Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 6. ISOLATION

6.2. Types of Isolation

The phenomenon of isolation is extremely diverse, which is why it is not always possible to determine the true causes behind THE ORIGIN OF a particular form. The restriction of interbreeding among individuals can be complete, resulting from some form of drastic isolation, or partial, when the divergence between the new and ancestral forms is established only gradually. In addition to restrictions on interbreeding in space, temporal restrictions are also possible. Some researchers distinguish between Primary and secondary forms of isolation, which further complicates the Classification of these phenomena.

Geographical isolation as the spatial Separation of individuals. Geographical isolation refers to the situation where individuals inhabit different ranges and are therefore unable to encounter one another. The operation of this factor does not inherently require the ancestral forms to possess obligate biological differences—such differences may arise later.

This type of isolation is characteristic of almost all organisms because the preconditions for its formation are ubiquitous: partial restriction of interbreeding is already achieved by the spatial distribution of individual organisms. The only exceptions are narrowly endemic forms with extremely restricted ranges, where panmixia may be nearly complete. It is precisely these forms that primarily arise As a result of the prolonged action of geographical isolation. Science knows numerous Examples (Darwin's finches, Hawaiian honeycreepers, Galápagos tortoises, etc.) where, on islands under conditions of unsaturated biogeocenoses, certain species evolved through adaptive radiation (The Development of a significant number of related forms adapted to diverse environmental conditions) accompanied by The formation of a vast number of endemic species.

Various populations within a species' range already exhibit a certain degree of geographical isolation from the very beginning, which hinders the EXCHANGE OF GENETIC material between them. The most striking examples of this phenomenon are relatively sedentary species with

large ranges. These include the Eurasian red squirrel (Sciurus vulgaris) or the common pheasant (Phasianus colchicus), which exhibit a high degree of subspecific diversity.

Spatial isolation is closely linked to the reproductive activity of organisms, which is determined by their mobility. If the radius of an individual's activity is small relative to the population size and the distance between neighboring populations, the pressure of spatial isolation is quite pronounced. Conversely, high individual mobility helps to mitigate the pressure of this factor. For instance, migratory birds exhibit low intraspecific differentiation (particularly diurnal raptors and waterfowl). Nesting site tenacity (philopatry in storks and certain other bird species) can also promote the formation of spatial isolation.

More apparent is barrier isolation, which involves the creation of specific geographical (mechanical) obstacles between populations. The scale of the obstacle required to trigger this elementary evolutionary factor largely depends on the behavioral activity and mobility of the species in question.

The most typical examples include isolation by Water bodies and mountain ranges for terrestrial, non-flying forms, by landmasses for aquatic forms, and by marine waters for freshwater forms. However, for species with low mobility, even minor barriers can become insurmountable obstacles.

The categorization of geographical isolation into the forms discussed above should be considered somewhat conditional, because the action of one often does not exclude, but rather implies, the action of another. For migratory birds, the distances they cover annually can act as a specific barrier causing increased mortality among individuals. For smaller organisms, this relationship between distance and obstacles becomes even more pronounced. For example, on the volcanic island of Oahu, there are 25 rainforest-covered valleys separated from one another by barren rocky ridges. Each valley harbors its own distinct species of land snails of the genus Achatinella, which is explained by the snails' inability to cross from one valley to another over the sun-warmed ridges. Their inability to traverse this short distance became an insurmountable barrier, as a result of which geographical isolation led to the formation of 25 independent species.

In the plant kingdom, geographical isolation is an even more complex phenomenon, since plant mobility is primarily restricted to dispersal during the seed or spore phase, while reproduction also depends on pollen vectors such as pollinators or wind. Therefore, the geographical isolation of plants is largely determined by the mobility of their animal pollinators.

Features of Biological Isolation

Unlike geographical isolation, which is associated with organisms inhabiting different ranges, biological isolation consists in making the interbreeding process itself impossible. All forms of biological isolation are divided into two major groups: pre-mating and post-mating.

Pre-mating biological isolation operates without the involvement of Gametes and aims to prevent copulation in one way or another. There are numerous mechanisms of such isolation, notably temporal isolation, which can be subdivided into phenological isolation and Morphology/12.html">ALTERNATION OF GENERATIONS.

