Speciation events are believed to happen under a number of conditions. Scientists generally believe it takes place because members of the same species have become isolated from one another. There are four types of speciation that evolutionists have identified as being the likeliest explanations for speciation.
In the case of allopatric speciation, an actual geographic boundary physically separates the species. A river or a mountain range, for example, may cause a species to diverge.
With parapatric speciation, a species is spread out across large areas with diverse environments. They adapt to these new areas and gradually become separate species. There's no defining geographical feature that separates these animals -- they may become separate species simply because of the distance that separates the groups.
Peripatric speciation occurs when a small group becomes isolated from the main body of the species. Since this cluster is just a small portion of the whole population of the species, all of the genetic differences which make the species robust may not be present. This presents what evolutionary biologists call a bottleneck. Some of the genes that flow within a species have been clipped off and separated from the gene pool.
In an evolutionary bottleneck, a small population is successful at producing subsequent generations. This has happened at various times in human history. Most Europeans, for example, are all descendents of only a few hundred ancestors, who lived in the midst of a bottleneck [source: The Polymath].
While a small group may be successful in repopulating, there are repercussions that may arise from a bottleneck. The genes present in this small population will become robust, since the genetic flow is much bulkier than it would be in a larger population, where genetic differences -- including abnormalities -- are spread out. This is called the founder effect. The genes of founding members of what eventually becomes a large population become much more frequent than they are in similar, larger populations. Several members of an Amish sect in Pennsylvania suffer microcephaly, a generally fatal disease where the brain does not attain its full size. These Amish are all descended from a single couple who arrived in Pennsylvania in the 18th century.
In the case of sympatric speciation, members of a species continue to live side by side, but still separate into different species. For example, some insects feed and reproduce on a single type of fruit. If some members of this species try another type of fruit, their offspring may be raised to visit that fruit as well. If this happens, the members of a species may diverge into two species based solely on the fruit they feed on and lay eggs in.
In each of these types of speciation, the species must undergo the process of reproductive isolation. For example, the damselfly evolved to form differing genitalia, and members became physically incapable of mating with one another. Reproductive isolation needn't be so drastic, however. Members of a single species can evolve to create different mating rituals, ones that won't attract members of the other species. They may also mate at different times of the day, month, season or year. Or they can evolve to mate in different places, for example on two different types of fruit, as in the example given above for sympatric speciation.
So when a population finds itself isolated in one way or another from the rest of a species, speciation occurs, right? Not necessarily. Find out more about the disagreement over speciation on the next page.