Biological Classification: Taxonomy, Phylogeny, and Anatomy

Taxonomy and Systematics: Classification Principles

Taxonomy is a scientific discipline concerned with the classification of living things, based on a hierarchical system of groups sorted according to their similarities.

The Linnaean System of Classification

The current system is based on that proposed in 1735 by Carolus Linnaeus. Living things are grouped into taxonomic categories:

• Species are included in genera.
• Several genera comprise the family.
• Families are grouped into orders.
• Orders are grouped into classes.
• The next category is the phylum (for animals and protists) or division (for plants and fungi).
• The highest taxonomic level is the kingdom.

Taxonomy helps establish relationships within different groups according to shared morphological, physiological, and genetic characteristics. The study of evolutionary relationships among organisms, or phylogeny, is the domain of systematics.

Systematics contributes to the construction of phylogenetic trees, which illustrate the evolutionary history of living beings, tracing major divisions from the kingdom level down to the species we know today.

Classification Based on Ancestry

The classification of organisms can be based on common ancestors:

• If all subsets of a taxonomic level share a common ancestor, they are called monophyletic.
• If, on the contrary, they are derived from various evolutionary lineages, they are called polyphyletic.

For example, it is believed that mammals evolved from three groups of Triassic reptiles.

Evolutionary Diversification

The first major evolutionary diversification (approximately 550 million years ago) produced many species, which subsequently diversified further. Other taxa became extinct and were replaced by others that emerged from different evolutionary branches. The phenotypic characteristics of extinct taxa were negatively selected by the environment. If environmental conditions on the planet had been different, other species, different from those we know today, would populate our environment.

Comparative Anatomy and Evolutionary Evidence

Basic similarities between groups of organisms often exist independently of the lifestyle they lead.

Richard Owen's Concepts: Homology and Analogy

Richard Owen developed two fundamental concepts: homology and analogy.

Homologous Organs

The front limbs of a man, a mole, or a bat, although superficially different, are made up of bones, muscles, and nerves in equivalent positions. In man, the hand functions for pressure; in the mole, for digging; and in the bat, for flight. These structures are called homologous organs: they have a common evolutionary origin but different functions.

Analogous Organs

Analogous organs have similar functions, but the structures that shape them are not related evolutionarily to each other. One example is the wings of a butterfly, which fulfill the function of flight, just like those of a gull or a bat, but do not share the same evolutionary origin. Another example is the lungs of mammals and the tracheae of insects.