Base and methodology of evolutionary morphology
Evolutionary morphology emerged at the intersection of three major biological disciplines. Its main task is to establish facts of the recurrence of ancestral traits (recapitulation), determine the modes of phylogenetic changes, and reveal the relationship between an organism's anatomy and the conditions of its existence.
The primary research tool is the method of "triple parallelism", which combines data from three sciences:
- Comparative anatomy helps distinguish between homologous and analogous structural similarities.
- Embryology reveals phenomena of embryonic resemblance (based on K. von Baer's law).
- Paleontology allows studying recapitulation, i.e., the repetition of ancient ancestral traits during development.
Comparative characteristics of organs
When studying the evolution of organ systems, it is crucial to understand the difference between two types of structures that may resemble each other in appearance or function.
Homologous organs form as a result of divergence. They share a common structural plan, a single origin, and develop from the same embryonic primordia. However, their functions can differ drastically. A classic example is the human forelimb, bird wing, and seal flipper.
Analogous organs, conversely, are the result of convergence. They develop from different primordia and have completely different origins, but look similar because they perform the same task. For example, a bird's wing and an insect's wing, or a fish's gills and a crayfish's gills.
Pathways and modes of organ transformation
During evolution, organs do not remain static. Their phylogenetic transformations follow two main pathways:
- Differentiation is a process in which an initially uniform structure divides into heterogeneous specialized parts. A classic example is the differentiation of the mammalian dentition.
- Integration is the reverse process of uniting parts into a single whole with increased interdependence, such as the fusion of individual vertebrae into the sacrum.
In addition, the functional load on the organ itself changes. A change of function is possible, where a secondary task becomes primary, as well as expansion or narrowing (intensification) of the initial function.
Correlative development: correlations and coordinations
Organs develop not in isolation, but in close connection with one another. During individual development (ontogeny), such interrelations are called correlations. For example, the development of the optic cup induces the formation of the lens. Correlations can be genomic, morphogenetic, and ergontic.
In historical development (phylogeny), stable interrelations are called coordinations. They are divided into three types:
- Topographic — associated with the spatial proximity of organs.
- Dynamic — based on direct functional interdependence.
- Biological — unite organs that are not directly connected anatomically, but are jointly necessary for survival in a specific environment (e.g., teeth structure and gastrointestinal features in herbivores).
Mechanisms of organ disappearance and appearance
In the course of evolution, organs may lose their significance or, conversely, appear as "echoes" of the past. Two concepts are used to describe these phenomena:
- Rudiments are underdeveloped structures that have lost their main function during phylogeny. The most important characteristic of a rudiment is that it is present in all representatives of a given species. In humans, these include the appendix, coccyx, and nictitating membrane.
- Atavisms are the sudden appearance in individual specimens of traits that were characteristic of their distant ancestors but have long been lost by the species as a whole. Examples in humans include the development of a tail, polymastia, or continuous body hair.
Briefly: answers to topical questions
What is the difference between morphogenetic and ergontic correlations?
Morphogenetic correlations belong to the type of ontogenetic interrelations in a developing organism. In contrast, ergontic correlations appear at later stages of ontogeny in a growing or adult organism and are caused by the direct interdependence of functions of various organs or body parts. For example, the degree of development of a specific muscle directly determines the degree of development of the bone process to which it attaches.
What vestigial organs are present in the human body?
Human vestigial organs include:
- cecum and appendix;
- nictitating membrane of the eye (semilunar fold);
- body hair;
- auricular muscles;
- wisdom teeth;
- coccyx;
- male nipples.
Rudiments are preserved in sexually mature individuals in an underdeveloped state and do not perform their original function.
Which human developmental abnormalities are classified as atavisms?
Human atavisms include traits characteristic of distant ancestors but lost by the species and appearing in individual specimens:
- polymastia (polythelia);
- caudal appendage (tail);
- continuous hair cover (hypertrichosis).
How is Karl Ernst von Baer's law of embryonic resemblance formulated?
According to Karl Ernst von Baer's law of embryonic resemblance, an organism's development proceeds from general to specific. During embryogenesis, the traits of the phylum, class, order, family, genus, and species appear sequentially; genus and species traits appear last.
What is the principle of organ substitution in phylogeny?
Organ substitution in phylogeny is an evolutionary process in which one organ is replaced by another, taking over its function. In this process, the replaced organ either disappears completely during evolutionary development or becomes vestigial. Classic examples of such substitution are the replacement of the notochord by the vertebral column or the replacement of the pronephros in vertebrates by the mesonephros.
To avoid confusing terms: Homologous organs — shared genetic origin (but different functions). Analogous organs — analogous work (identical functions, but different origin).
Class and exam questions
What is the main difference between a vestigial structure and an atavism?
Rudiments are normal and present in all individuals of a given species (e.g., the human coccyx). Atavisms are genetic abnormalities, a reversion to ancestral forms, found only in rare individual specimens (e.g., the birth of a human with a tail).
What are divergence and convergence in morphology?
Divergence is the divergence of traits in related organisms leading to the formation of homologous organs. Convergence is the independent acquisition of similar features by unrelated organisms due to the same habitat, leading to the appearance of analogous organs.
What is the triple parallelism method?
This is a comprehensive approach to studying evolution that simultaneously uses data from three sciences: comparative anatomy, embryology, and paleontology.
What else to study on this topic
- Modes of morphofunctional transformations (heterotopia, heterochrony, heterobathmy)
- Phyloembryogenesis according to A.N. Severtsov (anabiosis/anaboly, deviation, arhaellaxis)
- Types of ontogenetic correlations (genomic, morphogenetic, ergontic)
- K. Baer's Law of Embryonic Resemblance and its role in embryology
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