There are many aspects of nature to explore. The following topics may provide you with insights into how nature works and how the forces of natural selection operate to shape it. Species by definition are a group of organisms that can produce fertile offspring among themselves but not with any other group. This means they possess a unique gene pool and that their genetic make-up is so different from any other organism that they are incompatible (unable to reproduce viable offspring in other words.) While this group of organisms may be physically distinguishable for others by what one sees, this is not always the case. There are several species of Zebras, for example, that resemble one another quite closely in appearance. There are visible characteristics of these species, however, that one can observe if one looks closely. They are separate species even though one would have to look at details of their patterns of stripes to tell the difference physically. The can not interbreed and that is what really makes them separate secies. Differences in physical appearance within a species (a subspecies or race by definition) may exist. These differences may be as great or greater than those we find in the Zebras. However, members of a subspecies still can produce viable offspring and therefore are not species.
The following is intended to help you understand the process that makes a group of organism a separate species. As well, we offer a look at the problems posed by species differences in terms of how anthropologists might look at the fossil record.