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The isolating mechanisms put a check on the inter breeding amongst related species and serves as external barriers. There are a great many ways in which free interbreeding may be interfered with. These are as follows:

1. Geographical isolation.

2. Spatial isolation.

3. Reproductive isolation.

1. Geographical isolation

When two populations or two parts of a population are separated by geographic barriers, it is called geographic isolation. Wagner (1868) was the first who mentioned that geographic isolation was a factor of condition in the formation of every species, race or tribe of animals or plants on the earth.

The organisms are separated by geographical barriers, such as land masses in seas, bodies of water over land, mountains, deserts etc.

For example in the central Arabian desert, two races of rodent, Meriones serous inhibit different stream valleys separated by only a mile of base limestone plateau. In the same way, high mountain-ranges, deserts, dense forests and extremes of temperature also act as effective barriers.

Such populations are completely cut off from one another genetically. Such a type of geographic barrier is found between the fauna of an oceanic island and that of mainland. In Britain, a famous example of geographic isolation among birds has been given by Salomonsen. The Wren, Troglodytes troglodytes forms distinct subspecies, on St. Kilda in the shetlands and on the other Outer Hebridean Island.

Another interesting example of geographic isolation was given by Gulick as the distribution of Hawaiian land-snails. Each variety of snails occurs not only in one island but in definite valley on the island. Gullick’s findings were confirmed by Crompton.

Two species of mosquito (Genus-Anopheles) in Africa are separated by salinity of the water the larvae of both can live in fresh water but one of them can endure brekish water and perhaps to avoid competition usually does live in such water.

Another example of geographical isolation that described by Darwin for finches. Darwin found that there were 26 groups of finches among the Galapago Islands (which lie a few hundred miles west of South America). Only five of these groups were the same as the finches found on the mainland.

The other twenty-one were types peculiar to the groups of islands. Some of the twenty-one groups interbed quite freely, while others did not. Apparently, each of the groups became isolated by migration. Each group evolved separately from the continental forms as well as from other isolated groups on the islands, forming a series of species and subspecies.

2. Spatial Isolation or Isolation due to Sheer Distance

If a species ranges all over a wide extent of territory which is unbroken by barriers, there may be isolation because of great distance between extreme outlying section of the range. Its example is wrens (birds) of South America. Wrens are found all over the continent but the wrens of one region differ from those of the other in colour patterns, size proportions and habits. It shows that without any barrier, sheer distance apart tends to produce local races (subspecies).

3. Reproductive isolation

The mechanisms of reproductive isolation can be varied. Most modern evolutionists such as Mecham (1961), May (1948, 1970) and Stebbins (1966, 1971) have classified the reproductive isolating mechanism into two classes:

(a) Prezygotic mechanisms, which either prevents contact between species at the time of active reproductive period or else the fertilization is checked even if pollination may be effectively brought about.

(b) Postzygotic mechanisms, where fertilization is brought about, but the hybrids are either inviable or weak, or the hybrids are sterile or in still other cases the F1 may be fertile but F₂ progeny will be sterile.

Both types of isolating mechanisms include many sub-types, all of which has been given in the table.

(a) Prezygotic Isolation Mechanisms

Mechanisms that prevent interspecific crosses (i.e., fertilization and zygote formation).

1. Habitat isolation. The populations live in the same regions, but occupy different habitats, so that potential mates do not meet.

2. Seasonal or temporal isolation. The populations exist in the same regions, but are sexually mature at different times, so that, potential mates remain unable to mate.

3. Ethological isolation. The populations are isolated by (in animals only) different and incompatible behavior before mating so that, potential mates meet but do not mate.

4. Mechanical isolation. Cross fertilization or pollination is prevented or restricted by differences in structure of reproductive organs (genitalia in animals, flowers in plants), so that, copulation is attempted but no transfer of sperm takes place.

(b) Postzygotic Isolating Mechanisms

Fertilization takes place and hybrid zygotes are formed, but these are enviable, or give rise to weak or sterile hybrids.

1. Game tic mortality. Sperm transfer takes place but egg is not fertilized.

2. Zygotic mortality. Egg is fertilized but zygote dies.

3. Hybrid in viability or weakness. Zygote produces and F1 hybrid of reduced viability.

4. Hybrid sterility. Hybrids are sterile because gonads develop abnormally, or meiosis breaks down before it is completed.

5. F₂ breakdown. F, hybrids are normal, vigorous, and fertile, but F₂ contains many weak or sterile individuals.

1. Habitat isolation

Populations of different races or of a species may occupy different races or of a species may occupy different habitats in the same geographic region because of their reference to different ecological factors such as soil composition, soil water, altitude, etc. Such habitat isolation gives the individuals of the species little or no opportunity to meet and interbreed.

