A group of organisms in any of these categories can be generically referred to as a taxon, if you don't wish to specify which level of the hierarchy you're talking about, or if you're speaking in general terms. (For example, you might say, "Related taxa are descended from a recent common ancestor." without needing to specify which taxonomic level.)

The scientist who arranges organisms into these groups is engaging in TAXONOMY (from the Greek tax, meaning "arrangement" and nomy, "the science of"), and is known as a TAXONOMIST.

The scientist who studies the evolutionary relationships between organisms in these taxonomic groups is a BIOSYSTEMATIST (and does biosystematics).

Most Biosystematists are also Taxonomists.

• Genus is capitalized
• species is lower case
• the entire name is italicized (it's in a "foreign" language)(

Other familiar species:

• Canis familiaris
• Felis cattus
• Mus musculus
• Oryctolagus cuniculus

WHY DO WE CLASSIFY LIVING ORGANISMS INTO THIS TAXONOMIC HIERARCHY?

1. An aid to memory: It's easier to remember the characteristics of an entire group of organisms than to recall the individual characters of individual organisms over and over!

2. An aid to prediction: If you know that all members of a particular taxon have a particular set of characteristics, then there's a good bet that a *new* member of that taxon may have some of those (useful?) characteristics, too. (such as antibiotic production, edibility, etc.)

3. An aid to explaining evolutionary relationships

4. Provides a stable, relatively unchanging system of INTERNATIONALLY RECOGNIZABLE NAMES.

Why bother with long, Latinized scientific names?

• Common names can be confusing! Many names in different languages exist for the same organism, and many different organisms share the same common name.

• With Latinized scientific nomenclature, there's one species, one name and NO CONFUSION.

In the earliest studies of biodiversity

• classifications were based on subjective logic

• system was to order "like" organisms which reflected a "natural order" (kosmos)

• scientific names were long, cumbersome sentences that described the organism.

1758 - Swedish botanist Carl Linne (a.k.a. Linnaeus) published Systema naturae,outlining a new system of binomial nomenclature. It's still in use today.

In the Linnaean system, every scientific name consists of an organism's Genus and species, the names of which are always GREEK, LATIN or LATINIZED versions of other languages or terms. (In fact, Linne even re-named himself Carollus Linnaeus, to "underline" his insistence that all scientific names be Latinized. Today, we all remember him as Linnaeus, the Father of Modern Taxonomy.)

In most cases, the name is descriptive of the organism. Example: Oryctolagus cuniculus - In Greek, orycto means "digger"; lago means "hare"; cunicul means "underground passage". The Greek words are given Latin endings to finish the job: a rabbit is a hare-shaped creature that digs underground tunnels.

Eleutherodactylus planirostris, the Greenhouse Frog:

• eleutheros is Greek for...

• dactyl is Greek for...

• plani is Greek for...

• rostris is Greek for...

Many species are at least partially named after people, but these proper names, too, must be Latinized:

Example: Chilomeniscus savagei
• chilo is Greek for...

• meniscus is Greek for...

• And "savagei" is the Latinization of the last name of Dr. Jay Savage, eminent herpetologist (and professor emeritus at the University of Miami).

Let's look at an example...

Early 1900's - sporadic, localized outbreaks of malaria (caused by
• Plasmodium, a protozoan blood parasite, and transmitted by a vector species, female mosquitoes of the genus Anopheles) were believed to be caused by Anopheles maculipennis. Puzzling: This was a widespread species. Why were outbreaks so localized?

• Work by two systematists (Hackett in 1937 and Bates in 1940) revealed that "A. maculipennis" was actually several related species, each of which had a distinct ecology, diurnal periodicity and habitat.

• Knowing this, those in charge of eradication efforts were able to specifically target the responsible species and wipe out the problem.

Another example:

Let's illustrate this with the "traditional" primate families Hylobatidae (Gibbons), Pongidae (Great Apes) and Hominidae (Humans).

• Gibbons (Hylobates spp.)
• Orangutans (Pongo pygmaeus)
• Gorillas (Gorilla gorilla)
• Chimpanzees (Pan troglodytes)
• Bonobos (Pan paniscus)
• Humans (Homo sapiens)

This phylogenetic tree (from Human Molecular Genetics - Evans, 2004) is based upon amino acid sequences in a protein known as ASPM, which is believed by some to be involved in the formation of the central nervous system (brain and spinal cord) in vertebrates.

• K_s = frequency of synonymous changes in the gene (mutations that don't change the amino acid sequence of the protein)
• K_a = frequency of nonsynonymous substitutions in the gene (mutations that do cause a change the amino acid sequence of the protein)

A high K_a/K_s ratio means that the number of amino acid changes in that lineage is greater than would be predicted by random chance. This suggests that the protein sequence has been under selective pressure, though no one knows for certain (yet) what ASPM does.

SYMPLESIOMORPHIES help us establish that a study group shares characters with a hypothetical ancestor, but only the SYNAPOMORPHIES exhibited by each group tell us about how closely related they are to each other, and allow us to classify them into smaller, less inclusive taxa.

The more synapomorphies two groups exhibit, the more recent their common ancestor. For example, if the phylogenetic tree of the Great Apes above is correct, you can surmise that humans and chimpanzees exhibit more synapomorphies (shared, derived characters) than do humans and gorillas. Similarly, Gorillas, chimpanzees, and humans exhibit more synapomorphies (as a group) than do Gibbons and humans.

Any taxon's evolutionary history and phylogenetic relationships can be diagrammed with a phylogenetic tree such as the one we saw on the first day of class...

Modern Systematics is done using the Cladistic System devised by Willi Hennig. (clad is Greek for "branch")

In this system...

• all taxa must be monophyletic. That is, all members must share a common ancestor, and all descendants of a particular common ancestor must be included.
• only derived characters are informative in determining evolutionary relationships.
  Let's revisit the Great Ape Cladogram to discuss this:
  

• The more derived characters two taxa share, the more recent their common ancestry.

• The branching of a single, ancestral taxon into two new taxa is known as CLADOGENESIS.

• degree of specialization after a branch point gives no further useful information in terms of evolutionary relationship to other taxa.

• All cladogenesis now taking place is at the species level.
  Let's think about this for a moment:
  Example: What if Homo sapiens were to give rise to a new species that could no longer breed back with the original Homo sapiens individuals. Let's consider the taxonomic grouping of such a new species....

• The Cladist uses shared, derived characters to devise a phylogenetic tree that is (hoped to be) based on recency of descent from a common ancestor.

• Such a phylogenetic tree is called a CLADOGRAM.

• More than one cladogram may be consistent with the data. In this case the systematist chooses the most PARSIMONIOUS (i.e., the simplest; the tree with the fewest steps, which is thus the simplest explanation of the relationships) to be the "working hypothesis."

• This doesn't mean that the cladist believes that evolution is always parsimonious. But until more data become available, the simplest explanation is the model of choice.