CLASSIFICATION, THE CONCEPT OF SPECIES, AND TAXONOMIC CATEGORIES
Since ancient times, scholars have sought to bring order to the diversity of living forms by grouping them based on specific criteria, enabling them to distinguish one form from another. This activity gave rise to the science of organism classification, known as taxonomy.
A primary division of organisms can be made based on the type of cells that make up their bodies: this allows us to distinguish Prokaryotes from Eukaryotes.
The former consist of primitive cells lacking a nucleus, while the latter are made up of cells equipped with a nucleus containing hereditary material, separated from the rest of the cell by a nuclear membrane.
Another classification criterion is based on how organisms obtain the substances necessary to build and sustain their cells. Some organisms produce their own food by taking inorganic substances from the environment and transforming them into organic substances. These organisms are called autotrophs or producers, such as plants, algae, and phytoplankton. Other organisms, like animals, obtain the energy and nutrients they need from organic substances derived from other living beings. These are called heterotrophs.
Some organisms, like bacteria and microscopic fungi, obtain the matter and energy they need by breaking down organic substances into simpler ones; these organisms are called decomposers.
A third distinction can be made based on the type of reproduction, which can be sexual or asexual, and whether the organism develops from an embryo. Finally, morphological and functional characteristics of organisms are considered.
None of these characteristics fully resolve all classification challenges, and classification is subject to change as new knowledge is acquired.
The first attempt to develop an analytical method for classifying living organisms is credited to Carl Linnaeus. The famous Swedish naturalist based classification on key characteristics, or morphological traits, that enable the reliable identification of individual species. Linnaeus also introduced binomial nomenclature, which is still used in biology today. This system assigns each organism a name composed of two words: the first word indicates the genus, and the second indicates the species. For example, certain plants with capitulum inflorescences are grouped under the genus Helianthus. The common sunflower, which belongs to this group, is identified with the term annuus, making Helianthus annuus the full scientific name of the sunflower.
Linnaeus based his classification system on identifying characteristics such as shapes, sizes, body structures, and colors. For Linnaeus, all living beings were the result of divine creation and had undergone no transformation since their creation. His concept of a species was synonymous with the idea of a "creature of God."
With the advent of early evolutionary theories, methods of classification underwent significant changes. The Darwinian theory applied to systematics encouraged the study of phylogenetic relationships between species, investigating the processes through which species descend from one another. By studying evolutionary mechanisms, it was recognized that all living beings share a common, albeit distant, origin and that different living species have arisen from one another over time.
Using the evolutionary approach, we can affirm that whales are mammals and not fish because they share many characteristics with other mammals (e.g., they nurse their young and breathe through lungs) and because whales and other mammals share a common ancestor.
The studies of Gregor Mendel and the advent of genetics further refined the concept of species. Members of the same species share similar genetic makeup, resemble each other morphologically, and can interbreed to produce offspring that are also capable of reproduction.
In some cases, individuals from different species can interbreed but produce sterile hybrids—individuals that cannot reproduce. For instance, mules, which are the offspring of a horse and a donkey, are sterile. In other cases, offspring resulting from the interbreeding of different species may not survive.
Conversely, individuals of the same species may face difficulties in interbreeding. For example, dogs like Dobermans and Chihuahuas struggle to mate due to their significant size difference; however, they can mate with intermediate breeds, and thus they are grouped within the same species.
Taxonomic Categories
Structures in different organisms that share a common embryonic origin, even if they serve different functions in adulthood, are called homologous structures. For example, the limbs of mammals and the fins of fish and aquatic mammals are homologous because they originate from the same embryonic structures, even though they serve different functions in adult individuals. Similarities due to homology are clear indicators of relatedness and point to derivation from a common ancestor. Based on their homologies and descent from common ancestors, individuals from different species are grouped into broader categories. For example, lions, leopards, and tigers are different species but share many homologies and are grouped within the same genus, Panthera.
Related genera are then grouped into the same family. Taxonomists place the genus Felis, which includes cats, and the genus Panthera into the family Felidae. Dogs and wolves resemble cats, lions, panthers, and tigers in some respects, sharing common ancestors with them, but they differ in certain characteristics. Taxonomists highlight these differences by placing dogs and wolves in a different family, Canidae. Bears differ from felines and canines in various ways, and they are grouped into yet another family, Ursidae. These three families (Felidae, Canidae, Ursidae) share the characteristic of being fur-bearing predators and are therefore grouped within the order Carnivora.
Wolves, bears, and lions differ significantly from horses, so horses are assigned to a different order, Perissodactyla. However, horses, like monkeys, rats, cattle, and many other animals, share certain traits with wolves, lions, and bears—most notably, they nurse their offspring. These various animal orders are grouped into the same higher category: the class Mammalia.
Other animal classes, such as birds, frogs, fish, and reptiles, share with mammals the characteristic of possessing a notochord—a rod of cells providing support—during their embryonic stage. These classes are grouped into the phylum Chordata. Finally, all chordates, along with snails, butterflies, sponges, and thousands of other animals, are grouped within the Kingdom Animalia.
Summary of Taxonomic Categories
We have defined the following taxonomic categories for the classification of animals: species, genus, family, order, class, phylum, and kingdom.
Today, living species are divided into five Kingdoms: Monera, Fungi, Protists, Plants, and Animals.
The same taxonomic categories previously defined for the Animal Kingdom also apply to the other four kingdoms. However, for plants, in some classifications, the term division is used instead of phylum.
