Organ reduction, vestigial organs, and atavisms

Essence and causes of reduction

The reduction process implies either the complete disappearance of an organ or a significant simplification of its morphology throughout phylogeny. The main cause of such changes lies in the fact that a previously useful organ becomes relatively useless to the organism, and in rare cases, even harmful.

The key factors triggering reduction are:

• Environmental changes requiring morphological adaptation.
• Substitution is a phenomenon in which a new structure forms, functionally replacing the previous one. A characteristic example of substitution is the reduction of the head kidney (pronephros) in anamniotes, which occurs after the formation of a more advanced trunk kidney.

An important clinical characteristic of reducing structures is their increased variability. For example, the dimensions of the vermiform appendix in humans can range from 2 to 20 centimeters, which surgeons must take into account in their practice.

Degrees of reduction

Evolutionary underdevelopment can proceed with varying depths. Two main variants are distinguished:

1. Complete reduction: the structure disappears completely without a trace. It cannot be detected either in adult sexually mature individuals or at the stage of embryonic development.
2. Embryonic reduction: an organ does not develop in the adult organism, but at the embryonic stage it is present in the form of a specific embryonic rudiment. The classic example here is the formation of the pronephros (pronephros) in higher vertebrates.

Vestigial organs

Vestiges are organs whose genetic developmental program is strictly fixed hereditarily, but the morphogenesis process itself stops at a certain stage. As a result, such organs persist in sexually mature individuals, but are in an underdeveloped state and are incapable of performing their original function.

Humans possess a number of vestigial structures:

• Cecum and vermiform appendix.
• Plica semilunaris (remnant of the nictitating membrane of the eye).
• Body hair.
• Muscles moving the auricle.
• Third molars (wisdom teeth).
• Coccyx.
• Male nipples.

Patterns of underdevelopment and atavisms

The process of evolutionary underdevelopment of organs strictly obeys the rule of reverse sequence of morphogenesis. The essence of this rule is that the sequence of loss of developmental stages is the exact opposite of the normal course of organ formation.

First, the final (late) stages of development are blocked and dropped, and only then does reduction affect earlier primordia. For example, during limb underdevelopment, the primordia of the phalanges of the first and fifth digits disappear first, since normally during embryogenesis they form last.

Closely related to the concept of reduction are atavisms. Some structures undergo such deep reduction that normally they exist solely as short-lived embryonic primordia. A striking embryological example is the development of 9–10 tail vertebrae in the human embryo, which are reduced during further normal development.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation and clinical significance of high variability in vestigial organs
• Evolutionary transformation of the excretory system (from pronephros to metanephros)
• Genetic causes of developmental arrest in vestigial organs
• Examples of deep embryonic reduction in vertebrates

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

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