Homology, Analogy and Convergence

Homologous organs

Homologous organs are understood to mean the organs of organisms that can be traced back to a common blueprint, but that can clearly differ in function and appearance. In our example, the homology of the hand bones in different mammals becomes clear (I = human, II = dog, III = pig, IV = cow, V = tapir, VI = horse)

The anatomical similarities are thus due to a common ancestor in evolution, from which developed the extant species. Homologous organs - unlike analogous organs - have different functions (for example, hand bones in horses and humans have completely different functions)

The biologist Adolf Remane set to three homology criteria based on the respective relationship can be concluded:

1. Location criterion (organs are homologous when they are in the same situation, for example, the structure of the heart is identical in almost all mammals)

2. Criterion of specific quality (organs are homologous even if they are similar in many complex individual features)

3. Criterion of continuity (organs are homologous, if their development can be explained by the combination of intermediate forms)

Analogous organs

Despite their similarities, the wings of pterosaurs (1), bats (2) and birds (3) have developed independently of each other and thus represent an analogous organ.

Analogous organs are organs that have the same function, but their origin not in a common ancestor, but by similar environmental conditions that led to a similar development. Accordingly, analogies do not allow any conclusions to be drawn about any relationship relationships. It is believed that similar ecological niches with similar selection pressures lead to the formation of analogous organs in different species.

Further examples of analogous organs are, for example:

 - Forelegs of Mole and Mole Cricket

 - Hydrodynamic body shape for dolphins, penguins and sharks

 - Fins in fish and whales (ancestors of whales were land-living mammals)

Convergence

Convergence describes the independent development of analogous organs in different species due to similar environmental conditions. For examples s.o. according to analogy.