Speciation

__//**A. Introduction**//__

Speciation is a gradual process of macroevolution as it occurs over long periods of time. Evolving continuously ultimately leads to the formation of new species. There are various definitions of species, based on different point of views. A very common definition would be a population that has the capability to interbreed, and produce viable offspring; this is according to the biological species concept. Reproductive isolation is being used to have a better understanding of the term species as speciation starts occurring as a population becomes reproductively isolated from the other populations of the species. During this process, two isolated population starts to separate under selective pressures in different environments. If a population is different from its ancestral species and there aren’t any genetic changes among them, speciation is said to have occurred. **//__Speciation occurs in two ways: Allopatric Speciation and Sympatric Speciation.__//**

__//**B. Allopatric Speciation**//__ Allopatric speciation is a result of geographic isolation, such as separation by mountains, canyons, rivers, lakes, glaciers, altitudes or longitude, just to name a few. It is considered to be the most common type of speciation that works for almost all evolution of new animal species. When a small population migrates and settles in a new area away from the range of the original species, allopatric speciation occurs as well. Speciation is more likely to occur in small populations as genetic drift and the founder effect is more effective in a small population. Islands provide the geographic isolation that is needed for the rapidly occurring allopatric speciation and as a result, they offer excellent opportunities to study these evolutionary mechanisms. The Porto Santo Rabbit is an example of an animal that has undergone allopatric speciation. These rabbits used to live in the small island off the coast of Portugal where they didn’t have any predators or competitors. Around the 19th century, they were noticed to be completely different from their European ancestors as they adapted a different color pattern and a nocturnal lifestyle. But all scientists don’t agree that the Porto Santo Rabbit is a new species but actually a subspecies while other think of it as a new species that doesn’t interbreed with other rabbits in natural conditions.

__//**C. Sympatric Speciation**//__ Geographic isolation isn’t always a requirement for evolution even though it is key element for allopatric evolution. Sympatric evolution may occur without the geographic isolation and it’s very common in plants. This speciation may not be very important in animals as allopatric speciation is, in fact, it has been very difficult to demonstrate in nature. Some examples of this speciation would be polyploidy, habitat isolation, behavioral isolation, temporal isolation and reproductive isolation.

Polyploidy is the possession of more than two sets of chromosomes and it’s a very important factor in plant evolution. Autopolyploid and Allopolyploid are two kinds of polyploidy. Autopolyploid contains sets of chromosomes from one species while allopolyploid contains multiple sets of chromosome more species. Two polyploids can’t breed with others of the same species that are not polyploidy and as a result they are functionally isolated from each other.

In habitat isolation, two organisms live in the same area but breed in different habitats. For example, flycatchers are very identical and each species stay in its particular region within its range during breeding season.

Courtship behavior illustrates behavioral isolation and many species exchanges different signals before mating. For example, male fireflies give signal to the female by a particular pattern of blinking the lights on their tails. Female respond to the male that are from the same species by blinking back to attract males. If anything goes wrong while blinking, no mating occurs and they become isolated.

Temporal isolation demonstrates the genetic exchange between two groups that reproduce at the different times of the day, month or season. As flowers in warmer region becomes sexually mature earlier, they become isolated from the flowers grown in cool and shady areas.

Reproductive Isolation prevents interbreeding between two different species if their ranges overlap. Also it helps preserve the genetic integrity due to the prevention of gene flow. There might be a difficulty in breeding among closely related species due to the anatomical incompatibility. Prezygotic barriers prevent mating by stopping fertilization from taking place. In this case, male and female gametes never come into contact. Prezygotic barriers include temporal, habitat, mechanical, behavioral, and gametic isolation. Mating between a small male dog and a large female dog wouldn’t be possible because of the size differences. On the other hand, Postzygotic barriers help in the prevention of gene flow during the fertilization. Sometimes, fertilization might occur between gametes even after the existence of the prezygotic barrier and that’s when postzygotic barriers are needed. Genes from different species can’t function together properly for normal development and in this case reproductive isolation occurs by hybrid inviability. An example would be when bull frogs’ eggs are fertilized with leopard frogs’ sperms; all the hybrids die out in the embryonic stage. But if even an interspecific hybrid lives, they might lack the ability to reproduce. They may show courtship that is not compatible reducing the chance of mating. If two parents have different sets of chromosome numbers, hybrid sterility might occur causing the gamete of an interspecific hybrid to be abnormal. Also, there might be some defects that might cause the second filial generation to reproduce unsuccessfully leading to a hybrid breakdown //**__Sources:__**//

http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html.

[|http://www.globalchange.umich.edu/globalchange1/current/lectures/speciation/speciation.html