Speciation
Speciation is the formation of a new species by dividing one species into two. Two species are considered to be separate if the individuals cannot reproduce to produce viable offspring (those that are fertile and cannot produce offspring of their own). Thus, if two individuals are reproductively isolated, they are considered different species.
Speciation often begins when members of one species are geographically isolated from each other. Geographical isolation can occur because of physical barriers such as a mountain, river, forest, desert, or island preventing movement of individuals and genetic mixing of members of the species. For example, a population of fish in a large lake might be split in two during a great drought. When water evaporated and two separate smaller lakes formed, some individuals would be isolated from the rest of the population. The two populations of fish would be geographically isolated from each other.
Two geographically isolated populations of the same species can evolve separately over time so that they are each able to survive in the specific conditions under which they live. For example, the hypothetical fish populations described above might each live in areas with different sources of food or different predators. One population could diverge from the other to take advantage of the different conditions. While one population might remain the same as it always was, the other population could evolve adaptations to exploit the different sources of food or to survive in the different environmental conditions of its lake.
If enough time goes by, and the populations diverged to a great enough extent, the two species might no longer be able to reproduce and have viable (fertile) offspring; they would be reproductively isolated. Reproductive isolation can occur because of changes in anatomy, behavior (different mating times or seasons, different courtship behaviors), or physiology (the gametes are incompatible).
If in the hypothetical situation discussed above there were great rains after many thousands of years, the two populations could be once again living in one large lake. Because enough changes have occurred, even though they might no longer be geographically isolated, they would be reproductively isolated. Since they are reproductively isolated speciation has occurred. Individuals can no longer mate, and the two populations of the same species have evolved into two separate species.
In some cases, speciation occurs even though members of a population were not geographically isolated. Genetic changes such as polyploidy (more than two complete sets of chromosomes per cell) in plants can on rare occasions result in speciation if two individuals with the same genetic changes are able to mate and produce viable offspring.
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