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Inter-relationships of Classification
The aim of classification is to put together organisms on the basis of their similarities. But the question is what type of similarities? You know that fish and whales live in water; birds and butterflies live in air. Should we place fish and whales, in one group and birds and butterflies in another? Perhaps you are tempted to do so because fish and whales have organs to swim; birds and butterflies possess wings to fly. One of the early biologists, Pliny, in fact, classified animals on the basis of such analogous organs, that is organs that serve the same function and have similar appearance but have different evolutionary origins However, with the advancement in knowledge about the structure of animals it was revealed that the fish and whales or birds and butterflies have more differences than similarities between their body characters.
For example, fish respire using gills while the whales have lungs for this function. Similarly, birds have an internal skeleton whereas insects like butterflies lack it; instead they have an exoskeleton. Karl Linnaeus recognised these basic differences and based his system-of classification on the principle of homology. Homologous organs are those with the same evolutionary origin but which may not necessarily have the same appearance or serve the same function. For example, if you examine the fore-limbs of a man, the wing of a bat and the flipper of the whale (all mammals) you will see that in each case, though the function and the appearance of each of the fore-limbs are different, their basic skeletal plan is the same and they are homologous.
Furthermore, fossil records show that they all have a common evolutionary origin from the forelimbs of ancient amphibians. Thus, we conclude that all animals with pentadactyl forelimbs are related. Thus classification based upon homology reveals an inherent relationship among groups. In other words, it tells us that all creatures sharing basic structural features are related to each other and have evolved from a common ancestor.
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