In panmictic organisms (sexually reproducing), a species is a set of organisms that are similar in a whole range of characteristics, capable of freely interbreeding with the formation of fertile offspring. The concept of isolation is used in the context of microevolution or, as it is also called, speciation. Reproductive isolation begins the process of formation of a new species and ends it. But not all forms of this phenomenon will lead to the emergence of a new species, just as not every isolation leads to the final reproductive separation of populations.
Isolation mechanisms in evolution
Inside one species, individuals exist in groups - populations. It is populations as a unit of microevolution that serve as material for the formation of new species that are different from the original ones. Within a species, an exchange of genetic material occurs between populations in the process of reproduction. What is calledreproductive capacity of organisms. When for some reason this exchange between populations within the same species is limited or completely impossible, they say that reproductive isolation has set in. The definition of this evolutionary mechanism is that individuals of different populations cannot produce offspring. The history of the emergence of a new species is a chain of different types of reproductive isolation, replacing or overlapping each other, making the division of populations irreversible.
Reproductive isolation: classification
There are several concepts in the classification of varieties of population isolation. Various criteria taken as the main feature add some confusion to this issue. Let us take as a basis that reproductive isolation as a permanent restriction of panmixia (free crossing) is the last stage that completes speciation. Proponents of this view of species-forming isolation were well-known evolutionary biologists F. G. Dobzhansky (1900-1975) and E. Mayr (1904-2005). The mechanisms of reproductive isolation in this approach will be divided into three groups:
- spatial division (geographic);
- environmental isolation (environmental mechanisms);
- proper reproductive mechanism, including pre-copulation (before the formation of a zygote) and post-copulation (mating occurs, but the egg is either not fertilized or dies, or the hybrids are sterile) barriers.
Any mechanism achieves a limitpanmixia: full or partial. Let us briefly consider forms of reproductive isolation in each group. Examples illustrating them will help you understand the essence of a particular form.
Spatial mechanisms of population isolation
The mechanism of isolation is associated with various changes in the landscape (the appearance of a barrier in the form of mountain ranges or rivers) or with the spread of the species over vast territories. When the flow of genes between the separated populations is disturbed, reproductive isolation occurs. The most famous example of such a phenomenon with the formation of new species can be considered the island species of bindweeds of the Galapagos Islands, which became one of the examples for Charles Darwin's work "The Origin of Species by Means of Natural Selection". Or an example of a blue magpie, one population of which lives in China and the other in Spain.
Ecological isolation mechanisms
The reasons for this type of reproductive isolation are related to the difference in the conditions for the reproduction of populations of the same species living symmetrically, that is, in the same territory. For example, the breeding or flowering seasons do not coincide. On the California coast, two types of pines exist symmetrically: one species sheds pollen in February, and the other in April. Seasonal ecological isolation has become reproductive for them. An example of reproductive isolation, but resulting from different food bases, is demonstrated by three species of Antarctic seals descended from the same phylogenetic ancestor. The Weddell seal eats only fish, the leopard seal eats penguins and seals, and the Ross seal eatscephalopods.
Precopulatory forms of reproductive isolation
Mechanical isolation - the ineffectiveness of mating caused by the different structure of the reproductive or copulatory organs. For example, different types of sage have different flower shapes and are pollinated by other bees. The same correlation exists between orchids and hummingbirds. Drosophila flies interspecies mating leads to injury or even death of the partner.
Ethological isolation - not mating due to differences in sexual behavior (in courtship, singing, dancing, glowing, or differences in pheromones). For example, closely related species of fireflies, when inviting a female for mating, blink differently (with different frequency and duration). We can also mention species-specific songs in sparrows and frogs in this context. And everyone knows about the mating rituals of birds.
Gametic isolation - lack of gamete interaction or death of gametes. The existence of this type of isolation has been proven experimentally. For example, two species of sea urchins with external fertilization were crossed by American geneticists Denis and Brachet. The egg was fertilized, but the embryo died in the early stages of gastrulation.
Post-copulation forms of reproductive isolation
This refers to the non-viability of the fertilized egg and the death of the embryo in the early stages of ontogenesis. Or the death of a born cub (or individual) before reaching puberty. Very close concept to gameticinsulation.
Sterility of hybrids
In most animals, the surviving interspecific hybrids are sterile, that is, they are not capable of producing offspring. An exception may be semi-sterile hybrids. The mechanism of this phenomenon is quite complex, based on gene, chromosomal or cytological causes. We will give only examples of interspecific hybrids that are known to everyone.
Hybrid donkey and mare - mule. It is larger than a donkey and smaller than a horse, and besides, the animals are easier to keep. Hybrids of closely related subspecies of a dog and a wolf (wolfdog, half-wolf) have a more developed instinct and endurance than ordinary dogs. Many of the aquarium fish are hybrid forms (colorful aulonocars). They are beautiful, larger than parental forms, but when buying, you should specify the origin of the fish, otherwise you will not wait for offspring. Everyone knows that the seeds of cultivated plants (tomatoes, cucumbers) marked with the symbol F-1 are hybrid forms. The fruits of these plants are not left to seed.
