Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 16. Modern Problems of Evolution

16.7. Problems of Discreteness and Continuity

Discontinuity and continuity are distinct yet interrelated characteristics of any developmental process. Structure/149.html">The problem of their correlation in evolution has evolved into a separate issue. For a long time, there was a stark opposition between tichogenesis (Darwinian evolution via gradual, minor transformations) and saltationism (abrupt evolutionary changes).

The classical Darwinian concept views speciation as a gradual, gradualistic process where drawing a clear line between parent and daughter species is practically impossible. At the same time, even C. Darwin anticipated that speciation rates could vary. Saltationist views were among the first to be formulated by S.I. Korzhinsky in 1899, when he introduced METABOLISM/2.html">THE CONCEPT OF heterogenesis, suggesting that plant species could originate abruptly. In 1903, H. de Vries proposed the mutation theory of speciation driven by macromutations.

Scientific evidence concerning The Role of polyploidy and Hybridization in the speciation of plants first, and later animals, accumulated rather quickly. J. Huxley proposed the term "quantum speciation" to denote rapid speciation (these issues were discussed in detail in the previous chapter).

Such views find confirmation in paleontology, the data of which are often fragmentary, with most transitional forms yet to be discovered. Nevertheless, a growing number of scientists conclude that discreteness and continuity should not be opposed; rather, they Complement one another.

At the same time, nature exhibits both gradual speciation (primarily allopatric) and abrupt speciation (most forms of sympatric speciation, where a biologically isolated new form is established quite rapidly).

Furthermore, such phenomena are characteristic of the macroevolutionary level as well, manifesting as alternating periods of rapid morphological diversification and relative stability. The theoretical foundation for these views is I.I. Schmalhausen's rule on the alternation of the Morphology/3.html">MAIN DIRECTIONS OF evolution. Arogenesis is invariably a rapid process of forming a fundamentally new body plan, enabling the respective organisms to occupy a novel adaptive zone. It is periodically replaced by allogenesis, which is conceived primarily as a process of slow speciation within that adaptive zone.

Certain contentious issues in systematics are closely tied to this problem, concerning both the species level (as discussed earlier) and other taxonomic categories. Systematics, or Taxonomy, is the science of organismal Classification. Its core tasks are as follows:

1) determination (the primary, analytical stage) - sorting the natural diversity of individuals into easily recognizable groups characterized by a set of so-called diagnostic features, and assigning scientific names to these groups;

2) classification (the synthetic stage) - establishing an ordered system of higher categories;

3) studying speciation and evolutionary factors. In accordance with these tasks, three levels of systematics are distinguished: α-systematics - the stage of describing and naming species; β-systematics - grouping species into a natural system of lower and higher categories; γ-systematics - analyzing intraspecific variation and investigating evolution.

During the early period of The Development of systematics, the type concept, rooted in Platonic ideas, predominated. According to this concept, all members of any taxonomic category correspond to a specific type (bearing certain similarities to essentialism), which leads to an overestimation of the constancy of certain categories and an underestimation of variation.

Today, classification utilizes about 20 categories, which are divided into three types:

1) species;

2) population groups within a species - intraspecific categories;

3) species assemblages - supraspecific, or higher categories. At the same time, titans of systematics (Mayr and others) caution against introducing a subjective element into species delimitation: researchers frequently classify samples from natural populations rather than actual species. The consequence is a classification based on subjective perception,

an approximation of the reality of an unobserved morphological unit. While species were previously viewed independently of time and space, the modern species concept is multidimensional and lacks the precision that characterized the "ideal" dimensionless species.

The subspecies is the sole intraspecific taxonomic category defined as a geographically isolated group of local populations that differs from other similar subdivisions of the species. The term "race" lacks a precise taxonomic definition and can be used in various Senses (a subspecies may be regarded as a geographic and ecological race).

A superspecies is a monophyletic group of closely related and largely or entirely allopatric species. There is no special nomenclature for this category, and it gains its greatest significance in biogeography and speciation research.

A genus is a taxonomic category consisting of a single species or a group of species presumed to share a common phylogenetic origin and separated from other similar units by a distinct gap. As C. Linnaeus put it, characters do not make the genus, but the genus gives the characters. As a phylogenetic category, a genus has its own ecological niche, which is broader than that of a species. Generic characters are either the CHARACTERISTICS OF THE ancestral species or a complex of traits acquired simultaneously by the entire group of species.

A family is defined as a taxonomic category consisting of one or more genera of common phylogenetic origin, separated from other families by a noticeable gap. It is the highest category that, according to international rules, is tied to specific lower taxonomic categories. The type genus serves as the basis for the Concept of the entire family (it is generally considered that genus and family differ quantitatively). Families are typically distributed globally, with their representatives occupying similar habitats in different regions.

Taxonomic categories above the genus level do not rely on type species or genera; they represent major phylogenetic branches devoid of specific adaptive traits and characterized by a general body plan. Higher categories are quite widespread, with adaptive changes (adaptive radiation) occurring within each of them.

Higher categories are collective concepts, which is why their designation is often subjective. Examples of such subjectivity include recent frequent attempts to split large categories (including kingdoms) into smaller ones. Since the next section will deal directly with taxonomic categories, let us note at outset that, as an adherent of classical views on systematics as a fundamentally conservative branch of science, we will not focus on extravagant approaches.

A brief excursion into the issue of identifying taxonomic categories of various levels is provided to emphasize the absence of clear boundaries between discreteness and continuity: where one taxon ends and another begins can only be established conventionally, because everything in nature is interconnected.

Thus, a final reconciliation between proponents of different views is possible only after a certain Synthesis of the latter. According to N.N. Vorontsov, the prerequisites for a new scientific synthesis in evolutionary theory have already matured.

Food for Thought

The purpose of this section is to convince readers that, despite the worldwide recognition of the core Darwinian evolutionary concept, many issues remain problematic. At the same time, proponents of any viewpoints should handle the material accumulated by their predecessors with care, so as not to throw the baby out with the bathwater.