Classification

 It is a set of categories to which species are assigned on the basis of their evolutionary relationship to the other species. 

Need Of Classification:

 Over 1.5 million species of animals and over 0.5 million species of plants are known . To deal with such a large number of dissimilar forms , certainly we need some system by which species can be classified in a reasonable way . Many types of classifications are possible . We could , for example , classify flowering plants according to their colour , height , or any other character . This type of classification is not meaningful since it does not provide any information about the basic differences and similarities among different individuals . All living organisms are related to each other at some point in their evolutionary histories . However , some organisms are more closely related than others . Sparrows are more closely related to pigeons than either to the insects . Classification is based on relationship amongst individuals , that is similarity in form or structure.

What are the basis for classification and units of classification? 

Ans . 

Basis for classification.

 Biologists have classified all living things into groups showing similarities , based upon homologies , comparative biochemistry , cytology and genetics . Large groups are divided into smaller groups upto species level.

Units of Classification:

1) Species:

A group of natural population which can interbreed freely and among themselves and produce fertile offsprings, but are reproductively isolated from all other such groups in nature " . However " interbreeding " cannot be used as a criterion for species recognition among predominantly asexually reproducing organisms . Each species has its own distinct structural , ecological and behavioral characteristics , hence species are independent evolutionary units . Different species do not exchange genes between them . 

2 . Other Units:

 Since long the living things are classified in two kingdoms : Plants and animals . Each kingdom is further divided into smaller groups called phyla ( also divisions for plants , algae and fungi ) . A phylum , in divided into classes , classes into orders , and an order into families . A family contains related genera , and a genus consists of one or more species . Species is the basic unit of classification Conversely speaking , the organisms are grouped into larger , more tum , inclusive categories ( taxa ) , each category is more general than the one below it and has emergent properties . The taxonomic categories from speceies to kingdom form a hierarchy as shown in the following classification of corn.

Biological  classification of Corn , ( Zea mays )

Kingdom        plantar

Phylum           tracheophyta (anthophyta)

Class.               Monocotyledonae

Order.             Poales

Family.            Poaceae

Genus.             Zea

Species.           mays

Members of a lower catogery resembles one another more than do the member of a higher taxon.

Binomial Nomenclature:

Need of Binomial nomenclature:

From the earliest times both animals and plants have been given common names by the people. Since no system was used in choosing common names in many cases various region had their own names for the same plant and animal e.g onion its common Urdu name is piyaz but in different regions of Pakistan it is also called ganda ,bassal, or vassal. There are different names for onion in different countries.

Similarly ' amaltas ' , ' argvad ' , ' gurmala ' , ' golden shower ' ' purging cassia ' are commone names for the same plant . Thus the same plant may have different names. 

2) In some cases , a single name refers to several different plants or animals e.g .. Dozen of the plants with bell shaped flowers are called ' blue bells ' . 

3) Similarly the word ' black bird ' would mean a crow as well as a raven . 

4). The common names have no scientific basis . For a biologist , a fish is a vertebrate animal with a backbone , fins and gills . But ' silver fish ' is an insect , and a ' cray fish ' , ' Jelly fish ' , and starfish ' do not fit the biologist's definition of a fish.

5) Thus , the common names had caused confusion .




Pre-medical Ist year biology Notes