With the reproductive isolation of populations of one species begins, it continues and complete isolation ends, the formation of new species of any organisms on our planet. Reproductive isolation is the alpha and omega of the evolution of life on Earth.
Isolation theory
For panmictic plants and animals (those who reproduce sexually), the concept of “species” unites a set of organisms that are similar in many ways, living in a certain territory and capable of full reproduction. This implies free crossbreeding and the emergence of prolific offspring. Reproductive isolation is a complete or partial restriction on free crosses. Also, under the isolation associated with the processes of reproduction, they also mean the reproduction of sterile hybrid offspring.
Classification of isolation in biology
Today, various evolutionary scientists offer different systems for classifying species, forms and mechanisms of biological isolation. There are systems based on the time factor or the completeness of the non-crossing that arose. Let's take as a basis the most common principle of systematizing knowledge about isolation, proposed by the famous biologist - evolutionist F. G. Dobrzhansky (1900-1975).
All types of insulation differ in the mechanism of their occurrence:
- geographical isolation of populations;
- environmental;
- actually biological.
Let us consider in more detail the forms of reproductive isolation.
Geographic restriction
Geographical reproductive isolation is a restriction in the free crossing of organisms caused by changes in the geological nature (the appearance of mountain barriers, changes in the riverbed) or the spread of the species over large areas. In this case, a generalization of a group of organisms occurs, and drift and gene exchange is closed within a separate group. The most famous example is the Galapagos finches, the different types of which are characteristic of each island. It was these birds that prompted Charles Darwin to write his work, The Origin of Species through Natural Selection.
Environment and habitat isolation
It is environmental factors that underlie the ecological reproductive isolation of populations of the same species that exist on the same territory. Such factors may be a mismatch in the breeding time or feeding characteristics of individuals. Some scientists believe that this form is not reproductive isolation. This is a partial restriction that does not make crossbreeding impossible. And although the opinions of scientists differ, there are examples in nature. Three species of seals live on the Arctic coast . All of them descended from a single phylogenetic ancestor. But Ross seals feed exclusively on cephalopods, sea leopards are fierce hunters of penguins and other seals, and Witt seals eat only fish. And they do not interbreed with each other, although they are very similar in appearance.
Reproductive isolation itself
Reproductive isolation itself is a form that is directly tied to the characteristics of the reproductive process. And here they distinguish pre-population and post-population mechanisms that impede the reproduction of their own kind.
Some scientists call all types of isolation biological or reproductive and include mechanisms of geographical and environmental isolation in the group of pre-population reproductive isolation.
Pre-population mechanisms
These are isolation mechanisms associated with the impossibility of mating. There are three forms:
- Ethological reproductive isolation associated with the characteristics of sexual behavior. Examples: ritual mating behavior of birds, different rhythms of flashing in fireflies, the nature and timbre of mating sounds in frogs, the difference in pheromones (chemical communication substances) of large cats.
- Mechanical isolation is associated with differences in the structure of the reproductive organs. For example, differences in the structure of the orchid flower lead to the possibility of pollination with a specific species of hummingbird. The same is true for different types of sage familiar to all. It is rare in animals. For example, in Drosophila, mating of different species leads to injury or even death of a partner.
- The lack of interaction of germ cells (gametes) is called gametic isolation. It is proved experimentally in laboratories and zoos. In this case, mating occurs, but the zygote is not fertilized.
Types of post-copulative reproductive isolation
The concept is close to the last type of pre-populative isolation, but in this case the embryo is formed. However, it either dies in the early stages of ontogenesis, or dies before puberty, or hybrid individuals are sterile. The mechanism of death and sterility of hybrids is complex, associated with gene, chromosomal or cytological features. Examples of hybrids of different species are known. Related hybrids, as an exception, may be reproducible. But then in biology the phenomenon of degeneration of hybrid forms is observed - each subsequent generation of hybrids becomes more weakened and gradually degenerates.
The biological significance of reproductive isolation
Not all forms of population isolation lead to the formation of new species. But the partial or complete isolation of populations or groups of organisms always becomes the first step in speciation. Over time, when the exchange of gene material locked in a separate group accumulates genetic differences from other groups of this species, reproductive isolation becomes more significant and complete. With the accumulation of a sufficient number of differences in the genomes, they say that a new species has formed. In biology, it is not so simple; sometimes it can be quite difficult to separate a group of organisms into a new species. Nevertheless, each catch of deep-sea trawls brings researchers-oceanologists representatives who are not yet known to modern taxonomy.