Biology Reference
In-Depth Information
from pure chance, the simplest hypothesis for explaining similar characteris-
tics in a set of evolving objects is that the similarity is due to common ances-
try
. It is according to this rule that one can infer that, considering
Char. 24 in Fig. 2 is a “T” not only in most land plants, but also in the
alga
Euglena gracilis
, it was most probably a “T” in the cenancestor of
all these organisms.
It must be stressed that this is a practical, operational hypothesis;
however, natural processes, and in particular evolution, are not necessar-
ily simple or parsimonious. Most often, they are chaotic and recurring,
and their results are largely indeterminable, unpredictable, and redun-
dant — there are complex, random forces at work, and the life factory
often appears as a giant apparatus where organisms are assembled by
blind workers in a somewhat haphazard manner, fine-tuning being then
left to the forces of natural selection. Before addressing the difficulties
that this reality may entail for an analysis of a large number of characters,
it is necessary to clarify the basic phylogenetic concepts as applied to a
single character.
2.3. Ancestral or Derived — Qualifying the State
of a Character
In a phylogenetic analysis, the state of a character at a given node can be
considered, relatively to an ancestral node, either as unchanged (thus,
still ancestral), or as changed (thus, derived).
f
In most cases, it is through
a comparison with the state of the character in an outgroup (a group
phyletically external to the group being studied) that the analyst can
establish whether the state is ancestral or derived.
A character in an ancestral state is said to be plesiomorphic, whereas
one in a derived state is said to be apomorphic (Greek
plêsios
(“neigh-
bor”),
apo
- (“far from”),
morphê
(“form”)).
g
f
The terms “primitive” and “evolved” are also used, though they can be confusing
since they are often associated with a subjective notion of progress; furthermore,
a descendant can regress to a quite “primitive” state.
g
While, strictly speaking, the terms “ancestral” and “derived” qualify the state of
a character, they are often used to qualify a character as such; in this case, they are
synonyms for “plesiomorphic” and “apomorphic”, respectively.
