Sex and species, two related concepts

Taxonomic categories

The species is the basic taxonomic category used by biologists to classify living things. The other categories (genus, family, order, class and phylum) are considered artificial and arbitrary. On the other hand, we tend to regard the species as natural, obvious, similar to a concept when the represented objects are living beings. But we will not enter here into the famous problem of universals, nor wonder on whether concepts (and species) really exist or are mere constructs of the human mind.

The classic definition of a species is: a set of living beings that share common characteristics and can interbreed, giving rise to fertile offspring. Notice that the use of the word interbreed implies that the living things in question use sexual reproduction. This leads us to ask whether the concept of a species should be restricted to living beings with this type of reproduction, or it can be extended to those that reproduce otherwise, such as prokaryotes and some eukaryotes. This question can be answered in several ways:

Classifying living beings: Cladistics or complexity levels

The tree of life

Since Aristotle, living beings have been classified in kingdoms. At first there were two: plants, practically unmoving, and animals, capable of active movement.

When Antony van Leeuwenhoek discovered microorganisms, biologists tried to maintain this two-fold division, integrating some with animals (amoebas and Paramecium), others with plants (bacteria and microscopic algae and fungi). But at that level, the separation between animals and plants is blurry, and in the mid twentieth century a third kingdom was added to the other two: protists, unicellular living beings.

A little later, biologists came to the conclusion that the kingdom of plants should be divided into two: fungi at one side, all the other plats (metaphyta) at the other. By 1975, therefore, there were four different kingdoms.