The species and population level of life. The concept of species. Species criteria
A species is a set of populations whose members are similar in structure, physiology, behaviour and heredity, interbreed freely to give fertile offspring, are adapted to definite conditions and have their own range. The term “species” was introduced into science by the English botanist J. Ray; C. Linnaeus held it unchangeable, while Ch. Darwin showed that species arise, change and disappear. A set of criteria is used to identify a species. The morphological criterion is similarity of external and internal structure; but sexual dimorphism (males and females of one species differ) and sibling species (almost identical outwardly but not interbreeding) limit it. The physiological criterion is similarity of life processes and reproduction: members do not interbreed with other species or their offspring are sterile (horse × donkey = mule, sterile), but some species (dog and wolf, poplar and willow species) produce offspring. The biochemical criterion is the make-up of DNA, RNA and proteins. The geographic criterion is the range, though ranges of species may overlap. The ecological criterion is the environmental conditions and ecological niche. The genetic criterion is the karyotype: number and structure of chromosomes (two sibling species of black rat have 38 and 42); but cabbage and radish both have 18. The ethological criterion is behaviour (song, courtship). No single criterion is enough, so all are used together.
“Criteria table”: groups compare two species (for example horse and donkey) on the seven criteria and find which criterion is not enough.