SCHOOLS OF SYSTEMATICS
First of three lectures on Systematics. We are following a natural
progression from the variation and dynamics of genes within populations,
to divergence of populations and speciation to systematics
= the scientific study of the kinds and diversity of organisms and their
A systematic or phylogenetic perspective on diversity of life
itself follows logically from the fact that there is a phylogenetic
tree that relates all organisms: from one generation to the next there
is a pedigree that relates the parents to offspring. Within a population
at any one time there is a complex pedigree or network of the ancestry
of genes that describes who received what genes from whom. Among populations
of a species there is a tree indicating which populations diverged
from others and the sequence of branching events of population separation.
At the species level, there is another tree of relationships that
describes the sequence of branching events that led to the formation of
descendant from ancestral species. Thus, just as a kind of population
thinking is required to appreciate the evolutionary significance of
variation among individuals, a kind of tree thinking is required
to appreciate the evolutionary significance of the history of ancestor-descendant
relationships that unites all levels of organization: genes, individuals,
populations, species, higher taxa.
What are the goals of modern systematics? 1. Differentiate individual
organisms and establish the basic units: species 2. to arrange these
units in a logical hierarchy that permits easy and simple recognition in
the basis of similarity = classification 3. to keep the details
of 1 and 2 separate = nomenclature 4. determine the evolutionary
(ancestor-descendant) relationships between all levels of the hierarchy
Identification is not classification. Identification is to place
an individual into an already existing classification scheme. Classification
is to assemble groups into larger groups. There are conflicting goals of
systematics: static classification of organisms into pigeon holes
for easy reference; but this should reflect a dynamic history of
common descent = phylogeny. A systematic solution to the problem
of diversity incorporates both of these goals, but this result is not always
Terminology: Taxon (taxa) = a group of organisms of any taxonomic rank that is sufficiently distinct to be worthy of being assigned to a definite category. Category= rank or level in hierarchic classification. Taxa = robin, thrushes, songbirds, birds, vertebrates, animals
Categories = species, family, suborder, class, subphylum, kingdom
How do you classify? Historically: Downward classification by
logical division. Analogous to "20 questions". In Aristotle's
time things were either animals or plants. One could start by asking oneself:
is this an animal or a plant? Does this have feathers or not? and so on
down until it was properly placed in its category.
Linnaeus believed in the reality of the genus. He used downward classification
through his Linnaean hierarchy (kingdom, phylum, class, order, family,
genus, species [recall: King Philip Came Over From Germany Speaking]) to
reach the genus and then make the final division into the appropriate species.
This approach lead to the Binomial nomenclature: Genus + species:
Homo sapiens, etc.
This methodology gave way to Upward classification by empirical
grouping. It became apparent that the groupings of Linnaeus were not Natural.
The bottom of a downward classification process often lead to groupings
where members had clearly the wrong affinity.
Darwin's discovery forced the thinking towards Descent from a common
ancestor. It became apparent that these were the Natural groups
that had been sought. What ultimately is the basis for upward classification?
Characters. Can take varied forms: morphology, chemistry, behavior,
ecology, physiology all could provide good characters.
Characters have character states: we all have hair, but our hair
is different color; we all have eyes but our eyes are different color.
Character states may vary together in Character complexes, or they
may vary independently = Mosaic evolution of characters. Skin, eye
and hair color all vary together in humans. Is this three characters or
one (pigmentation)? Morphological and molecular characters may not evolve
together, in a mosaic fashion (reading in section next week). Different
characters may suggest different patterns of relationships (see fig. 14.3,
pg. 377), again an example of mosaic evolution.
Homologous characters = characters sharing a common genetic and
developmental history. Ancestor and descendant are linked by intermediate
forms having the same character. These characters are the basis of determining
a true phylogeny
Analogous characters = homoplasious characters: two characters
not sharing a common genetic and developmental history and usually
attained by adaptation to a similar ecological or functional challenge.
Bats and Birds forelimbs and wings: they are homologous as forelimbs,
but analogous as wings (a simple but crucial distinction).
Analogous characters are attained by convergent evolution where descendants resemble each other more than they do their respective ancestors. Ichthyosaur, Fish, Porpoise; desert plants. Parallel evolution e.g. marsupials (M) and placentals (P). Ancestors are viewed as different but related. Point is: two possible evolutionary trees could be drawn:
(P,M) (P,M) (P,M) (P,M) or (M,M,M,M) (P,P,P,P)
If the characters that a set of organisms have could be either analogous
or homologous characters, the systematist is faced with several problems:
1) attempting to identify which are which, and 2) deciding whether (or
how) to perform character weighting. Excluding characters is an
extreme form of weighting (weight = 0). Placentals have a placenta, marsupials
a pouch where the immature young finish their development. These are major
characters, should they carry more weight in our assignment of relationship.
