Who are the microorganisms
Those of us who do not work with or study microscopic organisms have a
tendency to clump the living world that is invisible to the naked eye into a large, nondescriptive categories: __, microbes, or even simply germs. Biologists themselves have
historically been guilty of tossing these diminutive to into catch-all groups irrespective
of their actual relationships. You may remember learning the five-kingdom classification
in school. This system split single-celled organisms into two groups - Kingdom Monera
for the Bacteria and associates and Kingdom Protista for every other unicellular creature.
While this classification overlooked the great diversity of microorganisms, it was actually
an improvement over the previous system, one that labeled every living thing as either
Animal or Plant!
Starting in the 19??’s, researchers used improvements in genetic techniques to
piece together a classification system that more accurately reflects evolutionary history
and the vast genetic diversity in the microbial world. By focusing on the similarities and
differences in the DNA code used by all living things, scientists could build a “family
tree” based on true relatedness, rather than on superficial resemblance.
The concept is fundamentally simple. The Tree of Life, like any family tree,
begins with a common ancestor. Distantly related groups that took on separate lineages
long ago branch off near the base of the tree, while closely related groups that have
diverged only recently appear close together. To fill in the tree, taxonomists have
exploited the fact that all organisms share the DNA molecule as genetic blueprint and that
changes to this molecule are what make each species unique. Mutations in the genetic
code are passed down only in the lineage in which they occur and thus genetic
relatedness gives an accurate representation of evolutionary relatedness. It turns out that
certain regions of the genetic code are more useful in assembling the Tree of Life than
others. One of the most useful regions encodes instructions for cellular organelles called
ribosomes. All living things rely on ribosomes to translate genetic information into
functional proteins within cells but their specific sequences vary enough that they can be
used to compare even closely related species.
The picture that emerged from such “phylogenetic” investigations was radically
different from that of traditional classification. No longer did the macroscopic plants and
animals dominate as in the five-kingdom system. Instead, the fundamental division of
living things became the three domains: Bacteria, Archaea and Eukarya. To the casual
observer this system may seem to give disproportional weight to microorganisms, which
occupy all of the first two domains and much of Eukarya, as well! But…
genetic /physiological/ ecological differences
relative sizes