (analogy)

• Similarity not due to common ancestry

• Reversal – loss of new (apomorphic) feature, resembles ancestral (old) feature.

• Convergence (parallelism) – gain of new, similar features independently.

Convergent evolution:spines of cacti & euphorbs

Cactus EuphorbEuphorb

Both examples of reversal within Tetrapods: loss of a derived feature – forelimbs.

Leg-less lizards Snake

Example of convergence relative to one another!Independently evolved.

snakesleg-lesslizards

leggedlizards

**

*= loss of legs

gain of legs (Tetrapods)

Aims/Objectives of Classification

These are given below:

1.To determine similarities and differences between different organisms.

2.To arrange organisms on the basis of similarities and differences.

3.To identify the organisms on the basis of their structure and other prominent characters and study them systematically and logically.

4.To find out inter-relationships of organisms.

The classification is not static, nor has only one system of classification been followed rather it is dynamic.

Whenever any new knowledge is available about organisms, it is used in classification.

Therefore, many systems of classification have been used.

Two Kingdom Systems

All organisms were classified into two kingdoms before present time.

1.Plant Kingdom (Plantae) - It includes all the small and large plants.2.Animal Kingdom (Animals) - It includes all the animals.

Important Characters of Plants and AnimalsPresence of cell wall and ability to prepare their own food were considered the most important characters of plants.

Lack of cell wall and inability to prepare food and characteristic mode of nutrition and especially the ability to locomote were considered the most important characters of animals.

Morphological (Linnaean) System

The biological classification system used today is based on the work of the great Swedish biologist Carolus Linnaeus (1707-1778).

The Linnaean classification system was developed before biologists were aware of the length of evolutionary time and when only a few methods for comparing organisms were available.

Consequently, organisms were grouped primarily on the basis of their gross morphological similarities and differences.

In the Linnaean system, each species is assigned two names, one identifying the species itself and the other the genus to which the species belongs.

This two-name system, referred to as binomial nomenclature, is universally employed throughout biology.

A reference to more than one species in a genus is expressed with the abbreviation spp. after the generic name (for example, Drosophila spp.); the abbreviation sp. is used after a genus name when the identity of the species is uncertain.

In the Linnaean system, the species are grouped into higher taxonomic categories.

The number and limits of these categories are somewhat arbitrary, but there are some guiding rules.

One is purely practical: If every species were put into its own genus, or conversely, all species were lumped into one genus, the genus would not carry any information not already present in the designation of the species.

A second consideration is the relative amount of similarity or dissimilarity among the organisms.

A higher taxonomic category may have a single species in it if that species is very different from all other species.

Some genera, on the other hand, contain hundreds of species.

Evolutionary Studies

The major goal of the biological classification systems in regular use today is to reflect evolutionary relationships.

Evolutionary history has two important components.

One is phylogeny, the pattern of genetic linkages between ancestors and their descendants.

The other is the rate of evolution of traits among groups of organisms.

Examine the history of evolutionWhat has evolution done in the past?Understanding how living organisms are

related and how they have changed over time

Examine the process of evolutionWhat drives evolution?Understanding mutation, gene flow and

natural selection

Because the rates of change in the structures of organisms differ among lineages, some organisms that are very similar to one another have been evolving independently within their lineages much longer than organisms in other lineages that are structurally much more different from one another.

Classification systems that combined both phylogeny and rate of morphological change preceded in time those that emphasize only one of these factors.

Phylogenetics

Phylon = tribe/race,

genetikos = relative to birth

Thus, it refers to the study of evolutionary relationships among organisms, sequences, or anything in between

Phylogenetics is the area of research concerned with finding the genetic relationships between species.

The basic idea is to compare specific characters (features) of the species, under the natural assumption that similar species (i.e., species with similar characters) are genetically close.

Classic phylogenetics dealt mainly with physical, or morphological features – size, color, number of legs, etc.

Modern phylogeny uses information extracted from genetic material – mainly DNA and protein sequences.

The characters used are usually the DNA or protein sites (a site means a single position in the sequence).

The relationships between species are then deduced from well conserved blocks in the alignment of several sequences, one from each examined species.

An interesting example is a research project that used phylogenetics in order to trace the origins of the human population on earth.

Researchers investigated the mitochondrial DNA of 182 people all over earth (the mitochondrial DNA is especially good for phylogenetic research since it is copied completely from mother to son, without recombining with the father’s DNA).

The phylogenetic analysis provided evidence that all humans have a common female ancestor who lived in Africa (”African Eve”).

Significance of Biological Classification

Classification systems serve four very important roles.

1.They are an aid to memory. It is impossible to remember the characteristics of a large number of different things unless we can group them into categories, whose members share many characteristics.

2.Classification systems greatly improve our predictive powers. If, for example, we know that females of all known mammalian species have mammary glands with which they produce milk for their offspring, we can be quite certain that a newly discovered animal with other typical mammalian traits, such as hair and a constant, high body temperature, will also have this method of provisioning its offspring, even if the first individuals we happen to find are males, and hence lack functional mammary glands.

3. Classification systems improve our ability to explain relationships among things. For biologists, this is especially important when we attempt to reconstruct the evolutionary pathways that have produced the diversity of organisms living today.

4. Taxonomic systems provide relatively stable, unique, and unequivocal names for organisms. If those names are changed, the systems provide means of tracing the changes. Common names, even if they exist (most organisms do not have common names at all), are unreliable and often confusing.

For, example, there are plants called "bluebells" in England, Scotland, the eastern United States, and the Rocky Mountains - but none of the bluebells in any of those places is closely related to the bluebells in any of the other places

Conclusion:

The different biological classification systems all are designed to express relationships among organisms, but they differ with respect to the kinds of relationships they attempt to express.

Classifications are based on features selected according to the goals of the system.

If, for instance, we were interested in a system that would help us decide what plants and animals were desirable as food, we might erect a classification system based on palatability, ease of capture, and the edible parts each organism possessed.

One ancient system divided animals into (1) those with placentas, (2) those formed from eggs, (3) those that generated spontaneously, and (4) those born of vegetable matter.

Another system divided animals into (1) those born of moisture and heat, (2) those that bear live young, (3) those that lay eggs, and (4) those that burst forth from the ground.

We do not use such systems today, but they served the needs of the people who developed them.

It is inappropriate to ask whether those classifications, or any others, including contemporary ones, are right or wrong.

Classification systems can be judged only in terms of their utility and consistency with their stated goals.

To evaluate any classification system we must first ask: What is it trying to accomplish? Then we can ask: How well does it accomplish those objectives?