If we looked for other characters in the animals we could probably find
many that would link the dog-dog, squirrel-squirrel, cat-cat, anteater-anteater,
etc. Since characters are the data we will use to do systematics other
questions arise: 1) should we use single or many characters?, 2) what are
legitimate characters (morphology, ecology, etc.)? 3) how do we weight
those that are chosen?
Analogous/homologous problem revolves around the distinction of the
similarity of characters with adaptive or genetic bases. This leads to
the distinction between grade and clade. Grade = level of
adaptation; organisms of similar grade due to similar adaptations due to
convergence (e.g., the "dog" grade or the "anteater"
grade that goes across marsupial/placental distinction). Clade = a
group descended from one common ancestor; a genetic lineage (e.g., the
placental clade vs. the marsupial clade).
There are different schools of systematics: different schools
place different emphasis on the goals of systematics. Some will emphasize
classification over phylogeny (grade over clade); another emphasizes phylogeny
over classification (clade over grade).
Phenetics (Numerical taxonomy) classification based on overall
similarity of organisms. Treat all characters of equal weight and amass
as many character as you can. Enter the characters into a computer that
runs an algorithm that gives you a number reflecting the degree of similarity
between different taxa. Assumes: homologous and analogous characters
will be in there together, but rate of character change is roughly proportional
to evolutionary distance and the homologous characters will carry the day.
Results plotted in a Phenogram showing evolutionary relationships.
See fig. 14.1, pg. 373, 14.4, pg. 378.
Cladistics (Phylogenetic systematics) Clade is everything. Define
a hierarchical series of dichotomous branching events reflecting ancestor-descendant
relationships. Seeks to identify monophyletic groups that, by definition,
are derived from a single common ancestor. Defines these groups as taxa
sharing derived characters (synapomorphies). Assumes that
speciation is dichotomous producing two sister taxa and that the
ancestral taxon disappears at the speciation event. See handout for examples
and terminology; see fig. 14.1, 14.2, 14.6, pg. 373, 376, 382.
Evolutionary systematics uses homologous characters but will
commonly weight characters differently depending on the "importance"
of the character. A good evolutionary systematist is one who "knows"
the group and can thus decide which characters to weight more heavily.
Criticized as being highly subjective and not scientific because decisions
are not testable hypotheses, but statements of faith about the importance
of the characters. Acknowledges grade as relevant to the study:
crocodiles and birds are different classes to evolutionary systematists,
but sister taxa to cladists.
Which approach do we use? Ideally a classification should be objective in that the criteria use to classify are not subject to the whim of the person doing the classifying. Objectivity is important if classification is to be a scientific endeavor: someone else ought to be able to step in and repeat your "experiment" in classification. Moreover a classification should be natural and not artificial so that if a set of characters were used to assign relationships, these relationships should also be apparent in other characters not used in the analysis. There are natural groups that have been generated during the history of life and systematists should attempt to discover these groups. In recent years cladistics has become the dominant school of systematics as it meets these two criteria well. However, phenetics is still very active and character weighting is still being used. Note that natural groups might generate many more hierarchical levels than the classical Linnaean hierarchy (see fig. 14.8, 14.9, pg. 386-387).
Terminology (See fig. 14.6, pg. 382 and note the different terms used for synapomorphy, symplesiomorphy and that analogy = homoplasy).
Monophyletic - referring to a group of taxa descended from a single common ancestor (e. g. angiosperms or seed plants)
Apomorphic - a derived character (seeds in angiosperms and gymnosperms relative to ferns)
Plesiomorphic - an ancestral character (stomata in angiosperms and gymnosperms)
note: apo and plesiomorphic are relative terms: vascular tissue is apomorphic to the monophyletic group above the bryophytes, but plesiomorphic to the angiosperms)
Syn - shared Aut - "self", unique to a group
synapomorphies are shared derived characters and are what define monophyletic groups because the members of that group have the character because they are descended from a common ancestor (seeds in angiosperms and gymnosperms)
symplesiomorphies are shared ancestral characters (chlorophyll in the angiosperms and gymnosperms)
autapomorphies are derived characters unique to one group (flowers in angiosperms)
autplesiomorphies can't exist, by definition
Paraphyletic group includes some but not all of the descendants of a common ancestor. Incomplete grouping based on symplesiomorphies (e.g. the non-natural group of ferns and gymnosperms based on the presence of chlorophyll and stomata; angios have these but are not in the group)
Polarity distinguishes between the plesiomorphous and apomorphous state of a character by comparison to an outgroup (a taxon know to lie outside the hierarchy of the groups being considered, e.g., the outgroup algae determines the polarity of the evolution of seeds and secondary growth with respect to the other taxa)
Sister taxa are the two lineages that descend from a common ancestor following a splitting event; can be considered at any level of hierarchy in a cladogram