Biology Reference
In-Depth Information
Under the Linnaean system, a taxonomist begins by assessing the physical
(phenotypic) characteristics that a set of species shares, then selects the most
representative species to be the “type” for each genus, then the most repre-
sentative genus to be the type of the family, and so on. Individual specimens
are deposited in museums to serve as a reference for that species and genus.
When new specimens are found with similar traits, they are categorized as part
of a known species, as a new species, or as a new genus (or family or order)
depending on how closely the new specimens resemble the “type.” This reli-
ance on types results in dramatic changes if a systematist reevaluates a group
and decides that some members don't belong. Removal of these individuals
can mean that the group's name must be changed, which often is disruptive to
other biologists.
In the 1980s, an assessment of methods occurred after a classification method
called
cladistics
, which is based on the evolutionary histories of organisms, was
proposed. The cladistics approach is based on phylogeny, whereas traditional
Linnaean methods (
phenetics
) are not. Most current systematists now take the
phylogenetic approach with classification based on evolutionary relationships.
However, the use of DNA-based methods created new concerns about appropri-
ate methods of analysis and whether molecular and traditional morphological
methods provide equivalent answers. The enormous amount of DNA sequence
data also requires that new methods of handling large datasets be developed
(
Hall 2011, Fan and Kubatko 2011
).
Systematists who have concluded that the fundamental Linnaean binomial
system of nomenclature is obsolete have added additional ferment (
De Queiroz
and Gauthier 1994, Ereshefsky 2001
). The
PhyloCode
was proposed as a solution
to the perceived problems (
De Queiroz and Gauthier 1994, Pennisi 2001
,
www.
ohio.edu/phylocode
). Advocates of PhyloCode want to replace the Linnaean sys-
tem to make species names more stable. Under this system, genus names might
be lost and species names might be shortened, hyphenated with their former
genus name, or given a numeric identification. The debate over which is the
better system has generated much heat and only time will tell which approach
has the fewest shortcomings (
Pennisi 2001, Nixon et al. 2003
).
Systematics encompasses the study of both
phylogeny
and microevolution-
ary change.
Molecular evolution
encompasses 1) analyzing the evolution of
DNA and proteins and the mechanisms responsible for such changes and 2) deci-
phering the evolutionary history of genes and organisms. A more recent topic,
available only since the complete genomes of a variety of organisms have been
sequenced, is comparative genomics.
Comparative genomics
compares the
