Speciation and Creationist Mythology
The first step to understand the concept is to define what a species is. A good general definition of species is “the basic unit of biodiversity”. However, we need to get a little bit more specific. A working definition for the post, at least for the sexually-reproducing species, is “a set of actually or potentially interbreeding populations, which always generate fertile offspring.” Using this last definition makes us easily visualize that, for example, the dog, in spite of all its different morphologies is a species. A Fox Terrier, can breed with a Great Dane and have fertile offspring. On the other hand, a horse and a donkey can breed, but their offspring is not fertile.
One creationist myth is that we have not seen a new species being generated since humans populated the planet. This is not true. Species have been created artificially. For example, domestic sheep were created by hybridization, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended. The best-documented creations of new species in the laboratory were performed in the late 1980s. Scientists bred fruit flies, using a maze with three different choices such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies which came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring would not interbreed. Evolution can happen in front of our eyes.
The second step to understand the concept is to understand the mechanisms by which this happens. I am not going to go deep into this, however, I will explain the simplest theory which is the allopatric speciation.. During allopatric speciation, a population splits into two geographically isolated allopatric populations (due to geographical change or emigration). The isolated populations then go through genotypic and/or phenotypic divergence, through mutations, as they become subjected to different selective pressures. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of interbreeding. This is what happened in the experiment mentioned in the previous paragraph.
Creationism implies that the species we see are the species that were created 6000 years ago by a divine being. This can be seen in websites such as Answer in Genesis. Intelligent design, a refined form of creationism, argues that a superior intelligence engineered the morphology of the current species and not different selective pressures through to random mutations in the genetic code. Unfortunately for the ID supporters, that would not be able to be proven since someone superior would have to have designed the designer, then the designer of the designer and so on until we reach the level of the supernatural designer, not provable, since it is outside nature. On the other hand, proper scientists have shown that new species can be formed and that the key for this is differential selective pressure combined with geographical isolation for a given period of time, which can be short like in the example of the fruit fly or longer like in the example of domestic sheep.