Mechanisms and pathways of speciation
The process of formation of new species can follow several scenarios. Allopatric (geographic) speciation has been studied in the most detail. It includes three strictly sequential stages:
- The appearance of an insuperable physical or geographical barrier that divides the original population into isolated parts.
- The accumulation of unique mutations in each of the isolated groups. Under the influence of genetic drift and natural selection, allele frequencies gradually change, adapting organisms to local conditions.
- The formation of reproductive isolation. If organisms from different groups meet again, they will no longer be able to interbreed and produce fertile offspring.
In addition, other pathways are distinguished:
- Phyletic speciation (anagenesis): this is the slow, gradual transformation of one existing species into another over historical time. No divergence (splitting into multiple branches) occurs.
- Hybridogenic speciation: the emergence of a completely new species as a result of crossing (hybridization) of two already existing ones. In botany, this process is often accompanied by polyploidy.
Forms of phylogenesis
Phylogenesis, or the historical development of organisms, can proceed in different directions depending on environmental conditions and the relatedness of groups:
- Divergence is the divergence of traits in evolutionarily related groups. This process leads to the formation of homologous organs (having a common origin, but potentially performing different functions).
- Convergence is the convergence (approaching) of traits in completely unrelated taxonomic groups that inhabit similar environmental conditions. As a result, analogous organs are formed (different in origin, but similar in function).
- Parallelism is the process of independent development of very similar traits in groups of organisms that are closely related.
Directions of evolution and biological progress
The evolutionary success of any taxonomic group is described by the concept of biological progress. According to A.N. Severtsov, progress is characterized by three main criteria: a steady increase in the number of individuals, an active expansion of the habitat range, and an increase in the number of subordinate taxa (new species or subspecies).
Biological progress is achieved through two main pathways:
- Aromorphosis (arogenesis). These are large-scale evolutionary changes that radically increase the general level of structural organization and metabolic intensity. Aromorphoses open the way to conquering fundamentally new habitats. Examples: the emergence of a four-chambered heart, the development of warm-bloodedness, the formation of pulmonary respiration in vertebrates, and the appearance of the flower in plants.
- Allogenesis (idioadaptation). This is the pathway of forming specific adaptations to strictly defined, concrete environmental conditions. During allogenesis, there is no increase in the general level of organization—organisms simply adapt to their niche. Examples: the formation of differently shaped beaks in finches depending on diet, and the development of various types of limbs in mammals.
The most important principle of macroevolution is the rule of irreversibility (Dollo's law). According to it, no organism or population is capable of returning to a state already passed in the line of its evolutionary ancestors. Even if environmental conditions become exactly the same as in the past, once-lost organs will never be restored in their original form.
Briefly: answers to topical questions
What are the pathways for achieving biological progress according to A.N. Severtsov?
There are three main pathways for achieving biological progress according to A.N. Severtsov.
- Arogenesis is progressive evolution through aromorphoses (major changes that raise the general level of organization).
- Allogenesis is evolution through idioadaptations (specific adaptations to environmental conditions without raising the level of organization).
- Catagenesis is evolution through general degeneration (simplification of organization), often upon transition to parasitism or a sedentary lifestyle.
What is sympatric (ecological) speciation?
Sympatric speciation is the process by which a new species arises within the original population in the same geographic area.
Its characteristic feature is ecological specialization without the formation of a physical barrier. As a result, reproductive isolation arises within the initial population. The process occurs instantaneously (in quanta) over a short period of time.
What is general degeneration (catagenesis) and how does it affect biological progress?
General degeneration (catagenesis) is a secondary simplification of organization, leading to the loss of organ systems and structures. It occurs during the transition to a sessile or parasitic lifestyle.
Despite the simplification, general degeneration increases the level of biological fitness, allowing the group to adapt to new conditions and widely spread, which represents a pathway of achieving biological progress.
What are the criteria for biological regression?
Biological regression is defined as a decrease in the level of adaptation to environmental conditions.
It is characterized by the following signs:
- A decrease in the number of individuals in the group.
- A narrowing or breakdown of the habitat range.
- Mass catastrophic elimination (death).
- Complete extinction of the group.
What are some examples of parallelism in evolution?
An example of parallelism in evolution is the repeated development of saber-toothedness within the cat family (Felidae).
Parallelism is convergence based on homologous organs, where a similar appearance is formed in initially divergent, but genetically close groups.
To memorize forms of phylogenesis: Divergence — Tree (branches diverge from a single trunk, organs are homologous). Convergence — Wedge (lines converge at a single point in unrelated groups, organs are analogous).
Class and exam questions
What is the main difference between aromorphosis and idioadaptation?
Aromorphosis always leads to overall complexification and an elevated level of organization (e.g., the appearance of warm-bloodedness), enabling the colonization of new environments. Idioadaptation is merely a specific adaptation to a particular environment without changing the general level of organization (e.g., different beak shapes).
Can a species regain a lost organ if it returns to its former habitat?
No. According to Dollo's law (the rule of evolutionary irreversibility), a return to the previous state is impossible, and lost organs are not restored in their former form.
How does phyletic speciation differ from the others?
In phyletic speciation, there is no splitting of a single species into multiple new ones (divergence). The original species is simply gradually transformed into a new species over time.
What else to study on this topic
- Detailed analysis of the stages of allopatric speciation and the role of geographical barriers.
- The impact of polyploidy on hybridogenic speciation in plants.
- Comparison of homologous and analogous organs using classical biological examples.
- Analysis of the criteria of biological progress according to A.N. Severtsov.
- The unity of micro- and macroevolution within the framework of the Modern Synthesis (Synthetic Theory of Evolution).
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