Speciation is the process whereby a particular species gives rise to two or more distinct species, neither of which can reproduce with the other or the original species. This process is also known as cladogenesis, and it can take place due to geographic isolation (allopatric speciation), within the same region (sympatric speciation), or in adjacent regions (parapatric speciation).

Only a single species existed initially, but that species became two or more over millions of years. Imagine a species separated into two populations due to the appearance of a geographic barrier, a mountain, for example. Some individuals are trapped on one side of the mountain, living in a warm, humid climate like a rainforest. Those that stay behind live in a drier environment, like a desert.

Much time has to go by, millions of years, but the 2 populations can become different species, unable to interbreed. Each of the populations will adapt to its environment. Individuals best adapted for surviving in the jungle-like habitat will be selected on the warm, humid side. The larger individuals will survive since they can feed on the many types of plants available.

They will evolve into a diurnal species, active during daytime hours. On the dry side, individuals that adapt to the desert habitat will survive; maybe only the smaller individuals will survive and reproduce, feeding on insects and evolving into a nocturnal species. The differences will lead to genetic drift after many years, and the frequency of certain alleles will differ in each population.

In our hypothetical example, the individuals living on the warm, humid side will have a higher frequency of genes that code for larger bodies. They will evolve herbivorous teeth and a digestive system that can digest plants. On the dry side, the individuals will have a higher frequency of genes that code for smaller bodies, carnivorous teeth, and night vision.

Over time, both species will have different behaviors and different physical characteristics. Suppose we add the possibility that each population undergoes a series of genetic mutations. In that case, the differences could increase even further, diminishing the possibility that they could successfully reproduce if they were ever to meet again in the same territory.