Species structure and spatial distribution
In nature, any organisms (including the species Homo sapiens) are distributed unevenly across the territory. There are areas with both increased and decreased concentrations of individuals. As a result, the species breaks down into separate groupings—zones of dense settlement consisting of independent populations.
Depending on the complexity of their internal organization, all species are divided into two large groups:
- Monotypic species. These are species consisting of just a single population. They are characterized by a continuous range and extremely weakly expressed trait variability. Typical examples are island species and narrow endemics. A classic example is the western capercaillie (Tetrao urogallus).
- Polytypic species. They have a more complex structure, as they consist of multiple populations and several subspecies. Their habitats are relatively separate. A striking example is the yellow wagtail (Motacilla flava), the various subspecies of which are easily distinguished by head coloration.
Key characteristics of a population
A population is the minimal group of individuals of the same species capable of self-reproduction over a long period. For an isolated group of animals or plants to be considered a full-fledged population, it must possess three crucial characteristics:
- Ecological niche. A population must occupy a strictly defined ecological niche in its biocenosis.
- Genetic system. The group forms an independent genetic system that is stable over time.
- Panmixia. Random mating must occur within the population. Moreover, the probability of individuals mating within their own group is significantly higher than the chances of contact with members of neighboring populations.
Synthesis of Darwinism and genetics
Until the mid-1920s, classical Darwinism and the young science of genetics developed completely independently of each other. Charles Darwin correctly identified the basic factors of evolution: heredity, variability, and natural selection. In doing so, he essentially anticipated the future role of genetics in studying variability, although the mechanisms of trait transmission remained unknown in his time.
The historical synthesis of these sciences was carried out by the Soviet geneticist S.S. Chetverikov. In his foundational 1926 article "On Certain Aspects of the Evolutionary Process from the Standpoint of Modern Genetics", he drew the scientific community's attention to mutation processes occurring under natural conditions.
Main provisions of Chetverikov's doctrine:
- Saturation with mutations. Natural populations are literally permeated with mutant recessive genes. Chetverikov gave a well-known analogy: "Populations are saturated with mutations as a sponge is saturated with water." For the time being, these mutations are in a heterozygous state and are reliably hidden from natural selection by dominant alleles.
- Evolutionary material. It is precisely these continuous mutation processes at the population level that form the elementary evolutionary material for subsequent natural selection.
Theory of microevolution and paradigm shifts
Before Chetverikov's work, erroneous views dominated science regarding what constitutes the subject of evolution. J.B. Lamarck suggested that evolution proceeds at the level of the individual. C. Darwin, in turn, considered the entire species to be the evolving unit.
Having studied hidden mutation processes, Chetverikov disproved these old views and proved that all real evolutionary events take place exclusively within a population. The main conclusion of his work states that the population is precisely the elementary evolving unit in nature.
This process of evolutionary changes occurring at the population level was named microevolution. To assess microevolutionary processes, researchers analyze various population characteristics. For example, the ecological characteristic and total population size are determined by two key parameters: the occupied range and the abundance of individuals.
Briefly: answers to topical questions
What types of isolation prevent panmixia between neighboring populations?
Panmixia between neighboring populations is prevented by spatial and biological isolation.
- Spatial isolation is the separation of a population by physical barriers such as mountains, rivers, or large distances.
- Biological isolation consists of reproductive barriers that prevent individuals from mating even when located in the same territory. It is subdivided into prezygotic (including ecological, ethological, morphophysiological, and gametic) and postzygotic (manifested as inviability or sterility of hybrids).
What elementary factors of evolution, apart from the mutation process and natural selection, operate within a population?
In addition to the mutation process and natural selection, sources indicate the following factors and driving forces of evolution:
- Isolation restricts panmixia; spatial isolation is associated with physical barriers and range fragmentation.
- Genetic drift is a random, non-directional change in allele frequencies; it can lead to the fixation or loss of an allele.
- Population waves are fluctuations in abundance with which genetic drift is associated.
- Migrations are listed among the driving forces of evolution in the provisions of the modern synthesis.
- Struggle for existence is listed among the driving forces of evolution and the biological factors of anthropogenesis.
What criteria are used to determine whether individuals belong to the same species?
A combination of criteria is used to determine whether individuals belong to the same species, as no single criterion is absolute on its own. Sources highlight the following groups of criteria:
- Morphological criterion.
- Physiological and biochemical criterion.
- Ecological and geographical criterion describes the spatial distribution and the role of the species in the ecosystem; each species occupies a specific range and ecological niche. Limitation: for some species, ranges may overlap or be very extensive.
- Genetic criterion is the main criterion of a species; a species is characterized by a strictly defined set of chromosomes—the karyotype. Analysis of the chromosome set makes it possible to distinguish outwardly similar organisms.
To memorize scientists' views on the unit of evolution, use the scheme "Individual — Species — Population": Lamarck (individual) → Darwin (species) → Chetverikov (population).
Class and exam questions
This is the minimum self-reproducing group of individuals of the same species, possessing its own stable genetic system and a high level of random mating (panmixia).
This is the minimum self-reproducing group of individuals of the same species, possessing its own stable genetic system and a high level of random mating (panmixia).
What was the error of Lamarck and Darwin in defining the unit of evolution?
J.B. Lamarck mistakenly believed that the individual organism evolves, while C. Darwin believed that evolution occurs immediately at the level of the entire species. Modern science has proven that the true unit is the population.
What does Chetverikov's phrase mean that populations are "saturated with water like a sponge"?
This analogy describes the saturation of natural populations with hidden recessive mutations. Being in a heterozygous state, they are suppressed by dominant alleles, but serve as a reserve material for evolution.
What parameters determine the ecological characteristic of a population?
From an ecological perspective, population size depends on two main parameters: the range (area of distribution) and the abundance of organisms in the group.
What else to study on this topic
- Genetic characteristics and structure of populations
- Mechanisms of recessive mutation emergence in nature
- Ecological parameters: dynamics of range and abundance
- Differences between microevolution and macroevolution
- Factors influencing the level of panmixia
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