Coda
There are about 5,000 known species of prokaryotes, but scientists estimate that true diversity could range between 400,000 and 4 million species. Each has adapted to its particular environment and each performs many roles. Some of these roles are essential to sustaining entire ecosystems. But what is a prokaryotic species? Microbes, which reproduce asexually, cannot be thought of in terms of reproductive isolation. The advent of molecular genetics has brought with it new approaches to defining the concept of species. Some bacteriologists are differentiating prokaryotic species based on their rRNA sequences. If organisms possess rRNA sequences that differ by more than a certain proportion (usually three percent), these bacteriologists consider them different species. As new molecular genetic approaches to the study of microbes are developed, scientists will find additional ways of describing the vast diversity of organisms that make up a parallel, albeit invisible, part of our world.
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