In pan-mictic organisms (reproducing sexually), a species is a set of organisms that are similar in a whole complex of characters that can freely interbreed with the formation of prolific 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 finishes 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
Within 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 other than the original ones. Within the species between populations, genetic material is exchanged during reproduction. What is called the reproductive ability of organisms. When, for whatever reason, this exchange between populations within the same species is limited or completely impossible, they say that reproductive isolation has begun. The definition of this evolutionary mechanism comes down to the fact that individuals of different populations cannot produce offspring. The history of the emergence of a new species is a chain of various types of reproductive isolation replacing or overlapping, making the separation of populations irreversible.
Reproductive Isolation: Classification
There are several concepts in classifying varieties of isolation of populations. Various criteria, taken as the main feature, give some confusion to this issue. We take as a basis that reproductive isolation as a constant restriction of panmixia (free crossbreeding) is the last stage that completes speciation. The advocates of this view of species-forming isolation were the famous evolutionary biologists F. G. Dobrzhansky (1900-1975) and E. Mayr (1904-2005). The mechanisms of reproductive isolation in this approach are divided into three groups:
- spatial separation (geographical);
- environmental isolation (environmental mechanisms);
- the reproductive mechanism itself, including dopopulation (before the formation of the zygote) and post-population (mating occurs, but the egg either does not fertilize, or dies, or hybrids are sterile).
With any mechanism, the limitation of panmixia is achieved: full or partial. Briefly in each group, consider forms of reproductive isolation. Examples illustrating them will help to understand the essence of one form or another.
Spatial isolation mechanisms of populations
The isolation mechanism is associated with various changes in the landscape (the appearance of an obstacle in the form of mountain ranges or rivers) or when the species is spread over vast territories. If the gene flow is disturbed between divided populations, reproductive isolation occurs. The most famous example of such a phenomenon with the formation of new species can be considered island species of Galapagos Islands bindweed, which became one of the examples for the work of C. Darwin “The Origin of Species by Natural Selection”. Or an example of the blue magpie, one population of which lives in China and the other in Spain.
Environmental isolation mechanisms
The reasons for this type of reproductive isolation are related to the difference in the breeding conditions of populations of the same species living symmetrically, that is, on the same territory. For example, the breeding or flowering seasons do not match. There are two species of pine trees symmetrically on the California coast: in one species, pollen crumbles in February, and in the other in April. Seasonal environmental isolation has become reproductive for them. An example of isolation of a reproductive, but arising due to different food supply base, is demonstrated by three species of Antarctic seals, descended from the same phylogenetic ancestor. Weddel's seal feeds only on fish, sea leopards on penguins and seals, and the Ross seal on cephalopods.
Pre-population forms of reproductive isolation
Mechanical isolation - mating failure, caused by the different structure of the reproductive or copulative organs. For example, different types of sage have a different flower shape and are pollinated by other bees. The same correlation between orchids and hummingbirds. Interspecific mating of Drosophila flies leads to injuries or even death of a partner.
Ethological isolation is not mating due to differences in sexual behavior (courtship, singing, dancing, glow, or pheromone difference). For example, closely related species of fireflies flashing differently at the invitation of the female for mating (with different frequencies and durations). Species-specific songs of sparrows and frogs can also be mentioned in this context. And everyone knows about the marriage rituals of birds.
Gametic isolation - lack of gamete interaction or death of gametes. The existence of this type of insulation is proved experimentally. For example, two types of sea urchins with external fertilization were crossed by American geneticists Denis and Brache. The egg was fertilized, but the fetus died in the early stages of gastrulation.
Post-population forms of reproductive isolation
This refers to the non-viability of a fertilized egg and the death of the embryo in the early stages of ontogenesis. Or the death of a born calf (or individual) before reaching puberty. A very close concept to gametic isolation.
Sterility of hybrid individuals
In most animals, surviving interspecific hybrids are sterile, that is, they are not able to give offspring. An exception may be semi-sterile hybrids. The mechanism of this phenomenon is quite complex, based on gene, chromosomal or cytological reasons. We give only examples of interspecific hybrids that are known to all.
Hybrid donkey and mare - mule. It is larger than a donkey and smaller than a horse, in addition, animals are easier to keep. Hybrids of closely related subspecies of a dog and a wolf (wolf-wolf, half-wolf) possess a more developed instinct and stamina than ordinary dogs. Many of the aquarium fish are hybrid forms (colorful aulonocars). They are beautiful, larger than the parental forms, but when buying it is worth clarifying the origin of the fish, otherwise you will not wait for the offspring. Everyone knows that seeds of cultivated plants (tomatoes, cucumbers) marked with the F-1 symbol are hybrid forms. The fruits of these plants are not left on the seeds.