The biological species concept, which was first introduced by Ernst Mayr, defines a species as a population whose members have the potential to interbreed to produce viable, fertile offspring, but do not produce healthy offspring with other species.
How do species originate? 
Different Modes: 
There are different modes of speciation that can lead to the origin of species. We will focus on allopatric and sympatric speciation, but will also introduce two other modes: parapatric and peripatric. Gene flow, the loss or gain of alleles in a population due to the migration of fertile individuals or between gamete populations, is reduced in all of these speciation modes. A recent trend towards designating speciation events as ecological speciation or nonecological speciation is motivated by a desire to focus more on the process, rather than the geography, leading to speciation events. This dichotomy may be somewhat misleading, however, as empirical work indicates that ecological factors are important in driving speciation regardless of whether or not gene flow is present, leading many to question whether speciation is ever truly nonecological.
Allopatric Speciation: 
A geographic barrier is the predominant means by which allopatric speciation occurs.
How it works 
- There are two populations that interact by interbreeding, which allows for gene flow (genetic exchange), within a broad geographic range. The resulting offspring are fertile because these populations are of the same species.
- A geographic barrier arises, isolating the two populations. The newly isolated populations now go through a period of isolation. Some of the individuals within a population might exhibit a trait more suitable for the new environment. This variant allows for differential survivorship and reproduction. Those individuals that carry the trait are more likely to survive in the new environment. Therefore, they will leave more offspring. Gradually, the variant trait will become dominant. Theoretically, this would happen with both of the newly isolated populations resulting in reproductive isolation.
- The geographic barrier ends. The two populations now have the opportunity to interbreed again. If the populations do not recognize each other as potential mates or cannot produce fertile offspring, speciation has occurred because the populations are reproductively isolated. The two populations have become two different species. If speciation has not occurred, the populations can still interbreed successfully to produce viable, fertile offspring.
An organism's mobility plays a significant role in allopatric specition. Two populations of birds would not be hindered by an emerging mountain range because of their ability to fly. The size and remoteness of a population also play a major role in speciation. There is a greater chance of a species forming in a small, isolated population because natural selection and genetic drift are more likely to alter the gene pool there.
These barriers are not limited to the more recognizable ones, mountain ranges and rivers, though. Populations can also become reproductively isolated by members of a population colonizing a new remote area without the presence of their parent population.
Examples of Allopatric Speciation: Ring Species and Adaptive Radiation
- Ring Species
- A species distributed around some geographic barrier, with the populations that have diverged the most in their evolution eventually meeting where the ring closes.
In this example, gene flow occurs throughout most of the circuit except for where the two populations overlap. The idea behind a ring species is that given enough distance and time, populations will diverge in their genetic makeup hindering their ability to interbreed.
- Adaptive Radiation
- A process characterized by periods of isolation and dispersal. This example of allopatric speciation usually occurs when there are new areas for a species to occupy, after mass extinctions, or when natives have not colonized an area.
Adaptive radiation can be seen in the evolutionary history of the Hawaiian Archipelago, which consists of numerous volcanic islands. Each island is physically diverse, allowing for natural selection to occur, which can lead to reproductive isolation. This type of allopatric speciation is exemplified in the evolutionary history of honeycreepers. There are over 20 living species of honeycreepers that all evolved from a single species that had colonized an island. Honeycreepers dispersed to the different islands and evolved. Those that came back to their native island were not capable of interbreeding with the natives anymore because the visiting honeycreepers had adapted to their new environment.
Sympatric Speciation 
Speciation does not only occur when there is a geographic barrier separating two populations; Sympatric speciation occurs when divergent selection causes preferred mating with genetically similar individuals but not with the parent population, hence reproductively isolating one part of the population from another.
Sympatric Speciation in Plants 
Plants frequently undergo this type of speciation. The most common type of sympatric speciation is a mutant condition called polyploidy, which is caused by a doubling of chromosomes during cell division. Autopolyploid mutations occur when an individual has more than two chromosome sets, all derived from a single species. A more common type of polyploidy species is an allopolyploid, which forms a hybrid from two species. Sympatric speciation generally occurs in plants through hybrids of related species and errors in cell division, producing fertile polyploids.
The parent species of a population is diploid (2n). A meiotic error during gamete production doubles the chromosome number from diploid to a tetraploid (4n) zygote. This zygote can now self pollinate or mate with other tetraploids, but cannot interbreed with the diploids of the original population. The offspring of a tetraploid with a diploid would create a triploid (3n), which would be sterile because of unpaired chromosomes. Gene flow has been reduced.
This type of speciation was first discovered by Hugo de Vries, a botanist. He studied variation among primrose species. During his breeding experiments with Oenthera lamarckiana (a diploid, 14 chromosome species of primrose), he discovered a new tetraploid (4n) species with 28 chromosomes. He named this tetraploid species Oenthera gigas. This species could not interbreed successfully with its parent species.
Two different species interbreed, combining their chromosomes to form a polyploid hybrid. A meiotic error during gamete production results in sterile hybrids because the haploid chromosomes from the two species cannot pair up. There are some mechanisms that can allow for a sterile hybrid to become fertile though. A sterile hybrid might be fertile with another sterile hybrid of its kind, but not successful when interbreeding with the parent species. Gene flow has been reduced.
Polyploid plant species are very common. Scientists have recorded many new species of plants due to allopolyploidy: Tragopogon (goastsbeard), T. dubius, T. pratensis, and T. porrifolius. Many of the plants we eat are also polyploids: cotton, potatoes, and wheat. Wheat is one of the best examples of polyploidy. Bread, which comes from a species of wheat called Triticum aestivum, results from allopolyploidy. Geneticists create hybrids and use chemicals that result in meiotic and mitotic errors to create more polyploids.
Sympatric Speciation in Animals 
Sympatric speciation is less common in animals than plants, but still exists. Animals can become reproductively isolated by becoming dependent on different resources than those used by their parent species. It is possible for there to be a genetic change that selects for different resources. Sympatric speciation in animals, which isolates the species reproductively, results from preferences for different habitats, food, resources, and breeding partners.
One of the best examples of sympatric speciation in animals is with the 200 different species of cichlids that inhabit Lake Victoria in East Africa. Sympatric speciation has occurred here as a result of variations in available resources, as well as selective mating based on coloration. This idea was tested by a group of scientists at Holland’s University of Leiden using P.pundamilia (blue back) and P. nyererei (red back). Although females of the P. pundamilia species only mated with males of the same species in normal lighting, they mated with the P. nyererei males in monochromatic orange lighting which made the two species appear identical. The resulting hybrids between the two species were still fertile. Mate choice based on coloration led to sympatric speciation within the cichlid species in this case.
Parapatric Speciation 
In the absence of a geographic barrier, parapatric speciation can also lead to the emergence of a new species. In this type of speciation, gene flow is reduced because individuals are reproducing with the mates closest to them. Therefore, mating is not random. The phenotypes are gradually divergent from the extreme forms, but are all equally present within the population. Gene flow is reduced within the population. Parapatric speciation cannot produce separate species in the absence of divergent selection.
Peripatric Speciation 
This type of speciation correlates (?) with allopatric speciation. It occurs in a few members of the population, which exhibit a different appearance than the majority. Genetic drift plays the major role in speciation. This mode is the hardest to support with evidence.
Campbell, Reece. Biology, Sixth edition. Benjamin Cummings. 2001.