One is known as evolutionary systematics or simply the classic or traditional method because it is the oldest of the two used.
It attempts to show relationships that depict the lineage or history of descent of a particular group.
It uses as its basic unit or taxa the species. Each species has a name consisting of two words. Note the first word in the examples below are capitalized and the entire name is italicized or underlined.
Canis lupus
Panthera pardus
Eublepharis macularius
Groups form a collective hierarchy (nested grouping) from species to domain. Each higher or larger unit contains one or more groups from lower level.
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Canidae
Subfamily: Caninae
Tribe: Canini
Genus: Canis
Species: C. lupus
Taxa distinctions are made following the basic tenet "Taxonomy should reflect phylogeny (evolutionary history=relatedness)." Two species in one taxa at any level must be more closely related (share more in common from descent) to each other than to species in other taxa at the same level.
So, recognizing and using characters that are homologous is important.
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Homologous: Structurally similar due to common ancestry. Having the same evolutionary origin but not necessarily the same function.
Example: vertebrate limbs.
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from (http://evolution.berkeley.edu)
as opposed to analogous because of convergent evolution. Analogous structures have separate evolutionary origins.
Visit this webpage https://evolution.berkeley.edu/evolibrary/article/evo_09
Be able to explain for the diagram below why bat wings and bird wings are examples of convergent evolution or analogous and not homologous.
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But remember scientists can construct taxa following homology even if some are older, or some are larger and contain more species than equivalent taxa.
There are more differences between some taxa of equal standing in traditional or evolutionary systematics than accepted by the next school of classification we will discuss. .
Example: The Vertebrate classes recognized in evolutionary systematics, such as Amphibia, Reptilia, Aves and Mammalia.
1. Aves (birds) and Mammalia are younger classes than Amphibia and Reptilia. Both birds and mammals descended from closely related ancestors yet there are more differences between mammals and some modern reptiles than birds and some modern reptiles
2. The platypus could be classified as a mammal or reptile. It has hair and mammary glands that are simple slits. It lays eggs and the skeleton, if only known from fossil limbs and vertebrae, would appear reptilian.
3. Should Chordata be a phylum, given the small number of species in it? Arthropoda contains about 1.25 million described species, Chordata contains only 43,000 species.
So one does not know by simply looking at a tree in evolutionary or traditional systematics whether branching arrangements are based mainly on certain characters or the ancestor shared.
It is this lack of limits, and so in some sense predictive and hypothesis testing value of the trees constructed, via evolutionary systematics, that has led to the greater acceptance today of the second method or cladistics/phylogenetics.
The second school attempts to be more "scientific" and uses more rules in constructing groups, which it terms clades. It is formally known as the phylogenetics approach or more commonly as cladistics.
Phylogenetics or cladistics
This is now the accepted way to classify organisms. It is slowly replacing the evolutionary approach in textbooks. Cladistics is a type of systematics developed by Willi Hennig, who attempted to develop a more scientific method of classifying organisms. This scheme only uses one type of homology (derived) for construction of "clades". Molecular phylogenetic analysis was being at the same time practiced by biologists who were using DNA and/or RNA sequences to group animals but with the same objectives postulated by Hennig. The two approaches have merged for the most part in what is now know formally as phylogenetics or informally as cladistics. Most cladists in fact use molecular data to construct cladograms.
It uses strict monophyly as the only criterion for grouping related species. In cladistic taxonomy, evolution is seen as a process of progressive bifurcations of lineages. Every species, therefore, has a sister species whether recognizable or not, and this pair is derived from an ancestral species.
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Cladistics depends on identifying shared derived characters. or synapomorphies. Cladists then distinguish between different types of homologies.
A derived homology or synapomorphy is unique to a particular group of species (and their ancestor), while a shared ancestral homology is found in the ancestor of a group of species but only in some of its descendants.
