Isolation, in evolutionary term, means segregation of different populations into smaller units by certain mechanism so as to prevent interbreeding among individuals.

1. Geographical isolation

When the populations are separated by a geographical barrier, such as river, sea, mountain, deserts and for aquatic animals land, they are physically prevented from interbreeding. Such populations are termed as allopatric and are forced to evolve independently and accumulate genetic differences. Geographical isolation may be different for different species. For example, a small stream may be an effective barrier for land insects and small mammals while for birds even mountain and oceans may not be barriers.

2. Reproductive isolation

It is the property of individuals that prevents interbreeding in populations that are actually sympatric (living in the same area). 

Classification of reproductive isolating mechanisms 

A. Premating mechanisms

They prevent interspecific crosses in sympatric populations.

1. Seasonal isolation: Also called temporal isolation, in which potential mates do not come in contact with each other because of differences in breeding seasons of two species, e.g. different flowering seasons in plants. Bufo americanus breeds in early rainy season (May), while Bufo fowleri breeds in late rainy season (July) in USA.

2. Habitat isolation: Also called Ecological isolation, in which also potential mates do not meet each other due to differences in habitats, requirements of food, space, climate etc. Potential mates live in different areas and therefore do not come in contact with one another. For example spawning grounds of riverine fishes are in different tributaries, which prevents interbreeding.

3. Ethological isolation: It is a behavioral isolation, in which potential mates meet but cannot mate, due to differences in courtship displays or other specific signals that are necessary rituals before mating. The signals may be of the following three types, which stimulate the opposite sex for mating:

(a) Visual stimuli: Feather displays and dancing in male birds is necessary to attract the female, e.g. peacocks, pheasants and birds of paradise. The colour and shape of the feathers as well as display pattern is so unique for each species that mating between two different species is not possible. Collection of the nest material and construction of the nest as by the weaverbird male is also a very specific display that cannot be imitated by the other species. 

(b) Auditory stimuli: Songbirds like cuckoos, mynas, nightingales, parakeets etc. use auditory signals to attract the opposite sex. Sometimes the singing goes on for several days before the pair can actually come together for mating. Auditory communication is used by a large number of animals, viz. frogs, toads, cicadas, gibbons, monkeys, jackals etc.

(c) Chemical stimuli: This includes odors of the animals that attract the opposite sex for mating. For example scent of musk deer and musthing in elephants attract the females. In insects, particularly Lepidoptera, females produce highly specific pheromones that can be detected by the highly specialized antennae of males from a distance of about 2 kilometers.

4. Mechanical isolation: In this case the above isolating mechanisms are not present and therefore mating is attempted but is not successful due to mechanical problems such as differences in the structure of genitalia. Dufour (1844) described “Lock and key mechanism” in the genitalia of insects. In the species of Drosophila the genitalia are so different that copulation is mechanically not possible.

B. Postmating mechanisms

These reduce the success of interspecific crosses. In case premating mechanisms fail to prevent mating then several postmating mechanisms prevent the success of mating and hybridization. There are 4 such mechanisms, which are given below:

1. Gamete mortality: Mating and sperm transfer takes place but egg is not fertilized. In Drosophila vaginal wall swells killing spermatozoa should interspecific crosses take place. If mating takes place between Bufo fowleri and Bufo valliceps, sperms cannot penetrate the egg membrane of each other, leading to mortality of gametes.

2. Zygote mortality: Egg is fertilized but the zygote dies. Eggs of many species of fishes may be present in the spawning grounds and some may be fertilized by the sperms of different species forming zygote but such zygotes fail to develop due to differences in chromosomes.

3. Zygote inviability: Zygote develops and hybrid is produced but is physically weak and inviable due to physiological disturbances in the body. It fails to survive for long and prematurely dies. Such cases have been recorded in different species of ducks.

4. Hybrid sterility: Hybrid is viable, physically strong and physiologically sound but is sterile due to differences in chromosomes and different gene arrangements. Mule is a cross between male donkey and female horse and Hinny between female donkey and male horse and both are sterile, albeit physically strong.

Sometimes all isolating mechanisms break, leading to fertile hybrids, which are generally not reproductively isolated from the parents and can produce fertile offsprings by Introgression (hybrids backcrossing with parents to produce fertile offsprings). This will be instant speciation.

Significance of isolating mechanism

  • Wasteful courtship is avoided. If isolating mechanisms are distinct and specific only individuals of the same species indulge in courtship.
  • Isolating mechanism protects gene pool of a species and prevents hybridization.
  • It prevents wastage of gametes and energy.
  • A weak isolating mechanism leads to production of new species through hybridization.
  • Absence of isolating mechanism leads to production of new species by instant speciation.
  • Geographical isolation followed by reproductive isolation ultimately leads to production of new species.
  • Isolating mechanisms protect the identity of a species, which all species fiercely guard.