Analogy (biology)

An analogy (Greek ἀναλογία, the analogy of " proportionality", " correspondence ", " proportionality ", also: Parallel evolution, parallelism, convergent evolution or convergence in molecular biology also: homoplasy ) describes in biology a similarity of function and / or structure of organs, proteins, genes or behaviors of different Lebewesentaxa whose common ancestor did not possess this characteristic. The development of analog characteristics at not more closely related species that have been trained in the course of evolution by adaptation to a similar functional requirement and similar environmental conditions ( similar ecological niches, "Imagine equivalence " ) is denoted by convergence. This implies that is a range of creatures observed similar features directly on their function attributed and not necessarily provide a conclusion on close relationship between two species.

On the contrary, common ancestors with appropriate facilities declining properties is referred to as homology.


Analogous organs are similar not only in function, but also partly external, partly even ( superficial ) anatomically. But they are phylogenetically different and arose independently. It refers to its origins as a convergent development or shortly convergence.

Analogous organs arise after the system theory of evolution by an interplay of convergent selection pressure and development corridors.

They do not represent relationships, but leave by analogy conclusions to similar environmental conditions and ways of life. Frequently form living things with analog organs similar, overlapping ecological niches.

Usually the term is used in zoology. An example of this are the fins training in fish and whales. Although the fins of whales have same function and similar in shape to that of the fish, but are phylogenetically originated from the limbs of the former land-dwelling mammals. Unlike analog organs homologous organs have the same evolutionary origin, but not necessarily the same functions.

In botany, there are also analogous developments of the plants. So spines are often confused with thorns by laymen. Unlike the spines spines but are formed only from the upper cell layers (epidermis, cortex tissue ). So you are only surface structures ( emergences ). Thorns, however, are transformations of the leaves or the stem axis. Such transformations for adaptation to special living and environmental conditions is referred to in botany as a metamorphosis.

The terms analogy and homology are also used in evolutionary arguments in molecular genetics and proteomics. For analog genes or proteins bases or amino acid sequences are indeed identical denominated sections, but that emerge, for example, lying by mutations of different or in different places ( loci ) genes.

Biological analogies thus each have a same shape and / or function, but are out of different precursors or precursors of different location.


A classic example is the skull of wolf and thylacine or the shape of anteater and aardvark. The cause of such convergent developments that led to analogies that are same selection factors that have led to comparable adjustments. " The most obvious example are the marsupials of Australia, [ ... ] there is no placental mammals in Australia were available, they developed adaptation types that correspond to those in the northern hemisphere. "

Another example are the adapted to locomotion underwater limbs of various aquatic vertebrates, such as turtles, whales and penguins, although they all represent variations on the basic blueprint of a five-membered extremity and thus homologous to each other, however, different due to their descent have evolved from legs or wings independently and thus are analogous to each other. It is therefore to make adjustments to similar environmental conditions that led to similar forms and functions. However, if you go back far enough in the family tree, reptiles, mammals and birds are descended from a common ancestor that has the five-pointed forelimb.

Even within certain taxa convergent evolution is assumed, such as the mouthparts of insects. Here it came from originally chewing mouthparts to various derived types of functions, on the one hand with flower-visiting insects, the formation of suckers that can accommodate nectar very efficient, on the other hand, the convergent training stinging- sucking mouthparts.

The louse developed independently despite their great morphological similarity in the evolution of each other twice. The big match of the parasitic body characteristics is thus the result of adaptation to the host animal, but not a sign of a close relationship among the animals.

Even in plants convergent developments are known. An example can be found in the succulents: The New World Saguaro Cactus Pachycereus weberi sees the triangular spurge Euphorbia trigona from Africa are very similar. The arrangement of the leaf spines and the flowers form to allow a distinction ( determination). The similarity is based on the adaptation to hot and dry location.

Convergence, it is not just in the area of ​​the body shape, but also on a molecular level. Although ruminants such as the domestic cattle and leaf-eating monkeys like langur Presbytis Slim entellus belong to distant systematic groups, but have a very similar lysozyme molecule that is produced in the stomach.

Other examples of analogous organs and structures are

  • The grave legs of mole crickets ( insects) and moles (mammals )
  • The internal skeleton of cephalopods ( cuttlefish ) and vertebrates ( notochord, spinal cord ).
  • The wings of birds, and pterosaurs Fledertieren.
  • The wings of a bird and the wings of a butterfly
  • The parrot -like beaks for cephalopods, and parrots
  • The duck -like beak in ducks, platypuses and Hadrosauriern
  • The snake-like body in eels, slow worms, amphibians and snakes crawl.
  • The aerodynamic body shape with fast floating vertebrates such as sharks, dolphins, ichthyosaurs and penguins (birds )
  • Eye lens for cephalopods and vertebrates.

Term accruals

Analogy and homology

  • Analog are structures which can not be traced back to a common plan. Your similar expression is explained by convergence. For example, the wings of birds and bats ( flying membrane ), are relative to the support surface - springs or flight skin - analogously. The bats have their flight skin but stretched between the fingers of their forelimbs, the birds fly with the whole arm, so the arm with the springs. One function equality is a very different plan based.
  • Homologous structures, which can be traced back to a common plan. Their different expression is explained by divergence. An example: The front flippers of a dolphin and the front legs of an elephant with respect to the skeleton homologous, since the order of the bone, ie humerus, radius and ulna, etc. is the same. An almost identical blueprint is a completely different function against.

The analogous structures or behaviors serve the same purpose in the individual organisms, and are therefore in terms of their function is equivalent, but not due to common ancestry. In simple terms, homologous features original same, analogous features are functionally identical. However, these are not only functionally identical, but see also still very similar.

In some cases the distinction between " analog" and " homologous" but was dependent point: fins of dolphins and penguins represent homologous limbs, but the fin structures not go back to a common ancestor. Setting the Analogue Exaptationen dar. Similarly, the wings of bats and birds analogous developments on the basis of homologous limbs. In two sample pairs, the common structures of the common ancestor are the forelimbs of Sauropsida before about 310 million years ago.

Secondary education

Sometimes the function of an organ, which was re-formed in the course of evolution, later secondary fulfilled by an analog organ, when changing the living conditions back in the original direction, such as a land animal into the water returns (for example, the Fluke whales as secondary caudal fin). In rare cases such as the secondary jaw joint, the transition from primary to secondary organ also takes place directly.


A very different process than convergence or parallel evolution is co-evolution, which refers to the adjustments strongly interacting species. An example are some representatives of the genus Hippeastrum ( amaryllis ) whose blossoms form is specifically designed for pollination by some hummingbird species. The beak shape of hummingbirds in turn has adapted to the shape of the calyx over time. This joint development has advantages for both species. On one hand, it is ensured that the hummingbirds do not have to compete with members of other species for food, because no other animal can reach the nectar. On the other hand, is guaranteed by the body shape of the bird and the fact that he receives the nectar in flight that the pollen sticks to his chest and he dusted other flowers with it.