The biological-species concept, explained in an earlier section, classifies species based on their ability to mate and reproduce. That system is all well and good for organisms that mate and reproduce, but it leaves lots of other organisms out in the cold.
Bacteria are perhaps the best examples of organisms that reproduce without mating. They simply divide into two daughter bacteria, each of which goes happily on its way, dividing further. Mutations can occur in this process, just as in any other DNA-replication process; hence, bacteria evolve.
When microbiologists go out to the environment (such as some exotic foreign location the likes of which you see on the Discovery Channel or the back of your throat), they find that they can group the microbes they collect into different categories such as:
1 Escherichia coli (E. coli): Common gut bacteria
1 Neisseria meningitides: One of the many organisms that can cause meningitis
Because scientists can group bacteria into separate, recognizable categories, they give those groups names like E. coli and call them species. But the bacterial species doesn't mean the same thing as it does for animals, plants, and other sexually reproducing organisms.
E. coli isn't a species because all E. coli mate with one another and not with Neisseria. It's a "species" for some other reason. The key is determining what cohesive force differentiates one species of bacteria from another. What keeps the groups separate?
One possible cohesive force that could be responsible for the existence of groups of nonsexually reproducing but similar organisms is periodic selection. In periodic selection, natural selection favors a mutation that confers high fitness, which leads to a purging of genetic diversity within a group of nonsexual organisms.
The process works this way. Think about our friend E. coli. As one E. coli bacterium divides, different mutations begin to accumulate, and the population of E. coli bacteria gets more and more variable. Now imagine that a mutation arises that is especially beneficial for an E. coli bacterium, enabling it to outcompete all the other E. coli. It's more fit, and as it takes over, all the other E. coli bacteria are eliminated; as a consequence, genetic diversity within the species is reduced.
This process, called a selective sweep, may be the reason why scientists can identify species of bacteria and name them based on their overall similarity — something called phenetic species or ecological species. In this type of system, natural selection periodically reduces the variation found in a species and keeps, for example, all E. coli bacteria looking pretty much the same.
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