MODULE #1: Biology: The Study of Life
The Definition of Species
After reading the last section, you should have noticed a few things about classifying organisms. It's not very easy or clear-cut. To separate organisms into five separate groups, we have already run into exceptions. Kingdom Plantae, for example, is supposed to contain autotrophs. There are, however, some parasites that belong to that kingdom as well. In addition, we use the word “mostly” quite a lot, because although the majority of the members in a kingdom have a certain characteristic, there will be some members that do not. Thus, classification of organisms into kingdoms gets a little complicated.
As you might expect, classifying organisms in phyla, classes, orders, families, genera, and species becomes even more difficult. After all, as you move down the hierarchy in our classification scheme, you are getting more and more specific. While kingdoms have many, many members, those members are split into phyla. Thus, each phylum has fewer members than does the kingdom of which it is a part. In the same way, classes have fewer members than the phylum that they are in, orders have even fewer members, families have even fewer, and genera have still fewer. By the time you get to species, you have a very small group of organisms.
Since classification gets more and more difficult as you go down the hierarchy, splitting organisms into species becomes incredibly hard. If you thought that our definitions for what organisms go into each of the five kingdoms were bad, it is so hard to classify at the species level that biologists can't even agree on a definition for what the classification “species” really means! There is a lot of work going on right now in the field of biology trying to figure out a good way to define this difficult classification. For our purposes, however, we must have a definition, so we will go with the most commonly accepted one:
Species – A unit of one or more populations of individuals that can reproduce under normal conditions, produce fertile offspring, and are reproductively isolated from other such units
Although this definition is not perfect, it is the one that we will use for now. What does it mean? Basically, if organisms can reproduce and their offspring can also reproduce (that's what “fertile” means), these organisms belong to the same species. Any other organism with which this species cannot reproduce is said to be “reproductively isolated” from this species and therefore must belong to a different species.
Notice that in the previous section, we gave you the characteristics by which you can separate all organisms on earth into the five kingdoms of our classification system. Then, in this section, we skipped over all of the other classification groups except for species. For that classification group we gave a definition. Why did we leave out the other classification groups? Well, we didn't want to overwhelm you with information. There are (depending on whose classification system you use) nearly 100 different phyla in creation. Members of each phyla have their own characteristics, and we would have to go through each phylum individually to give you a good feel for how to classify organisms into these groups. Of course, since each phylum is split into several classes, there are even more of those. Thus, to go through and give you a view of each kingdom, phylum, class, order, family, and genus would be an incredibly long discussion! When we get to species, however, the classification is so specific that we can actually come up with a weak definition for it. That's why we skipped from kingdom all the way to species.