1 - Principles of Classification
1 - Principles of Classification
1 - Principles of Classification
Principles of Classification
What is the need for classification?
Systematics
Classification
1) Artificial system
• Characters of convenience without relation to physiological significance
• Animals classified into 1) animals that fly 2) animals that cannot fly
2) Natural system
• Classification based on characters which indicate natural relationships
• Organisms of a natural systematic category agree with another in so many
characters because they are descendants of one common ancestor
3) Phylogenetic system
• Evolutionary and genetic relationship of the organisms
• Enables to find out ancestors and derivatives of any taxon
Linnaean hierarchy
• This method is not considered an accurate classification as it does not account for
organisms that might belong in different groups.
• It does not take into account the genetic relationships.
• Organisms classified according to Linnaean classification might not be related
genetically.
Kingdom
Phylum
Order
• Each class is further divided into smaller groups knows as the order
• Eg : In Mammals, the organisms that can climb trees have been placed in the order Primata
Family
• Each order is further divided into smaller groups of families based on their similarities
• Eg: The order Carnivora includes many families such as Felidae, Canidae etc
Genus
• A genus is a group of organisms that have many similarities. Many such genera are grouped into
family based on similar characteristics
• Eg: Lion, Tiger, Leopard are grouped under the same genus due to their unique cranial formation.
Species
• Species were considered to be immutable, distinct entity, before the advent of Darwin.
• Defined by fixed morphological characteristics
• New organisms compared with the type specimen.
• Any differences observed were considered to be imperfections, rather than give them a logical
explanation
Biological Species
• Species defined as the basal grouping of organisms, distinct from other groupings, within which
there is a parental pattern of ancestry and descent.
• The evolutionary species concept considers any evolutionary changes in an organisms as one
species, whereas the phylogenetic concept also groups the evolved organisms into a smaller group.
• The phylogenetic species concept creates more species than any other concept, as it recognizes
every evolved species as a separate entity.
Some more definitions of Species
It has already been stated that individual organisms which have many features in common and are
able to breed only amongst themselves are encompassed by the term species.
Allopatric species: The species inhabiting different geographical areas.
Sympatric species: The species normally occupying the same geographical areas.
Morphospecies: These are ones established by the morphological similarity regardless of other
considerations
Bio-species and genetical species: A group of inter-breeding populations which are
reproductively isolated from other such group.
Sibling species: It is a term applied to pairs or groups of very similar and closely related species.
When applied to closely related species (in phylogenetic sense) this expression refers to
hypothetical species, these cannot be dealt with in taxonomy but can be useful in speculations on
evolution.
Taxonomic species: A species which has been provided a specific name under the International
Rules of Nomenclature.
Evolutionary species: These are lineages (ancestral descendent sequences of populations) evolving
separately from each other and with their own unitary evolutionary roles and tendencies.
Polytypic species: Polytypic species are those which consist of two or more subspecies.
Monotypic species: Monotypic species consist of a single subspecies.
Binomial Nomenclature
• The first part of the name consist of the genus and the second part of the name is the species
• The genus is always capitalized whereas the species is written in small letters
• The Binomial nomenclature ensures that there is no ambiguity in the organism being studied.
Adaptive radiation
Homologies
• Homologies are anatomical features of different organisms that have a similar function or
appearance due to a common ancestor.
Example 1:
• The forelimb of a bear, wing of a bird, human arm have the same functional type of bones, as
seen in the reptilian ancestor they share.
Example 2 :
• The mouthparts and the antennae of different insects such as the grasshopper, the honeybee, the
butterfly and the mosquito consists of the same structures but are used for different purposes.
• The grasshopper primarily bites and chews while the honeybee bites and the butterfly sucks
pollen.
The more homologies the organisms possess, the more likely that they have a genetic relationship
Homologous Structures
Example
• North American wolves and Tasmanian wolves
have similar appearance and predatory behavior.
They seem to have evolved from a common
ancestor, the dinosaur.
• North American wolves are placental mammals
whereas the Tasmanian wolves are marsupials.
• Fishes on both sides of the poles have developed
an anti-freeze mechanism to cope up with the
cold temperatures.
Analogies
Anatomical features that have the same form or function in different species that have no
common ancestor.
Example : Wings of a bird and wings of a butterfly.
Superficially similar in shape and function, but different on the inside.
Body plans that define the animal diversity and the common architectural theme
for the classification of organisms
Animal body plans are differ in grades of organization, Body symmetry, Number of
embryonic germ layers and Number of body cavities.
i) Spherical symmetry
• Any plane passing through the center divides a body into equivalent or mirrored
halves
• Chiefly found in unicellular forms
• Best suited for floating and rolling
ii) Radial Symmetry
a) Blastulation
• The cells of the endoderm form another germ layer known as the mesoderm that lies between the
ectoderm and the endoderm.
• Organisms having three germ layers are known as triploblastic organisms.
• They are usually biradially symmetric.
4 ) Methods of Coelom formation
i) Acoelomate
iii) EuCoelomate
A) Non-cellular components
• Body fluids
• Extracellular structural elements
i) Body fluids
• Body fluids are subdivided into two fluid compartments, those that occupy intracellular space,
within the body’s cells and those that occupy extracellular space, outside the cells.
• Extracellular fluids are further divided into blood plasma and interstitial fluid.
• Interstitial fluid also called tissue fluid, occupies the space surrounding cells.
• Most invertebrates have open blood systems
a) Epithelial tissue
• An epithelium is a sheet of cells that covers an external or internal surface. Outside the body,
epithelium forms a protective covering.
• Inside, it lines all organs of the body cavity
• Transport of molecules
• Modified into glands that produce mucous or hormones or enzymes
• Simple epithelia found In all animals
• Stratified epithelia restricted to mammals
b) Connective tissue
• Diverse group of tissues that serve various binding and supportive functions.
• Loose connective tissue
• Dense connective tissue
• Specialized connective tissue such as Blood , Lymph , adipose, cartilage and bone.
c) Muscular tissue
d) Nerve tissue
1) Animals are classified into groups based on their similarities and dissimilarities.
2) Linnaeus proposed the hierarchical form of classification and the binomial
nomenclature for grouping and unique identification of the organisms
3) Whitakker proposed the five kingdom classification to include the organisms that
cannot be included in either plantae or Animalia due to their unique characteristics
4) The classification of animals has been done based on their body plans, due to
symmetry, developmental stages, presence or absence of coelom.