One way of looking at synapomorphies is that they define nodes, groups of sister species and their closest relative. The very formal and rigorous way synapomorphies and nodes are used, make the trees drawn, true lineage trees.
In the diagram below the clades or groups are indicated at top, for example, Vertebrates and Tetrapods. The shared derived characters for a clade are indicated below the clade, so the tetrapod "clade" all have four limbs, but no other clade has this characteristic and it is a "new" or derived trait (not an ancestral trait or trait found in an remote ancestor to this clade). Again, each branch point is known as a node.
A more detailed tree
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A bit on terms used in the diagram above. You should be aware of vertebrate relationships. Please use the explanations below to learn more about the synapomorphies use to divide vertebrates into clades.
Archosaur--------Synapomorphies of archosaurs include
teeth set in sockets,
antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively) that make skull lighter,
and a fourth trochanter (a prominent ridge on the femur) which provides more surface for muscle attachment.
Anapsid (none), Synapsid (one) and Diapsid (two)
Synapsids contain one ancestral skull opening or temporal fenestrae
Diapsids contain their two ancestral skull openings (temporal fenestrae) posteriorly above and below the eye. This arrangement allows for the attachment of larger, stronger jaw muscles, and enables the jaw to open more widely
A better view of a diapsid skull, but note the fenestrae or holes in front of the eye of alligators and crocodiles have disappeared.
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Note: The classification scheme diagrammed revises what we believed formally about vertebrate evolution. Mammals did not evolve as portrayed in many popular movies and books from reptiles. Mammalia is an line as old as the line leading to reptiles.
An good example of why phylogenetics is considered more representative of "true" evolutionary relationships.
We will take as example the relationships between reptiles, more specifically dinosaurs, and birds. In traditional taxonomy, Aves, or birds, are considered a class equivalent to the class Reptilia. In Cladistics they are simple considered a type of extant reptile, because no synapomorphy divides them from the dinosaur line. In other words there were dinosaurs that had feathers, and the modifications to the limbs and skeleton that typify modern birds.
Visit these websites to learn more about the relationships between birds and dinosaurs.
In the top right corner of one of the websites you visited http://evolution.berkeley.edu/evolibrary/article/evograms_06,is a teaching resource on the evolution of flight.
Visit from there or the links below the following explorations in the teaching resource.
If you are not familiar with bird anatomy visit the first two modules
http://www.ucmp.berkeley.edu/education/explorations/reslab/flight/introduction/I1.htm
http://www.ucmp.berkeley.edu/education/explorations/reslab/flight/Skeletal/S1.htm
To answer questions on homework: http://www.ucmp.berkeley.edu/education/explorations/reslab/flight/index.htm
Please review modules 3-6.
The last three to four modules of this exercise on flight should convince you that what we consider dinosaurs had many of the adaptations we associate today with birds, and so in some sense, birds are just small feathered dinosaurs. There is no separation of birds into a separate clade in cladistics for this reason.
What if the group you are studying does not have a good fossil record?
In lieu of fossils, outgroups can be used. Outgroups are a group that branched from the ancestral group before the groups being classified branched from each other. Outgroups are used by many biologists who use molecular data to build trees because often they cannot determine which came first from an array of nucleotide sequences. Outgroups here can indicate the most primitive sequence and so can be used to "root" a tree.
Visit one of the websites below for more information not only on outgroups, but building trees in general.
http://evolution.berkeley.edu/evolibrary/article/phylogenetics_02 or http://www.ucmp.berkeley.edu/clad/clad2.html
Cautions on using textbooks in general during the transition between evolutionary systematics and cladistics.
Many textbooks superimpose the names of the evolutionary systematics (also known as the classic or traditional school) for taxa, although these do not formally exist in phylogenetics/cladistics. This right now just adds to the confusion, although in time it may help define what are know as crown clades or the larger groupings in the evolutionary school such as kingdoms, and phyla. That is if the cladistics school can get all involved to agree on which crown clades should be phyla, etc.
