Taxonomy and Biological Classification of Living Organisms
Unranked Systems
The advancement of knowledge led scientists to a conclusion: a series of common features are found in individuals of each group. These tri-fold categories are homogeneous. One of the group’s advantages is that it is able to include living beings until the moment they are unknown. A classification is good if it refers to characteristics that do not vary in people. According to the criterion, we get one or the other, and it is important that the classification criterion is objective and discriminatory.
Systematics, Taxonomy, and Nomenclature
Systematics is a science that aims to create classification systems that express the degree of similarity between human beings and reflect existing evolutionary relationships. The result is a general classification. Systematics uses nomenclature and taxonomy as tools to meet its objectives. Taxonomy is concerned with the ordinance of beings and provides the principles, rules, and procedures to conduct a classification. It handles nomenclature to name the different living organisms.
First Attempts at Classification
The first known classifications grouped living beings into artificial categories, according to the more or less arbitrary criteria established by naturalists of the time. Aristotle classified the animal and plant kingdoms into categories. Theophrastus classified plants into trees, shrubs, subshrubs, and herbs. Linnaeus established the basis of taxonomy. Later, he established the binomial system. He organized them into groups of increasingly lower levels. Each level contains the taxonomic categories.
Taxonomic Categories
The most important are species, genus, family, order, class, phylum, and kingdom. Similar species are grouped in the same genus. Categories can be divided into subcategories with the prefix “sub”. Groups of organisms are called taxa.
Embryonic Segmentation Development
The zygote divides by mitosis according to increasingly perpendicular meridian planes, causing smaller cells called blastomeres that stay together. The spherical mass of these cells is called a morula. When segmentation advances, the blastomeres migrate to the periphery, forming an outer wall, the blastoderm, which leaves an internal cavity filled with fluid called the blastocoel. This state is called a blastula. The amount of yolk determines the type of segmentation and the size of the blastocoel. When the egg has a lot of yolk, segmentation is partial.
Gastrulation
The blastula undergoes serious changes in the folding position of cells, ending in a state called gastrula. In the process, three layers or sheets are formed, which are the embryonic ectoderm, mesoderm, and endoderm. In this process, the first layer formed is the ectoderm, then the endoderm. Inside the gastrula, the endoderm has a cavity, the archenteron, which communicates with the outside through the blastopore. The formation of these layers can be by invagination or epiboly. Animals that only develop these two embryonic layers are called diploblastic. The rest continue their development with the formation of a third embryonic layer between the two, the mesoderm. These animals are called triploblastic.
Protostomes
Animals whose blastopore gives rise to the definitive mouth.
Cephalization
Concentration of sense organs on the front with nervous organs.
Phylogenetic Tree
Phylogeny can be represented as a tree that represents natural kinship relationships among organisms. These trees are called phylogenetic trees. On the base of the trunk is the common ancestor of all beings, from which branches start diversifying towards the other pole to reach the periphery, where the present species are. Edge relations are built from evidence of evolution, such as paleontology, anatomy, embryology, genetics, cytology, and ethology. A natural classification can be established in which organisms are grouped according to characteristics.
The Species
It is the fundamental taxonomic category and represents the basic unit of an unranked system. The concept of species was used at the end of the 19th century as a morphological species, that is, individuals with their own morphology. We now know that organisms do not have fixed and unchanging characters, but there is variability within individuals. The term “biological species” is used, which defines the species as a whole of organisms that have a significant number of characters in common, share a common genetic heritage, can cross between them giving fertile offspring under natural conditions, and do not share characters with the other organisms, implying reproductive isolation.
How a Species Forms
All species descend from others that are transformed gradually over time. The mechanism by which a species occurs from another is called speciation. There are two mechanisms:
- Allopatric: The most frequent mechanism. It occurs when a population is isolated because of geographic barriers. These barriers prevent interbreeding with other populations.
- Sympatric: It occurs when a species occupies a territory that is diversified into two populations because of mechanisms impeding their reproduction, such as the existence of different habitats within a territory.
Nomenclature of Living Beings
The diversity makes it necessary to appoint living beings clearly. Nomenclature has some codes for controlling and regulating the creation of scientific names for animals and plants. Linnaeus established binomial nomenclature. This system considers the species as the basic unit of classification, given a name made up of two words:
- Generic name: It is common to all species of the same gender, and the first letter is capitalized.
- Specific epithet: It is common for individuals belonging to the same species and differentiates species of the same gender. It is written in lowercase. In binomial nomenclature, the epithet cannot be used in isolation. The scientific name is usually accompanied by the surname of the scientist who discovered the species.
The scientific name is used because:
- Most species have no common name, either because there is no need or because they are microscopic.
- The scientific name is universal, while the common name is different in each language and can vary.
- The common name is sometimes inaccurate because the same name is assigned to different species.
Three Domains
Recent research recognizes the domain as a new taxonomic category above the kingdom. Carl Woese proposed three domains: Archaea, Bacteria, and Eukarya. Archaea and Bacteria include microbial organisms with prokaryotic cells. The Archaea form a group of primitive bacteria that would be more akin to the domain Eukarya than Bacteria. Eukarya is the line that encompasses the eukaryotic kingdoms: Protoctists, Fungi, Animals, and Plants.