Morphological criterion
This criterion is traditionally based on a detailed study of the external and internal structure of individuals. For a long time, this approach was considered the main one, but today its clear insufficiency is recognized.
The main problem of the morphological criterion is the existence of so-called sibling species. These are groups of organisms that are virtually indistinguishable externally and anatomically, yet are reproductively isolated from one another.
A telling example is the malaria mosquito (Anopheles maculipennis). It has been proven that at least 6 sibling species of it exist in nature. The biological boundaries between them can only be detected through a careful analysis of accompanying factors:
- Differences in the shape of laid eggs.
- Specifics of insect breeding sites.
- Dietary specialization (feeding targeted strictly at specific animal species).
Physiological-biochemical criterion
This criterion is based on the fundamental similarity of tissue chemical composition and the unity of physiological processes occurring in related organisms. However, taken alone, it is also not considered absolutely reliable.
The main limitation is intraspecific biochemical variability—individuals of the same species may exhibit different metabolic pathways. A striking example is the peculiarities of nitrogen metabolism in dogs:
- In the vast majority of dogs, the end product of metabolism is allantoin (in biochemistry, it is represented in two forms: a branched ring and a cyclic form).
- In some specific breeds, as a rare exception, the end product of metabolism is uric acid (a purine base), which is more characteristic of human physiology.
Ecological-geographical criterion
This approach describes the spatial distribution of organisms and the role that a particular species plays in its ecosystem.
Main provisions of the criterion:
- Each species historically occupies a specific range of distribution — areal. For example, the brown rat (Rattus norvegicus) has its own strictly specific habitat range on the map.
- Each species occupies its own unique ecological niche in the ecosystem.
The limitations of this criterion are directly related to the existence of cosmopolitan species. These are organisms whose distribution ranges can be extremely vast or even completely overlap with the ranges of other animals. Because of such ubiquitous distribution, a species cannot be characterized solely by geographic location. Typical cosmopolitan species include the housefly, sparrows, pigeons, and rats.
Genetic criterion
In modern evolutionary biology, this is recognized as the primary criterion of a species. A biological species is viewed today as a genetically stable system that strictly preserves its internal integrity.
The key characteristic within this criterion is the karyotype, i.e., a strictly defined and unique set of chromosomes for each species. Counting and analyzing the chromosome set makes it possible to unmistakably distinguish between externally similar organisms and differentiate sibling species.
Practical significance and examples of karyotypes:
- The human genome contains 46 chromosomes, whereas in chimpanzees the set consists of 48 chromosomes.
- Division of the common vole. For a long time it was considered a single species (Microtus arvalis), but based on chromosomal analysis it was divided into two groups (subspecies). It turned out that the subspecies Microtus arvalis contains 46 chromosomes, while the morphologically identical subspecies Microtus subarvalis has 54 chromosomes.
Briefly: answers to topical questions
What precisely is the difference between a geographic range and an ecological niche?
The difference is that a geographic range is the territory where a species is distributed, whereas an ecological niche characterizes the species' role in the ecosystem. The ecological-geographical criterion of a species describes its spatial distribution and role in the ecosystem.
| Characteristic | Geographic range | Ecological niche |
|---|---|---|
| Definition | Territory where the species is distributed | Role of the species in the ecosystem |
| Significance for the species | Shows what territory the species occupies | Describes the species' role in the ecosystem |
What mechanisms ensure reproductive isolation between sibling species?
The cited sources explicitly state that sibling species are morphologically indistinguishable yet reproductively isolated. General isolation mechanisms are described as barriers preventing free interbreeding.
- Biological (reproductive) isolation may include prezygotic barriers: ecological, ethological/behavioral, morphophysiological, and gametic isolation.
- Postzygotic isolation is associated with hybrid inviability or sterility.
- The source on sibling species for the malaria mosquito (Anopheles maculipennis) indicates that differences are revealed in egg shape, breeding sites, and dietary specialization.
- The genetic criterion makes it possible to distinguish externally similar organisms by their karyotype; the example of the common vole shows groups with 46 and 54 chromosomes.
For quick memorization of the four main criteria, use the acronym MEFG: Morphological, Ecological-geographical, Physiological-biochemical, Genetic.
Class and exam questions
Can a species be reliably determined using only the morphological criterion?
No, you cannot. There are sibling species that are completely indistinguishable externally and in internal structure, yet are reproductively isolated.
What are cosmopolitan species?
These are species with extremely vast distribution ranges that may overlap with the territories of other species. Examples: rats, pigeons, sparrows, housefly.
Why is the genetic criterion considered primary?
Because a species represents a genetically stable system. The karyotype (number and shape of chromosomes) is strictly specific and allows even identical-looking sibling species to be distinguished.
What else to study on this topic
- Mechanisms of reproductive isolation in sibling species
- Features of nitrogen metabolism: biochemical significance of allantoin and uric acid
- Formation and characterization of a species' ecological niche
- Karyotype analysis and chromosome differentiation methods
- Evolutionary significance of cosmopolitan species in the biosphere
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