Phenological isolation is associated with the existence of so-called spring and winter races, which is clearly observed in certain fish species. Winter races migrate to spawning grounds in autumn, and reproduction begins in early spring. Spring races, by contrast, only begin migrating toward spawning grounds with the onset of spring, causing their reproductive process to occur much later. As a result, they become temporally isolated from the earlier race. Similar factors also cause phenological shifts in the breeding times of birds and certain other organisms.

One form of the alternation of generations is the switch from sexual reproduction to Vegetative Reproduction, which is characteristic of both Protozoans and Multicellular Organisms (where a sexual process alternates with asexual division or budding). Such a strategy allows new genetic combinations—formed during the sexual process—to be tested by the prolonged action of natural Selection during vegetative reproduction, i.e., during a sustained increase in the number of individuals. This significantly dampens The impact of deleterious factors on individual combinations while not hindering further Variability through sexual reproduction.

In multicellular organisms, such alternations acquire special significance, particularly in the case of a sessile lifestyle in the vegetative generation (hydroids, coral polyps, bryozoans, certain ascidians, etc.). Sometimes this type of reproduction becomes established for long periods, which promotes the formation of large colonies and prevents the accidental extinction of a particular genotype. Vegetative reproduction gains even greater biological significance in plants.

The evolutionary consequences of the alternation of generations lie in facilitating the fixation of a given genetic combination during the initial, most vulnerable stages of securing its right to independent existence. The reproduction of the new form itself occurs only after its viability has been successfully tested.

Even more significant in an evolutionary sense are life cycles featuring the alternation of sexual generations and asexual reproduction (found in certain animals, primarily Apicomplexa (Fig. 6.1), as well as in most plants). This form of isolation is not limited merely to increasing the number of copies of the new form, but also includes the development of haploid organisms in which every mutation is expressed to the maximum extent, thereby subjecting them to harsher natural selection. Consequently, the haploid generation acts as a "sanitarian," shielding the species from an accumulation of deleterious hereditary changes. However, in the course of evolutionary development, the brunt of natural selection shifts primarily toward the diploid Organism, which provides a better environment for the realization and combination of new traits of varying degrees of expression. This is particularly evident in higher plants, where angiosperms have achieved the greatest evolutionary success (accompanied by the maximal reduction of the gametophyte).

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Fig. 6.1. Generalized diagram of the developmental cycle of parasites of the family Plasmodium (illustrator: O.V. Karnaukhov):

SP - sporozoite; T - trophozoite; Sh - schizont; M - merozoite; GM - gametes; X - copulation; Z - zygote; OOK - ookinete; SC — sporocyst

Similar in their consequences to previous forms of isolation across generations are the capacity for self-Fertilization in plants and parthenogenesis in animals. Most plants (with few exceptions) are capable of both cross-fertilization and self-fertilization. If the latter begins to predominate in certain cases, it promotes the rapid spread of a new form, provided it is viable and competitive. On the one hand, self-fertilization protects a particular Gene combination from disintegration, but on the other hand, organisms lose plasticity and may forfeit their capacity for further progressive evolution.

Parthenogenesis in animals differs in that most of its forms take the appearance of heterogony (the alternation of sexual reproduction with parthenogenesis). In rotifers, lower crustaceans (cladocerans), and certain insects (such as aphids), even a seasonal reproductive cycle has evolved, where sexual reproduction occurs predominantly in autumn with the formation of so-called winter eggs, whereas several parthenogenetic generations develop throughout the spring-summer period. In parasitic forms, such heterogony leads to the formation of complex life cycles, most clearly illustrated by trematodes (Fig. 6.2).

Fig. 6.2. Typical developmental cycle of trematodes using Paragnimus westermani as an example (illustration by O.V. Karnaukhov)

M - marita; Y - egg; MTs - miracidium; SPTs - sporocyst; MR - mother redia; DR - daughter redia; Ts - cercaria; MTTs - metacercaria

Any combination arising during cross-breeding is capable of multiplying to the scale of an entire population over the course of the summer season. As in all previous cases, such isolation eliminates the probability of the accidental loss of a new form, although a transition to permanent parthenogenesis may also lead the group's further development into a dead end.

