Ontogenetic and Phylogenetic Correlations

Role of natural selection

In the course of evolution, natural selection affects the interrelationships within an organism in various ways.

• Correlations of a specific character have limited significance and often disappear in the course of historical development.
• Correlations of a general character, conversely, are preserved even during profound structural changes, as they are useful across a wide range of conditions. A characteristic example is the interdependence between the development of an organ and the systems that support it (blood circulation, innervation, musculature).

Primary embryonic rudiments play an important role. For example, in all vertebrates, the chordamesoderm primordium strictly determines the subsequent development of the neural tube.

Types of ontogenetic correlations

Three main types of interdependencies are distinguished in individual development:

1. Genomic correlations. These form the basis for other forms and are driven by gene interactions. A key role here is played by Hox genes (mammals and humans have 39 of them), which regulate the formation of various body parts during embryogenesis by controlling complexes of other genes. The most important mechanism is pleiotropy (multiple gene effects), as seen, for example, in phenylketonuria, where a single gene causes intellectual disability, metabolic disorders, and pigmentary abnormalities. The evolution of Hox gene clusters proceeded via duplication (from Drosophila to vertebrates), preserving the homology of expression zones.
2. Morphogenetic correlations. Characteristic of the early stages of embryonic development, when organ functions are not yet formed. They are based on embryonic induction (e.g., the notochord induces the development of the neural tube, and the Wolffian duct induces the mesonephros) and the formation of morphogenetic fields. Earlier arising primordia determine the development of subsequent ones.
3. Ergontic correlations. Manifest later, during the growth period or in the adult organism. They are caused by functional interdependence. A striking example: the degree of development of a bone crest directly depends on the degree of development of the muscle attached to it (in gorillas, paralysis of the chewing muscles prevents the development of cranial crests).

Phylogenetic correlations (coordinations)

Coordinations are evolutionarily fixed interrelationships between organs. They are divided into three types:

• Topographic. Caused by spatial rather than functional arrangement. They are based on morphogenetic correlations and determine the general body plan of a taxon (e.g., the presence of a diaphragm, left aortic arch, hair, and mammary glands in mammals).
• Dynamic. Based on functional connection. Example: the correspondence between a highly developed peripheral olfactory organ and large olfactory bulbs in the brain of mammals.
• Biological. An indirect connection between organs that are united neither by function nor by topography. The habitat or diet acts as the connecting link. Thus, carnivorous animals combine developed canines, a short intestine, and claws, whereas herbivores have flat teeth, a long intestine, and hooves.

Class and exam questions

What else to study on this topic

• Mechanisms of embryonic induction
• Properties and functions of morphogenetic fields
• Evolution of Hox genes in vertebrates and invertebrates
• Pleiotropic effects of genes illustrated by hereditary diseases
• Formation of the general body plan of chordates

Other topics in the section "General patterns of organismal phylogeny"

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