Classification of Animals

What is the need to classify organisms present around us??? “There are an estimated 1.5 – 1.7 million named species on earth (E. O. Wilson, 2001). Of these, invertebrates account for >99%. In reality there may be 13 620 000 species (or more) in total according to the Global Biodiversity Assessment”.

1) To give specific names to the organisms.
2) To identify the different species and
3) For finding the evolutionary relationships between the organisms, for these reasons Classification of organisms becomes extremely vital.

Classification is not something recent, evidence from the days of Aristotle is available for Classification.There are many ways scientists classify organisms based on many criteria and this causes lot of confusion.

Classification based on Physical Characteristics- Number of cells, number of germ layers ( diploblastic or triploblastic), nature of digestive system (one opening or two openings), segmentation (anterior, posterior, dorsal, ventral), skeleton – type (chiten, cartilage, or bone), appendages (number and arrangement), symmetry (radial or bilateral).

Since the olden days classification based on morphological characters is widely used and other system based on germ layers Was also quite popular. Now Molecular data is used to find out the relation between various living organisms. Systems of classification are hierarchal i.e placing related groups together into larger groups.

Kingdom-phyla-Class-Family-Order-Genus-species.

It will be too lengthy to go into the history of classification so we just look at some important contributions from olden era which still has got some impact.

Science of Systematics dates to Linnaeus in the 18th century who devised the basic systems of binomial nomenclature and hierarchical classification which is also used today. Example : Fruit flies are Drosophila melanogatser Drosophila represents Genus and melanogaster indicates species.

One of the main aim of classification is to find evolutionary relationships between the species and phylogentic trees and Cladiograms are a way to represent them.Branching diagrams called phylogenetic trees summarize evolutionary relationships among organisms.These trees are constructed using characters which can be morphological or molecular. In a phylogenetic tree the tips of the branches specify particular species and the branching points represent common ancestors.

Cladograms are diagrams that display patterns of shared characteristics. Within a tree a clade is defined as a group that includes an ancestral species and all of its descendants. A cladogram and a phylogenetic tree are similar, but not identical. A phylogentic tree’s branches represent real evolutionary lineages and branch lengths represent time or amounts of evolutionary change.

Cladogram branches contain no such information. Branching order of cladogram should, however, match that of phylogenetic tree.

We first take a look at the classification based on the germ layers , symmetry of organisms and 18S RNA data. ( Phylogeny based on Phylogenomics will be discussed later)

All organisms can be classified into two broad groups depending upon the Germ layers:

Diploblastic: Two layers , No mesoderm e.g. Cnidarians, Sponges and Ctenophores (comb jelleys).

One important thing to be remembered is that according to all the criteria so far used to classify animals, Sponges (Poriferans) are placed as the sister group to all other animals meaning that they have diverged first from the rest of the animals in the evolution making them most primitive animal group on the earth. It also makes sense as sponges are among most simplest animals as they lack gastrulated embryos, extracellular digestive cavities, No nervous system, lack muscles and they posses choanocytes (feeding cells)which appear to be homologous to choanoflagellates, a group of unicellular and colonial protists that are believed to be the immediate precursors of animals. So The traditional conclusion is that sponges are the basal lineage of the animals

Triploblastic (Bilaterians): Three layers which includes mesoderm. Bilaterians are further divided based on presence or absence of coelom into animals with body cavity and Acoelomates ( Acoela) The animals with coelom can be further divided into Protostomes and Deutereostomes based on the fate of first opening of the development. In protostome development, the first opening in development, the blastopore, becomes the animal’s mouth. In deuterostome development, the blastopore becomes the animal’s anus.

Protostomes can again be divided into ecdysozoans , Platyzoa and lophotrochozoans. based on the molecular dat of 18s DNA .

* Ecdysozoa, e.g. arthropods, nematodes
* Platyzoa, e.g. platyhelminthes, gastrotrichs
* Lophotrochozoa, e.g. molluscs, annelids

Deuterostomes can be classified into : Chordata, Echinodermata, Hemichordata and Xenoturbella.

Chordata which includes Vertebrates ,Urochoradata, e.g. Ascidians, Cephalaochordata e.g. Amphioxus

The basal chordates lack a backbone and many other advanced structures possessed by their vertebrate relatives. They do, however, exhibit three basic features characteristic of the vertebrates:

1. A structure equivalent to a notochord
2. A dorsal nerve cord
3. Presence of pharyngeal gill pouches (gill slits)

Echinodermata,e.g. Star fishes, Sea urchins
Hemichordata ,e.g. Acron worm
Xenoturbella, e.g. Xenoturbella bocki, Xenoturbella westbladi (This phylum includes two marine species and occupies basal position in super phylum deuterostomes).

Additional reading:

For the phylogentic tree based on 18S DNA and hox clusters Read NATURE|VOL 402 |SUPP| 2 DECEMBER 1999 |The future of evolutionary developmental biology – Peter W.H.Holland

Broad phylogenomic sampling improves resolution of the animal tree of life.
Nature. 2008 Apr 10;452(7188):745-9.
Dunn CW, Hejnol A, Matus DQ, Pang K, Browne WE, Smith SA, Seaver E, Rouse GW, Obst M, Edgecombe GD, Sørensen MV, Haddock SH, Schmidt-Rhaesa A, Okusu A, Kristensen RM, Wheeler WC, Martindale MQ, Giribet G.

Image Credit : Openstax Biology