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Deciding what living things should be classified in
the same groups requires deciding what's related to what, and how
close those relationships are. Long ago, it was often done
by lumping together
analogous traits: features
that are used to do the same function. This is why, in Biblical
times, if they were streamlined and swam ("Beasts of the
Water"), or had wings and flew ("Creatures of the
Air"), they were classified in the same groups. Certain simple
forms were also used for grouping: by this
approach, snakes would be grouped with earthworms and eels.
As more and more people studied Nature in
detail, it became obvious that a butterfly's wings were very
different structures than a bird's wings. And sometimes, it
could be seen that two structures used for very different
functions - such as a human hand, a bat's wing and a whale's
flipper - all contained the same basic internal architecture, with
changes in parts producing the outward changes. Traits with
similar internal structure are called
homologous traits,
and it was eventually decided that these traits were a better
measure of
relatedness than analogous traits. Keep in mind,
however, that traits can be both analogous and homologous (like a
monkey hand and a human hand), it isn't automatically an either / or
situation.
There
are, in modern taxonomy, two somewhat different approaches to putting
together "family trees" of organisms. In systematics, branches
occur when one species splits into two - at the point of a common ancestor
to the new branches.
Cladistics is similar, but the focus is
on when certain "new," special traits arise - humans
might split from chimps, for instance, at the point that our ancestors
began walking upright. Both are applying fairly rigid "branchings"
to points that in real time probably were more spread out, and only sometimes do they seem
at odds with each other. Their representations also tend to differ - systematics, the older approach, usually uses fluid, naturalistic
branching diagrams, while
cladograms tends to branch at geometrical
angles and use a lot of straight lines (it was developed when computer
printers dealt well with straight but not so much with curvy lines). There
is a third approach that might be added in -
molecular evolutionary
taxonomy is concerned with when certain key genetic differences
arise. They tend to be an offshoot of cladistics, but that
connection may get less clear over time.
Much basic taxonomy is still done anatomically,
although the level of detail has gotten smaller through the use of
microscopes and broader through the discoveries of genetics and
biochemistry. These will be covered later as they come up in
the historical journey.
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