Speciation is the changing of individuals within a population so they are no longer part of the same species. This most often occurs due to geographic isolation or reproductive isolation of individuals within the population. As the species evolve and branch off, they cannot interbreed with members of the original species any longer. There are four types of speciation that can occur based on reproductive or geographic isolation, among other reasons and environmental factors.
The prefix allo- means "other". When paired with the suffix -patric, meaning "place", it becomes clear that allopatric is a type of speciation caused by geographic isolation. The individuals that are isolated are literally in an "other place". The most common mechanism for geographic isolation is an actual physical barrier that gets between members of a population. This can be something like as small as a fallen tree for small organisms or as large as being split by oceans.
Allopatric speciation does not necessarily mean the two distinct populations cannot interact or even breed at first. If the barrier causing the geographic isolation can be overcome, some members of the different populations may travel back and forth. However, a majority of the populations will stay isolated from each other and as a result, they will diverge into different species.
This time, the prefix peri- means "near". So when added to the suffix -patric, it translates into "near place". Peripatric speciation is actually a special type of allopatric speciation. There is still some sort of geographic isolation, but there is also some sort of instance that causes very few individuals to survive in the isolated population compared to allopatric speciation.
In peripatric speciation, it may be an extreme case of geographic isolation where only a few individuals are isolated, or it could follow not only a geographic isolation, but also some sort of disaster that kills off all but a few of the isolated population. With such a small gene pool, rare genes are passed down more often, which causes genetic drift. The isolated individuals quickly become incompatible with their former species and have become a new species.
The suffix -patric still means "place" and when the prefix para-, or "beside", is attached, it implies that this time the populations are not isolated by a physical barrier and are instead "beside" each other. Even though there is nothing stopping the individuals in the entire population from mixing and mating, that does not happen in parapatric speciation. For some reason, individuals within the population only mate with individuals in their immediate area.
Some factors that could influence parapatric speciation include pollution or an inability to spread seeds for plants. However, in order for it to be classified as parapatric speciation, the population must be continuous with no physical barriers. If there are any physical barriers present, it needs to be classified as either peripatric or allopatric isolation.
The final type of speciation is called sympatic speciation. Putting the prefix sym-, meaning "same" with the suffix -patric which means "place" gives the idea behind this type of speciation. Amazingly enough, the individuals in the population are not separated at all and all live in the "same place". So how do the populations diverge if they live in the same space?
The most common cause for sympatic speciation is reproductive isolation. Reproductive isolation may be due to individuals coming into their mating seasons at different times or preference of where to find a mate. In many species, choice of mates may be based on their upbringing. Many species return to where they were born to mate. Therefore, they would only be able to mate with others who were born in the same place, no matter where they move and live as adults.