Speciation is the process through which a new species is formed that can interbreed and produce viable offspring under natural conditions. New species form when barriers exist to prevent outbreeding: these are usually environmental (mountain ranges, climatic barriers, oceans) or biological (cannot produce an offspring).
The following is a fictional example of speciation-
The mongoose is an invasive species in Fiji and has been for some time now. The small Indian Mongoose (Herpestes javanicus) was introduced into Fiji in the late 1800s. It is found on 11 Fijian islands including the two main islands: Viti Levu and Vanua Levu. In 2007, a new species of large, red coloured mongoose was found when trapping mongoose near Suva for testing the spread of leptospirosis in Fiji. DNA testing revealed that this recently discovered mongoose is of a new species: the Red Mongoose (Herpestes jalabicus).
Mongooses prey primarily on rats, and occasionally snakes. On one island in Fiji called Humunaka Veru there was a disease which caused all of the rats to perish. This removed a vital piece in the food web: the mongoose was pressured since less food was available to them. The mongooses on this island grew more dependent on snakes as a food source, and in turn, they evolved to become better at catching snakes: evolution caused the mongooses on this island to grow larger, and stronger. Over generations, the coats of the mongooses got redder and redder: this allowed them to better camouflage when stalking the snakes which enjoyed sunbathing on the beach. As well, the natural mating times of mongooses changed from nighttime to daytime. Previously, the mongoose had been a nocturnal animal: it hunted rats in the dark. Now, the red mongooses on Humunaka Veru were active primarily during the day since their prey (snakes) enjoyed coming out and sunbathing during the day. This evolution also affected the times of mongooses` mating. The mongooses on Humunaka Veru still mated during the early spring, just like the Small Indian Mongooses. But, they mated during the daytime rather than the nighttime.
The evolution of the new species of Red Mongoose (Herpestes jalabicus) is an example of speciation through reproductive isolation. This new species underwent ecological isolation since it lived on a separate island separated by the Pacific Ocean, and temporal isolation since it mated at different times. These two types of reproductive isolation resulted in the evolution of a new species of mongoose: the Red Mongoose.