(By Dr Girish Chandra)
Speciation is the evolution of a new species from the pre-existing one by gradual modification. Since species are reproductively isolated populations, creation of a new species demands a mechanism that will produce reproductive isolation between two populations. Speciation can be classified into two categories as follows:
Convergent speciation: Two species that have not developed enough reproductive isolation but have maintained their independent existence owing to the geographical isolation can merge and produce a new species by hybridization should the geographical barrier separating them is removed. Sometimes migrations or accidental carriage by wind or storm can also make the two populations merge into one species. ·
Divergent speciation: When two or more species are produced by splitting of a single species, either by migration or adaptive radiation. Gradually each population gives rise to an independent species, as happened in the case of Darwin’s finches. When a population enters a new ecological zone evolution is generally quick to produce new species by adaptation to new environment. ·
Phyletic speciation: This is also called sequential evolution or transformation, in which a species changes gradually over a long period of time to become entirely different from the ancestor. Evolution of horse from Hyracotherium and elephant from Moeritherium are excellent examples of phyletic speciation. Darwin’s theory is based on this concept of gradualism. ·
Quantum speciation: This involves sudden formation of a new species by rapid changes or saltation, caused by megamutation and disruptive selection, hybridization or polyploidy. When new areas are colonized, all niches are vacant that forces the isolating mechanism to set in rather rapidly to produce new types. Based on the mode, habitat and geographical distribution, speciation can be of several types as follows:
1. Allopatric speciation: Also called geographical speciation. A physical barrier divides the population into smaller units, as happens during interglacial period when sea level rises and separates islands from the mainland. The separated populations on the islands evolve differently by constant genetic changes, translocations, inversions etc. They change into races, then subspecies and if time is long enough, into species.
Sometimes the populations can also be separated when they are accidentally carried into a new area (as Darwin’s finches) or some individuals cross over the barriers like mountain, sea or river and never return back. Fourteen species of Darwin’s finches found in various islands of Galapagos archipelago originated from a single population accidentally blown with storm from Ecuador in South America.
Reproductive isolation is produced by accumulation of genetic changes over long period of time. The above mechanism is traditional speciation and is termed as Dichopatric speciation but sometimes geographical barriers are not well defined and small populations do frequently wander out of the main range of distribution to the surrounding uninhabitable areas. Such peripheral isolates lose contact with the main population and establish themselves as new species. This kind of speciation is known as Peripatric speciation.
2. Parapatric speciation: Described by Endler (1977), this type of speciation takes place in widely distributed species, having continuous distribution and no geographical barrier separating populations. Large populations are usually broken into clines, which have no effective geographical barrier between them but the distance and habitat differences themselves may form loose barriers. Such populations have overlapping boundaries which serve as zones of hybridization or ecological escarpment. Eleven species of land snail, Partula, on the island of Moorea in Society Island are known to have originated by parapatric speciation.
3. Stasipatric speciation: This type of speciation was categorized by White (1968, 1978) and Key (1968). The new species arises sympatrically, first by chromosomal rearrangement within the geographical range of the parent species. The new population spreads within the range of parent species, spreading by parapatric distribution. White (1978) studied 7 species of Australian grasshopper (Vandiemenella) having parapatric distribution, with zones of hybridization 200-300 meters wide and believed that all species are chromosomally distinguished and that their differences arose sympatrically.
4. Sympatric speciation: This type of speciation takes place in freely interbreeding populations which have no geographical isolation but sometimes host preferences may create pockets within the same area. Reproductive isolation is therefore produced by polyploidy or hybridization. Polyploidy is very common in plants. Approximately 47% of all angiosperms are polyploids. But it is rare in animals and has been reported in some asexually reproducing shrimps, isopods, bagworm moths, weevils and flies.
Hybridization normally produces sterile offspring but sometimes fertility is possible by introgressive hybridization (offsprings backcrossing with the parents). Also if hybrids become polyploids, then they will have full complements of all chromosomes and therefore will be fertile and can create a new species instantly.
5. Quantum speciation: This is sudden emergence of new groups by saltation. This speciation is much more rapid and sudden, and produces new species or higher groups. In small, scattered populations or populations that have migrated to new area, genetic drift, rather than natural selection plays an important role in quickly changing the species. Sometimes chance events or mega mutations aid in quick speciation. Disruptive natural selection then makes the species distinct and diverse.
After mass extinction, surviving species find a relaxed natural selection, as most of the ecological niches are vacant. The species then split into small populations in new environmental pockets and slowly get diverged into new types.