Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 12. Evolution of Ontogeny

12.3. Embryonization of Ontogeny

Embryonization of ontogeny refers to the shifting of certain developmental stages to be protected within the mother's body or special protective membranes (eggs or seeds).

The Embryonic period of Organism development formed only in the course of evolution and was not an obligatory stage of ontogeny. The core meaning of this phenomenon can be considered as ensuring The Development of a more complex and demanding embryo (compared to ancestral forms) under more stable and protected conditions.

In plants, The process of embryonization consisted in The formation of the seed as a method of reproduction, which is significantly more effective compared to spores. Since seeds possess a protective coat and contain nutrients inside alongside the embryo, necessary for the latter's development, such an aromorphic acquisition can be regarded as the primary prerequisite for the modern success of seed plants on land. As we can see, the embryonization of higher plant ontogeny proved to be a necessary prerequisite for their progressive development in general.

Similar transformations of individual development also occur in the animal kingdom. The primary-larval type of development, typical of animals that develop from small eggs with a small amount of yolk, is accompanied by the presence of a free-living larva capable of independent existence. However, even in these organisms, There is a noticeable tendency to reduce the number of stages during which the larva contacts the environment. Thus, in parasitic nematodes, a larva of the second or even third age group rather than the first may hatch from the egg; in other species, host infection occurs at the egg phase or via a vector; in Fishes and amphibians, The amount of nutrient reserves increases, allowing the organism to reach a certain level of development before encountering the environment, and so on.

During the transition to non-larval development (cephalopods, reptiles, birds, etc.), the embryo remains protected by egg membranes for a long time, consuming a significant amount of the egg's nutrient reserves. However, the most important aspect in this case should be considered the possibility of direct development—metamorphosis becomes unnecessary, and all the most complex transformations shift to the embryonic period. For terrestrial animals, this very phenomenon can be considered the most important aromorphosis, as it frees ontogeny from dependence on the aquatic environment.

The secondary-larval type of development forms when the embryo develops under the protection of another organism's body, which may involve parasitism or viviparity. This phenomenon is accompanied by the reduction in egg size and the formation of specific adaptations that ensure the embryo's development at the expense of another organism (for example, the Placenta in placental mammals).

The most important consequence of embryonization can be considered the stabilization of the internal environment in which embryonic development takes place, as well as a decrease in the embryo's dependence on environmental influences.

This phenomenon can be reinforced by parental care and the Formation of the K-strategy, in which reproductive potential is expended on ensuring the maximum survival of a small number of offspring, compared to the allocation of resources in r-strategists toward producing the maximum number of offspring, the vast majority of which perish at the early Stages of Ontogeny. Thus, embryonization contributes to enhancing the Integrity and Stability of ontogeny.

Neoteny is the capacity for reproduction at early (larval) Selection/3.html">Stages of development. This phenomenon is quite widespread in nature, but a distinction should be made between facultative (optional) and obligate neoteny (Fig. 12.9). An example of facultative neoteny is the axolotl (Ambystoma mexicanum), which has not lost The ability to undergo metamorphosis and can transform into an adult form—the ambystoma. Other forms, such as the greater siren (Siren lacertina), have lost the ability to undergo metamorphosis (obligate neoteny).

The Evolutionary Significance of neoteny can be seen in the possibility of avoiding the narrow specialization characteristic of the adult organism, thereby increasing the evolutionary plasticity of the group. To some extent, the duration of ontogeny is shortened, and consequently, the number of generations increases. All of this influences the overall course of evolution in the respective groups of organisms. It is believed that herbaceous plants, insects, and even vertebrates formed through this exact path (more on this in the next chapter).

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Fig. 12.9. Examples of obligate neoteny (after A.G. Bannikov, 1985):

1 - Texas blind salamander (Typhomolge ratbuni); 2 - amphiuma (Amphiuma means); 3 - mudpuppy (Necturus maculosus);

4 - greater siren (Siren lacertina)

Fetalization is the retention in the adult state of traits characteristic of embryonic stages of development, caused by the slowed developmental rate of certain Organs or structures in ontogeny. This is also a common method of evolutionary transformation in organisms, examples of which include the predominance of cartilaginous elements in the Skeleton of modern amphibians. This specific form of change was of great importance in Human Evolution (slight Development of Body Hair, The ratio of the facial to the cerebral PARTS OF THE Skull—"childlike" traits).

Adultization is the accelerated development of certain organs or Organ Systems of an organism, As a result of which definitive features appear at Cytology/cytology/16.html">Early stages of ontogeny. Examples of such transformations include the formation of the auditory system in pinnipeds and neoteny itself (accelerated formation of the Reproductive System).