Biomedical Engineering Reference
In-Depth Information
the class that includes all bacteria; and eukaryotes (organisms with a
membrane-delimited nucleus) [14].
3.
The prokaryotes preceded the emergence of the eukaryotes, and the first
eukaryotes were built from the union of two or more prokaryotes [15].
4.
Every eukaryotic organism that lives today is a descendant of a single
eukaryotic ancestor [15].
5.
Every organism belongs to a species that has a set of features that charac-
terizes every member of the species and that distinguishes the members
of the species from organisms belonging to any other species.
Of course, it is difficult to garner unanimous agreement by scientists, and
every fundamental principle of taxonomy has been challenged at one time
or another. For those who would include viruses (Chapters 38
45) and
prions (Chapter 46) among the living organisms, statements 1 and 2 are
debatable. The validity of statements 3 and 4 has been questioned. Some
scientists have postulated that prokaryotes descended from eukaryotes,
shucking off organelles in favor of a more simple, casual life-style. Others
suggest that prokaryotes and eukaryotes arose simultaneously. In this case,
any genetic or metabolic homologies between eukaryotes and prokaryotes
result from a shared gene pool.
Statement 5 has a long and disputatious history. It has been argued that
nature produces individuals, not species; the concept of species being a
mere figment of the human imagination, created for the convenience of
taxonomists who need to group similar organisms. There are those who
would use computational methods to group organisms into various species.
Ifyoustartwithasetoffeaturedataon a collection of organisms, you can
write a computer program that will cluster the organisms into species,
according to their similarities. In theory, one computer program, executing
over a large dataset containing measurements for every earthly organism,
could create a complete biological classification. The status of a species is
thereby reduced from a fundamental biological entity to a mathematical
construction.
This view is anathema to classic taxonomists, who have long held that a
speciesisanaturalunitofbiologicallife, and that the nature of a species is
revealed through the intellectual process of building a consistent taxonomy
[16]. There are a host of problems consequent to computational methods
for classification. First, there are many different mathematical algorithms
that cluster objects by similarity. Depending on the chosen algorithm,
the assignment of organisms to one species or another would change.
Secondly, mathematical algorithms do not cope well with species conver-
gence. Convergence occurs when two species independently acquire an
identical or similar trait through adaptation; not through inheritance from a
shared ancestor. Examples are: the wing of a bat and the wing of a bird;
the opposable thumbs of opossums and of primates; the beak of a platypus
