Taxonomy Classification Criteria

Carlos Linneo, based his classification system on similarities in body structure. For example, he considered a bat to be a mammal because it had many structural similarities to other mammals. The mere fact that it had wings did not qualify it to be a bird. Other evidence shows that even though bats have wings, they are mammals and not birds.
Linnaeus' work on taxonomy was done more than a century before the work of Darwin or Mendel. After Darwin presented his Theory of Evolution, scientists saw the differences and similarities in organisms as products of evolution. Today, the major emphasis of taxonomy is the study of evolutionary relationships.
The classification of a species is based on the evolutionary history of that species. For example, at one time, rabbits and squirrels were classified as rodents. The study of primitive fossils of rabbits shows that they evolved from a different ancestor than the ancestor of rodents. For this reason, rabbits are now classified in a separate group from rodents.

Many of the ideas that support the Theory of Evolution provide a useful basis for classifying an organism as a particular species. Today, taxonomists who classify organisms study them in various ways:

1. The general structure of the organism is studied to try to find homologous structures. Remember that homologous structures are structures that have a basic similarity and have developed in organisms that may have had a common ancestor. For example, the wings of a bird and the leg of a tortoise are homologous structures.
2. The life cycle of the species is studied to look for a biological similarity with other groups of organisms.
3. The fossil record is studied, if available, to show relationships between organisms over time.
4. The degree of biochemical similarity between the species is determined. The order of amino acids in similar proteins differs from species to species. Similarities in the amino acid order of certain proteins can help determine how to classify organisms.
5. The genetic similarity between the chromosomes of different species is studied. Information about this resemblance can provide very important clues for classification. Relationships between different species can be found by studying and comparing segments of:
6. DNA. The more similar the order of the nucleotide bases, the more closely related the species are.