The probability of organism encounters is also reduced by ecological isolation, which is caused by the divergence of closely related groups into different ecological niches—namely, differences in the choice of breeding sites or The Use of specific food sources. In ecological isolation, the probability of mating partners meeting is reduced, but if an encounter does take place, the reproductive process proceeds without hindrance.

Initially, Mutations may arise that produce genotypes with differences in reproductive ecology or reproductive territory. For instance, birds frequently develop forms that differ in the Location of their nests in various PARTS OF THE nesting habitat (higher or lower in the tree canopy, etc.).

Ecological isolation associated with a preference for different biotopes is characteristic of many species. In particular, the Western European population of the great bustard (Otis tarda) transitioned to nesting in agricultural landscapes long ago. In Kazakhstan—following the plowing of the virgin lands—and in Russia, processes of synanthropization began relatively recently. For a certain period, two ecologically isolated populations formed: individuals of the first prefer nesting in virgin steppe areas, while those of the second prefer agricultural landscapes. Naturally, the mortality rate of offspring in the latter case is significantly higher than in the former, but the surviving population gradually replaces the original one due to the absence or shortage of traditional breeding sites. Ecological differences in living biotopes are also typical of the roe deer (Capreolus capreolus), which is colonizing agricultural landscapes, and the Eurasian blackbird (Turdus merula), which can be found in urban parks in contrast to its ancestral habitats.

If organisms can meet in time and space, a whole series of additional isolating mechanisms comes into play. Among them is ethological isolation, primarily associated with differences in the mating behavior of representatives of different species. It is quite typical for animals, which can be explained by species-specific courtship behavior understood only by members of a specific species. Furthermore, complex courtship rituals consisting of many components also serve to prevent interspecific Hybridization.

It is believed that ethological isolation is the primary mechanism preventing the formation of significant numbers of interspecific animal hybrids, which is why it is so widely distributed.

Every animal possesses an innate capacity both to emit specific signals characteristic only of its own species and to respond exclusively to them.

In nature, one can observe numerous instances of sexual reactions by representatives of one species toward another, but very few of these result in pair formation and fertilization. For example, in the grayling butterfly (Eumenis semele), males initiate courtship by pursuing a sexual partner in the air. This partner may be a butterfly of another species, some beetle, a fly, a small bird, falling leaves from trees, or even the male's own shadow. However, only the female grayling, if she is ready for mating, responds appropriately to the male's pursuit—she lands and remains motionless. Other species mostly flee. Even in cases of similar behavior, females of related species are unable to reproduce all the components of courtship behavior, which prevents mating from occurring.

A similar pattern is characteristic of other species, including fruit flies (Drosophila), where mating between different species is prevented at various stages of courtship due to errors made by either the male or the female. The courtship behavior of the three-spined stickleback (Gasterosteus aculeatus) and the nine-spined stickleback (Pungitius pungitius) is similar, though they differ slightly in the nuptial coloration of the males and their behavior in the nest during egg-laying, which is sufficient to prevent cross-breeding (Fig. 6.3).

Fig. 6.3. Sequence of courtship in the three-spined stickleback (Gasterosteus aculeatus), taking the form of "signal-response" reactions (after N. Tinbergen, 1993):

1 - zigzag dance of the male; 2 - courtship by the female; 3 - male leads the female to the nest; 4 - she follows him; 5 - male indicates the nest entrance; 6 - female enters the nest; 7 - male nudges the female, after which she lays eggs and swims out

One of the mechanisms forming ethological isolation is imprinting (the memorization of prominent features of objects during early life stages) regarding members of one's own species. Specifically, in Darwin's finches, reproductive

isolation is ensured not only by differences in ecological niches but also by the specific response of each partner exclusively to the beak shape characteristic of its species, which is linked to dietary adaptations. In some cases, conversely, imprinting may promote the appearance of hybrids: when the Canada goose (Branta canadensis) and the greylag goose (Anser anser) are raised together, mixed sexual pairs can form because they grow accustomed to each other from an early age.

Of particular importance in the formation of ethological isolation are so-called releasers—signals that elicit an intense behavioral response in individuals of the same species (Fig. 6.4). For instance, in male three-spined sticklebacks, such a releaser is the red patch on the belly, whereas in the nine-spined stickleback, it is the pitch-black coloration. In many birds, brightly colored patches in the plumage also act as releasers, with their arrangement, shape, and color always being species-specific.