Examples are: a fish and a seed-eating bird, which are environmentally isolated, although they live in the same locality. Similarly, beetles, which burrow in the ground, are environmentally isolated from beetles, which live on trees. In this case, environmental conditions are not similar. Moore (1949) said, “The difference between geographic and habitual solution is merely quantitative.” Stebbins (1965) mentions that ecological or environmental isolation is much more effective in plants than animals.

In some cases, evolution may be due to combined action of ecological and geographic isolation working together. This has been shown by Camin and Ehrlich for water snake, Natrix sipedon found in Florida. One race of it is found in freshwater and the other in salt water. They do not mate when come close to each other.

2. Seasonal isolation

Specific differences in breeding season or seasonal occurrence are extremely common in insects and are not rare in other groups. In Cicada of the United States there are two races, the 17-year race and the 13- year race. The numbers refer to the time in years spent as a subterranean nymph.

These two main races scarcely differ in structure, but do not appear to interbreed. Similarly, in the United State Rana calmitans, R. pipiens and R. syluatica may all breed in the same pond but they do not interbreed as individual breeding is determined by water temperature R. syluatica breeds when water temperature is as low as 44°F. R. pipiens starts breeding at 55°F, while R. clamitans requires 60°F.

In Cocklebures, the flowering time of two species have become so different that in the same area one species flowers only after the other has formed seed capsules. Thus, chances of crossing do not occur.

3. Ethological isolation

There are many instances in which behaviour differences particularly in regard to courtship, restrict copulation. The males of every species have specific courtship behaviour and females of the same species are receptive.

Courtship involves an exchange of stimuli (visual, auditory, tactile, olfactory or chemical) between male and female continuing until both have reached a state of physiological readiness in which successful copulation can occur.

The visual stimuli includes colour, colour patterns on the body of organisms, the shape, size and their movements as seen in birds, insects etc. The auditory stimuli include songs, calls and other acoustic signals. The chemical stimuli include the scents or specific odours as seen in insects and mammals.

It appears that unless the female recognizes the male as belonging to her species she will often eat him and the peculiar dances or scent or sound of males assist the females to avoid mistakes.

4. Mechanical isolation

Mechanical isolation is proved due to differences in the genital organs of different species in animals, so that copulation may not be easily brought about. However, this mechanical isolation in animals has now been shown to be fairly ineffective. According to L. Durfour (1844) the genitalia in insects are developed on ‘lock and key’ principle.

Certain type of key can fit and open a particular lock. According to Dufour the male and female genitalia are so exactly fitted to each other that even slight deviation in structure of either renders copulation impossible. No doubt mechanical isolation was considered to be an effective barrier to mating in some species, but it is not applicable to all.

(b) Post-Zygotic Isolating Mechanisms

If the prezygotic isolating mechanism fails then the postzygotic isolating mechanism prevents the successful hybridization. They are of following types:

1. Gametic mortality

The gametic mortality is seen in forms both with external and internal fertilization. Even when mating occurs, the sperms are killed in an antigenic reaction in the female genital tract before they could reach the eggs. Patterson found in cross-insemination between related species of Drosophila.

The mucous membrane of vagina swells markedly after mating. The swelling is stimulated by spermatic fluid, even without any sperm in it. If the mating has been with a male of same species the vagina returns to normal size within few hours. In an inter-specific mating, it may remain swollen for days. Eggs are destroyed in a swollen vagina.

2. Zygotic mortality

Sometimes the gametes of different species have succeeded in uniting; there still may be complete isolation of the contributory species. The development of hybrid zygotes is often irregular and the zygote may die at any stage during development (between fertilization and adulthood). For example in a cross between Datura stromonium and D. metel development takes place up to 8 celled stage and then stops.

3. Hybrid inviability

Many naturally occuring hybrids have been found to leave no offspring’s, even though they seen fully fertile. The weakness of hybrid is attributed to some physiological disturbance or ecological differences.

4. Hybrid sterility

Hybrids may be formed, may be vigour and appears to be normal, yet it is sterile. The mule is hybrid of jackass (Equus asinus) and a mare (Equus calabus). It is a very vigrous animal, superior in many respects to both of its parents, but it is a sterile one.

Whether sterility is gametic or zygotic (haplontic or diplontic), basically it will either be due to genie disharmony or due to chromosomal differences. The genie sterility may be caused due to genetically controlled spindle abnormalities, a synapsis or desynapsis and similar other abnormalities. Chromosomal sterility will be mainly due to lack of homology between chromosomes of two parents.

The chromosomes do not pair at all, thus resulting into meiotic irregularity leading to strong game tic sterility.

5. Hybrid break-down

Merrell (1962) describes that even vigorous, fertile F, hybrids produce hybrid breakdown in F₂ generation may be responsible for reproductive isolation. In such cases, subsequent generations may show reduced vigor or fertility or both.

An example of hybrid breakdown is provided by the F, hybrids, Gossypium arboreum X G.herbaceum this case, progeny is either missing or is very weak.