Classification of Living Things

Taxonomy - The science of classifying and naming organisms.

Throughout time, scientists have puzzled over how to group organisms. . .

•Do you put insects, birds, and bats together because they fly?

•How about putting squid, whales and fish together because they swim?

It was during Renaissance times that people started to categorize creatures according to their similarities.  The “best” system was developed by Carolus Linnaeus around 1750.  This system has been modified some, but is still in use.  Linnaeus did not know about evolution, and that concept is now incorporated in the classification system.

Linneaus developed a binomial system of nomenclature - two part names for organisms.  Organisms are identified by their Genus species name. 

The names are given in Latin, which is an international language that is no longer in use.  This language can provide very specific names.  It removes ambiguity.

Canis familiaris - is the scientific name for domestic dogs of all breeds in all parts of the world.

TAXONOMIC CLASSIFICATION IS HIERARCHIAL…

•Domain                                  Eukaryotic

•Kingdom                                Animalia

•Phylum (or Division)             Chordata

•Subphylum                             Vertebrata

•Class                                      Mammalia

•Order                                     Primate

•Family                                   Hominidae

•Genus                                    Homo

•Species                                   sapiens

When working with a classification system, some definitions are in order. . .

• A Taxon is a particular taxonomic grouping (e.g. Chordata, Mammalia)
• Subspecies (varieties, strains) - These are geographically distinct populations within a species often displaying certain consistent characteristics that distinguish them from other populations of the same species.  Subspecies are capable of interbreeding, however if they are reproductively isolated they may, in time, become clear-cut species.

Splitters and Lumpers. . .

Some organisms are easy to classify, others are difficult.

“Lumpers” are taxonomists that group organisms into already existing units; recognize 10 animal phyla and 4 plant divisions.

“Splitters” - taxonomists that establish separate categories for organisms that do not fall naturally into an existing classification; recognize up to 33 animal phyla and up to 12 plant divisions.

The Domains - taxonomic categories larger than Kingdoms.

•Prokaryota - Kingdom Monera (bacteria and cyanobacteria); lack a nucleus and membranous organelles.

•Eukaryota - Kingdom Protista, Fungi, Plantae, and Animalia; have cells with distinct nucleus and membranous organelles.

•At this time, biologists recognize five Kingdoms.

The system most commonly used is Whitaker’s Five Kingdom system.

Systematics is the study of evolutionary relationships - phylogeny.

Monophyletic taxon - this is a taxonomic grouping where all of the subgroups have a common ancestry.

A Clade is a monophyletic taxon containing all species descended from the common ancestor.

Polyphyletic taxon is a taxonomic group in which the subgroups do not share a common ancestry.

Example - mammals are thought to have evolved from at least three different groups of Triassic reptiles.

Determining phyletic taxon. . .

• Homologous structures imply divergent evolution from a common ancestor. 

The relationship of organisms is based on the extent of similarity between living species and on the fossil record.

Homologous structures are important in determining similarity; they are a result of divergent evolution.

(Analogous structures result from convergent evolution and are not as useful when determining taxa.)

Primitive and Derived characters. . .

• Primitive characters are common to a group of organisms, have remained essentially unchanged and indicate a remote common ancestry.
• Derived characters are those that have evolved more recently and indicate a more recent common ancestry.

Example:  Three bones in middle ear

• not all chordates have them so they are a derived character in chordates.
• All mammals have these three bones - so they are a primitive character in mammals.

Primitive and Derived are relative terms, in other words - a feature viewed as a derived character in a large group may be seen as a primitive character in a smaller taxon.

Choosing taxonomic criteria. . .

•Organisms are not grouped together that share analogous adaptations.

•Organisms are grouped together that share derived characteristics.

•Shared primitive characteristics serve as a basis for classification and indicate a common ancestry.

•Deciding the appropriate weights for various traits in determining taxonomic categories is not always simple.

•Organisms are usually classified by a combination of traits.

•DNA analysis is becoming an important component for classifying organisms.

•Taxonomy is a dynamic science.

Molecular Biology as a taxonomic tool. . .

•DNA evolution

•DNA evolves at a constant rate enabling biologists to use specific genes as molecular clocks.

•The number of differences in nucleotide sequences in two groups of organisms reflects the time since the groups branched off from a common ancestor.

•Protein evolution

•A given protein evolves at a constant rate enabling biologists to use specific proteins as molecular clocks.

•The degree of difference in amino acid sequence reflects the time that has passed since the groups diverged.

•Protein Similarities. . .

•These can be determined by serological techniques that involve the immunological comparison of proteins.  (The rabbit injected with rat blood example.)

•The amino acid sequencing techniques are more reliable.

•DNA similarities. . .

•Among related species, the DNA sequences of the same structural genes are very similar.

•Detailed restriction maps within large homologous regions of chromosomes are also very similar.

•The genome of mammals consists of thousands of copies of alu-DNA; differences in alu-DNA, even between closely related species are thought to reflect evolutionary changes.

•Hybridization techniques are used to compare DNA from different organisms.

There are three main approaches to taxonomy - phenetics, cladistics, and classical evolutionary taxonomy.

•The phenetic system is a numerical taxonomy in which organisms are grouped according to the number of phenotypic characteristics they share.

•Both homologous and analogous adaptations are computed.

•Evolutionary history is not reconstructed.

•Least popular method because it does not work very well.

•The Cladistic approach emphasizes phylogeny; focusing on how long ago one group branched off from another.  The taxa are monophyletic.

•This approach involves the construction of a diagram called a cladogram.

•The cladogram resembles a tree with organisms branching off at respective times delineating between primitive and derived characters.

•There are some problems with accuracy with this method.  There is evidence that some branches of cladograms may be anecdotal examples.

•(Cladistics would classify birds with crocodiles due to common ancestry).

•Classical Evolutionary Taxonomy is the most widely accepted taxonomic system of phylogenetic classification.

•This system is based on evolutionary relationships and the extent of divergence since branching occurred.

•Organisms are classified in the same category according to their shared characteristics only if those traits are derived from a demonstrable common ancestor.

•The significance of the adaptations possessed by related organisms is also considered.

•The classical taxonomist is more of a splitter than a lumper.

•(The classical taxonomist would separate birds “warm blooded, etc.” from reptiles).