Fig. 6.4. Examples of releasers that are species-specific and prevent interspecific hybridization (after R. Chauvin, 1972):

1 - male fiddler crab (Uca lactea) uses its claw as a releaser indicating its sex; 2 - great tit nestling (Parus major) displays colored patches in its beak (releasers) to its parents, stimulating them to feed it

An interesting example of ethological isolation is the formation of biological races in the common cuckoo (Cuculus canorus), which differ genetically in the coloration of their eggs, mimicking the eggs of various species of small passerine birds. Reproductive isolation between them is maintained by host species through the destruction of eggs that lack the appropriate camouflage coloration. At the same time, this example can also be viewed as an ecological form of isolation, since the individuals occupy different ecological niches.

Difficulties in cross-breeding between organisms also arise as a result of morphophysiological isolation. It is caused by specific mutations or morphoses that preclude the possibility of cross-pollination or mating. Initially, individuals that lost the capacity for cross-breeding did not possess significant genetic differences. In plants, morphophysiological isolation depends on minor mutations that alter pollen germination rates, cause partial heterostyly, or produce other changes in flower Structure that affect its interactions with pollinators.

In animals, morphophysiological isolation is primarily associated with structural modifications of the male copulatory apparatus. For instance, in many insects and mammals, the Morphology of the genitalia exhibits distinct differences that are even utilized in Taxonomy (Fig. 6.5). The core mechanism of such isolation is the inability of organisms to successfully mate due to structural incompatibility of their reproductive Organs or general anatomical differences. This type of isolation is clearly exemplified by artificial

selection, whereby various breeds of domestic animals (such as dogs) cannot interbreed due to profound differences in their constitution. Thus, morphophysiological isolation does not reduce the likelihood of individuals encountering one another; rather, it renders fertilization impossible due to significant morphological and physiological disparities.

Post-copulatory isolation mechanisms include proper genetic isolation, which may manifest as zygote mortality following fertilization, the development of partial or complete hybrid sterility, or reduced hybrid viability. The most obvious cause of genetic isolation is substantial karyotypic divergence between organisms. In plants, this is facilitated by polyploidy, while in living organisms generally, it is driven by chromosomal mutations.

Genetic isolation may also begin with the destruction of gametes post-copulation, even before fertilization occurs. In this scenario, biochemical incompatibility arises between the spermatozoon and the foreign Tissues of the organism, as well as between its Nucleus and the egg Cytoplasm. If fertilization does occur, the embryo may perish at early or later stages of Embryogenesis due to metabolic disruptions underlying ontogeny and vital life processes.

An example of the latter type of isolation is observed in the American and European minks (Mustela vison and M. lutreola, respectively), where hybridization does not occur due to embryo resorption. Well-known examples also include hybrids (both Artificial and natural) characterized by sterility or reduced viability.

Fig. 6.5. Glans Penis morphology in males of certain birch mouse species (after B.O. Kuznetsov, 1975):

1 - Chinese birch mouse (Sicista concolor); 2 - Caucasian birch mouse (S. caucasica); 3, 4 - Altai red-backed birch mouse (S. napaea), anterior and lateral views

In terms of their MECHANISM OF ACTION, all forms of isolation are fundamentally similar, serving to consolidate group differences resulting from disrupted panmixia under prolonged exposure. Initially, any form of isolation may arise under the Influence of External factors relative to the organisms (geographic isolation) or internal ones (biological isolation). The consequence of such a phenomenon is invariably the deepening divergence within the emerging populations, which may ultimately culminate in complete genetic isolation.

Food for Thought

Contemporary scientific evidence indicates that isolation is not merely a necessary, but an obligatory prerequisite for the formation of new life forms. Whether these forms evolve into new species or attain higher taxonomic ranks depends on numerous factors. Isolation itself provides the very foundation for The Emergence of such groups. However, genetic isolation ceases to be strictly mandatory if Other forms of isolation are operative.

A special status is held by agamic and parthenogenetic forms, where each individual is immediately genetically isolated from all others. Consequently, for these forms, individual adaptations may ultimately acquire evolutionary significance.