You pick up an article in the Journal of Vertebrate Paleontology on a new species
of ground sloth. The next thing you know, you're confronted with
symplesiomorphies, autapomorphic characteristics, and paraphyletic groups. And
some very funny looking branching diagrams, annotated with numbers and
accompanied by a table of ones and zeros. Before you know it, you're swamped
by the jargon, put the article aside, and go on to something else.
What is this branch of evolutionary biology called cladistics, and how can you
wade through the terminology to gain some understanding of how it's used?
Cladistics is a method of analyzing the evolutionary relationships between groups
to construct their family tree. It has been around for almost fifty years, but has
really become popular in the past two decades. The principle behind it is that
organisms should be classified according to their evolutionary relationships, and
that the way to discover these relationships is to analyze what are called primitive
and derived characters.
Primitive characters are those attributes of a plant or animal which all members
of the group possess. Having four legs is primitive for mammals; they inherited
this characteristic from their common ancestor (a proto-mammal or mammal-like
reptile). Primitive characters are of no use in analyzing the relationship of
organisms within a particular
group. Were you to try to
construct a family tree for all
mammals, it is not helpful to note
that they all have four legs. They
do, but it doesn't help you in
determining who is related to
whom. In the jargon of cladistics,
primitive characters are called
plesiomorphic. Primitive
characters shared by all members
of the group in question are called
symplesiomorphic.
Derived characters are advanced
traits which only appear in some
members of the group. Cladistics
is based on the assumption that
the appearance of derived characters gives clues to evolutionary relationships. In
our example, a derived character for some mammals might be loss of the tail,
which occurs in the great apes and man. It is assumed that loss of the tail
occurred only once, in the common ancestor of apes and man, and that none of
us has one because we inherited that trait from our common ancestor. Thus if
mammals are separated into groups which do and which don't have a tail, shown
by a fork on the evolutionary diagram (cladogram), this represents the point at
which a new species evolved which didn't have a tail. Man and the great apes
are assumed to have descended from this species (which may or may not remain
undiscovered at the present time).
Derived (advanced) characters are called apomorphic in the lingo of cladistics. If
they belong only to the group in question, they are called autapomorphic;
obligate bipedalism (two-footed walking) is an autapomorphy of hominids
(people), a characteristic which we do
not share with the great apes. If the
derived character serves to unite two
groups, it is called synapomorphic; loss
of a tail is a synapomorphy of the group
containing great apes and man. No
animal within the group has a tail, yet
our next-nearest relatives, the monkeys,
do.
It is important to note that the
designation of primitive and derived
characters has meaning only when
related to the group under study. A
character which is derived relative to
one group may be primitive for a less
inclusive group. The occurrence of fur is
a derived character if one is studying all
tetrapods (four-footed vertebrates), and serves to distinguish mammals from their
ancestors, the reptiles. However, it is a primitive character for the group
consisting only of all mammals, and is not useful for determining relationships
within the Mammalia.
The premise behind a cladistic analysis is that by examining suites of primitive
and derived characters, diagrams can be drawn which illuminate the evolutionary
relationships between the groups. Branching points (nodes) on the diagram are
generated every time a derived character (or group of them) is identified which
one group possesses and another does not. The two groups on alternate sides of
a node are called sister-groups. By analyzing enough different characters or
traits, eventually, it is hoped, a true picture of the family tree can be generated.
The goal is to create a diagram where all members of the analysis are
descended from a single, common ancestral species, and for which all
descendant species are included. This is called a monophyletic (or, sometimes,
holophyletic) group. If all members of a group are not descended from a single
common ancestor, the group is termed polyphyletic. If the group doesn't contain
every descendant of that common ancestor, it is called paraphyletic. An example
of a paraphyletic group is the reptiles. The Class Reptilia in its traditional sense is
a useful concept, but it doesn't contain all the descendants of a common
ancestor, because mammals and birds are generally placed in their own classes.
(Dyed-in-the-wool cladists are dead set against allowing classifications which
contain paraphyletic and polyphyletic groups.)
There are some factors which complicate a cladistic analysis. One is convergent
evolution. We said above that bipedality was a characteristic of man. But if your
analysis included all groups of mammals, you would probably note that
kangaroos are also bipedal. Does this mean that they should be considered
closer relatives of man than are the great apes? Intuition tells you they should
not. This problem is handled by including as many different characters as
possible in a cladistic analysis. When one, like this, clearly doesn't fit the pattern
of all the others, it is assumed to be an anomaly and overridden in the analysis.
Reversals can cause problems, too. We said above that all mammals have fur.
Whales don't (to any significant degree), but that is because the fur their
mammalian ancestors possessed has been lost in an aquatic environment.
Again, when anomalous characters like this are encountered, they are overridden
in the analysis. It is important to use enough characters in the study that things
like this will shake out.
Another problem arises when dealing with fossil material in that sometimes parts
of the animal are missing which are needed to evaluate a particular character. In
this case, the characters are simply scored as missing (frequently designated by
a question mark); they are just ignored (for that specimen) when generating the
cladogram. This problem is handled by gathering data on as many characters as
possible so that no one deficiency is likely to throw the analysis off.
Cladistic analyses are generally run on the computer using the principle of
parsimony. This basically means that the computer generates all possible family
trees which would fit the data, and you assume that the simplest one is probably
correct. To use a computer program such as PAUP or MacClade, you must first
convert the results of your study into a data matrix. That's the table of ones and
zeros you see in many papers. This is simply a shorthand for, does a particular
animal have a certain characteristic, or doesn't it? (Is it bipedal, or not? Does it
have a tail, or not?) Although multistate characters are sometimes used, it is
most common to split the analysis up into a series of either/or questions which
are then scored yes or no, one or zero. If you know which is the primitive and the
advanced state, you usually assign zero to the primitive and one to the
advanced, but this is not necessary. The computer can handle the unpolarized
data matrix and give you the simplest diagram without that information.
The more information you include in your analysis -- the more species, and the
more characters -- the more likely you are to come close to the "true" family tree.
Although cladistics is touted as the greatest thing since sliced bread and a totally
objective method, it is still no better than the researcher who decides what
characters are important to use in the analysis. It is also no better than the
completeness of the fossil material available. The real merit of cladistic methods
is in their use of shared derived characters to unite groups and in the ability of
the computer to handle large batches of data. Over time, things will change, and
new cladograms will be generated. Don't be intimidated by them the next time
you try to read a paper in the primary literature. They're really just highfalutin
diagrams which try to illuminate evolutionary relationships.