Why Classify Living Organisms?
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 one knows that all members
of a particular group have a certain set of characteristics,
then there's a good bet that a new member of that group may also
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 in every language, and no confusion.
- Remember: Always use your Dictionary of Word Roots and Combining Forms by Donald Borror when you
encounter a scientific name or term you don't know. It's the best way to
learn not just the term you're looking up, but the word root for the future.
Taxonomy - the science of naming and classifying organisms
Systematics - the science of determining evolutionary
relationships among organisms.
(Most systematists are also taxonomists.)
A species' or other group's phylogeny is its evolutionary history.
A phylogeny can be represented with a diagram called a phylogenetic tree
You can learn much more about how to read phylogenetic trees at the UC Berkeley Museum of Paleontology.
Another fantastic resource showing a variety of very up-to-date phylogenetic trees can be found at the Tree of Life Web Project.
In the earliest studies of biodiversity
- classifications were subjective
- superficially similar organisms were placed together in the same group
- scientific names were long, cumbersome sentences
that described the organism.
In 1735, 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.
Example:
Oryctolagus cuniculus
Each species is nested in every taxonomic level above genus in ever more inclusive
groups:
DOMAIN - Kingdom - Phylum - Class - Order - Family - Genus (plural =
genera) - species
(Insert clever mnemonic device here)
For the rabbit, this would break down as...
Domain Eukarya
Kingdom Animalia
Phylum Chordata
Class Mammalia
Order Lagomorpha
Family Leporidae
Genus Oryctolagus
Species cuniculus
In general, scientific names have a specific meaning, usually describing
the species. For example,
Eleutherodactylus planirostris, the Greenhouse Frog:
- eleutheros is Greek for...
- dactyl is Greek for...
- plani is Greek for...
- rostris is Greek for...
(So what do you suppose this frog looks like?)
Many species are at least partially named after people, but these proper
names, too, must be Latinized:
Example: Chilomeniscus savagei is named after Dr. Jay Savage.
- 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 of the University of Miami).
Check out some entertaining scientific names.
What is a Taxon?
"Taxon" is a generic term used to describe a group of
organisms that has been classified together at any given taxonomic
level (Kingdom, order, family, etc.) without specifying that level.
A Taxon Has Three Aspects:
1. Its name
For example, the name of the taxon containing all domestic dogs is Canis familiaris. The order to which all dogs belong (along with a host of other flesh-eating mammals with specialized cutting teeth called carnassials) is Carnivora.
2. Its rank
For example, the taxon Mammalia is assigned the taxonomic rank of Class. The taxon's rank has no true biological significance. It serves only to help the biologist locate the taxon within the hierarchy.
The relative rank of a given organism within its larger and smaller groupings is more relevant than the rank itself, which is subject to change.
For example, it's important to know that all members of Felis (cats) are classified within the larger taxon "Carnivora," (carnivores) and that all carnivores are classified within the still larger taxon "Mammalia" (mammals).
It's less important to know that "Carnivora" is an order and "Mammalia," is a class (especially since those ranks can change with new data).
(In fact, many institutions now use rankless system in publication, in which taxa are described only by their name, with rank being tacitly understood. An author using this system will write "Mammalia" rather than "Class Mammalia" to avoid confusion as the ranks shift and change along with new information.)
3. Its content
All the students in this class are members of the genus Homo and the species Homo sapiens. This is the biologically relevant aspect of the taxon. By grouping specific individuals within a single species, related species within a single genus, related genera within a single family and so on, the systematist tells us which organisms are believed to be most closely related to one another.
Rules of Nomenclature
Whenever a scientific name has a gender (masculine or feminine), both
genus and species must have the same gender.
- Masculine ending: -us
- Feminine ending: -a
- ...and there are many endings which are neither masculine nor
feminine.
1. A taxon has only one correct name.
2. No two species or genera may have the same scientific name.
3. The name must be Latin or Latinized
Why should you care about systematics?
Here's an excellent overview of
the importance of phylogenetic
systematics from the University of California at Berkeley Museum of
Paleontology.
How Do Systematists Construct Phylogenies?
A taxon has dimensions in space (its geographical range) and time (its evolutionary history). It also has physical characteristics that help us tell it apart from other taxa. First, it's important to know whether a characteristic you're comparing in two species is actually comparable.
Homologous and Analogous Characters
A homologous character found in two different species
- may or may not perform the same function in those two species
- but is inherited from the same common ancestor of those two species
Example: The bones in the forelegs of various vertebrates.
An analogous structure found in two different species
- serves the same function in two species
- but is not derived from a common ancestral structure
Example: The wing of a butterfly and the wing of a bat.
How do we know the common ancestor of the butterfly and the bat didn't pass on the wings to both species? You'll have to trust me on this, but if you follow the embryo development of a butterfly and a bat, you'll see that their most recent common ancestor probably looked something like this:
(A little, microscopic thing called a gastrula.)
No eyes. No head. No wings.
The wings of the butterfly and the wings of the bat evolved independently, long after their ancestral lineages diverged.
Primitive and Derived Characters
A primitive character is one that is relatively unchanged from its
original, ancestral form. (Also called a plesiomorphy)
Example: All vertebrates have a bony tail posterior to the anus.
It was present in the ancestral vertebrate, and though different vertebrates have evolved differently shaped tails, they are all evolved from the common ancestral tail. The PRESENCE of the tail is primitive.
A derived character is one that is relatively modified from its
original, ancestral form. (Also called an apomorphy)
Example: the modified shapes of various vertebrate tails. An extreme example: The loss of the external tail in great apes (including humans). The baboon shows the primitive condition, whereas the gorilla and Very Handsome Man show the derived condition of the post-anal tail.
(We still have a tail; it's just very reduced and tucked where it doesn't show.)
Note that these are comparative terms. You can't say something is
"primitive" or "derived" without comparing it to something else. In systematics, you'll be comparing two related species, perhaps an ancestor and its descendant, or perhaps two related species.
Systematists use the existence of shared characters to help reveal their common ancestry.
- a symplesiomorphy is a shared, primitive character
- a synapomorphy is a shared, derived character
The more recently two species diverged from a common ancestor, the more
synapomorphies they will share.
Here's how this works, using our own family (Pongidae; Great Apes) and our own species (Homo sapiens) as an example.
Here are the hypothetical evolutionary relationships of apes.
- 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
called ASPM, which is believed by some to be involved in the formation of
the central nervous system (brain and spinal cord) in vertebrates.
You'll notice that there's an animal on there who's not an ape: The Owl Monkey.
We use this species because we know it has more primitive characteristics than the Great Apes we're studying, so we use it as an outgroup to find out which Great Apes are the most primitive. If some great apes share primitive characters with Owl Monkeys, but others have the same characters in highly derived form, we usually can say that the ones more similar to the Owl Monkeys are more closely related to Owl Monkeys, so they branch off first.
The more symplesiomorphies a taxon within your study group shares with the
outgroup, the more likely it is that it shares a more recent common ancestor
with that outgroup. This means it may be more primitive than other members of the group you're studying.
Only synapomorphies help us establish recency of common descent
among related organisms.
The more synapomorphies two groups exhibit, the more recent their common
ancestor. Let's see how this works by using our TAILS again.
A taxon's evolutionary history/relationships can be diagrammed with a
phylogenetic tree such as the one above. Each branch point represents a hypothetical ancestor of all the taxa above it on the tree.
Thus, you cannot correctly say that any species evolved from another species. You can say only that two species share a common ancestor.
Let's try this together. True or false:
"Humans evolved from monkeys."
What do you think?