Reproductive isolation: definition, causes. Reproductive isolation forms: examples

In panmictic organisms (reproductive by sex), a species is a collection of organisms that are similar in a whole set of characters capable of freely interbreeding with the formation of prolific progeny. The concept of isolation is applied in the context of microevolution or, as it is called, speciation. Reproductive isolation starts the process of forming a new species and ends it. But not all forms of this phenomenon will lead to the emergence of a new species, as not every isolation leads to the final reproductive disunion of populations.

Isolation mechanisms in evolution

Within one species, individuals exist in groups-populations. It is the populations as a unit of microevolution that serve as a material for the formation of new species, other than the first-born. Within the species between populations, genetic material is exchanged during reproduction. What is called the reproductive capacity of organisms. When for some reason this exchange between populations within one species is limited or completely impossible, it is said that reproductive isolation has occurred. The definition of this evolutionary mechanism boils down to the fact that individuals of different populations can not give offspring. The history of appearance of a new species is a chain of different types of reproductive isolation, replacing or overlapping one another, making the separation of populations irreversible.

Reproductive isolation: classification

There are several concepts in the classification of varieties of isolation of populations. Different criteria, taken for the main feature, give some confusion to this issue. We will use as a basis that reproductive isolation as a constant limitation of panmixia (free crossing) is the last stage, which completes speciation. Proponents of this view of the isolation of the species-forming were well-known evolutionary biologists FG Dobrzhansky (1900-1975) and E. Mayr (1904-2005). Mechanisms of reproductive isolation with this approach are divided into three groups:

• Spatial separation (geographic);
• Environmental isolation (environmental mechanisms);
• The actual reproductive mechanism, including dopupulation (before the formation of the zygote) and post-copulation (mating occurs, but the egg either does not fertilize, or dies, or hybrids are sterile) obstructions.

With any mechanism, a panmixia restriction is achieved: complete or partial. In brief, in each group, we consider the forms of reproductive isolation. Examples illustrating them, will help to understand the essence of one form or another.

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 when the species is spread over vast areas. When the flow of genes between divided populations occurs, reproductive isolation occurs. The most famous example of this phenomenon with the formation of new species can be considered island species of Convolvulus Galapagos Islands, which became one of the examples for the work of Charles Darwin "The Origin of Species by Natural Selection." Or an example of a blue magpie, one population of which resides in China, and another in Spain.

Environmental mechanisms of isolation

Reasons of reproductive isolation of this type are related to the difference in conditions of reproduction of populations of one species that live symmetrically, that is, in the same territory. For example, the seasons of reproduction or flowering do not coincide. Symmetrically there are two types of pine on the California coast: one pollen is showered in February, and the other - in April. Seasonal ecological isolation became reproductive for them. An example of isolation reproductive, but arisen due to different food bases, demonstrates three species of Antarctic seals that have occurred from one phylogenetic ancestor. The seal of Veddel eats only fish, sea leopard - penguins and seals, and the seal of Ross - cephalopods.

Dopupulation forms of reproductive isolation

Mechanical isolation is the inefficiency of mating, caused by the different structure of the reproductive or copulatory organs. For example, different kinds of sage have a different flower shape and are pollinated by other bees. The same correlation between orchids and hummingbirds. Interspecies pairing of flies of Drosophila leads to trauma or even death of a partner.

Ethological isolation is not a mating due to differences in sexual behavior (in courtship, singing, dancing, glowing or pheromone difference). For example, closely related species of glowworms at the invitation of the female for mating in different ways blink (with different frequency and duration). Also it is possible to mention in this context species-specific songs of sparrows and frogs. And everyone knows about the marriage rituals.

Gametic isolation - the absence of gamete interaction or the death of gametes. The existence of this type of isolation has been proved experimentally. For example, two species of sea urchins with external fertilization were crossed by American geneticists Denis and Brashe. The egg was fertilized, but the fetus died in the early stages of gastrulation.

Post-copulatory 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 ontogeny. Or the death of a born calf (or individual) before reaching puberty. Very close concept to the gametic isolation.

Sterility of hybrid individuals

In most animals, surviving interspecies hybrids are sterile, that is, they are not capable of giving offspring. An exception may be semi-sterile hybrids. The mechanism of this phenomenon is rather complicated, based on gene, chromosomal or cytological reasons. Let's give only examples of interspecific hybrids, which are known to everyone.

The hybrid of an ass and mare is a mule. He is larger than an ass and less horse, in addition, animals are easier to maintain. Hybrids of closely related subspecies of the dog and the wolf (wolf cub, half-wolf) possess a more developed instinct and endurance than ordinary dogs. Many of the aquarium fish are hybrid forms (colorful aulonokary). They are beautiful, larger than the parent forms, but when buying it is necessary to clarify the origin of fish, otherwise do not wait for offspring. Everyone knows that seeds of cultivated plants (tomatoes, cucumbers), marked with the symbol F-1, are hybrid forms. The fruits of these plants do not leave on the seeds.