The morphological criterion involves a description of the external (morphological) characters of individuals that make up a particular species. In appearance, size and color of the plumage, for example, it is easy to distinguish a large spotted woodpecker from a green one, a small spotted woodpecker from a yellow one, a large tit from a crested, long-tailed, blue-tailed and from a gait. By the appearance of shoots and inflorescences, the size and arrangement of leaves, clover species are easily distinguished: meadow, creeping, lupine, mountain.
The morphological criterion is the most convenient and therefore widely used in taxonomy. However, this criterion is insufficient to distinguish between species that have significant morphological similarities. To date, facts have been accumulated that testify to the existence of twin species that do not have noticeable morphological differences, but which do not interbreed in nature due to the presence of different chromosome sets. So, under the name “black rat” there are two double species: rats with 38 chromosomes and living throughout Europe, Africa, America, Australia, New Zealand, Asia west of India, and rats with 42 chromosomes, distribution which is associated with the Mongoloid sedentary civilizations inhabiting Asia east of Burma. It was also established that under the name “malaria mosquito” there are 15 apparently indistinguishable species.