CLASSIFYING ORGANISMS

Taxonomy is the science of naming/classifying living things.

300 BC Aristotle developed the first biological system of classification –> plants & animals
Why classify?
Aid to understanding and communicating information about organisms or groups of organisms.
Assumption – “related” organisms have a common ancestor.

Species

Basic unit of biological classification
Definition - members of the same species are able to mate naturally to produce fertile offspring
There are inherent problems with using this definition to classify organisms:
- “natural” mating depends on their being of a similar size - varieties of organisms that are obviously members of the same species are not necessarily able to mate e.g. Irish Wolfhound X miniature Fox Terrier
- organisms that reproduce asexually e.g. bacteria, viruses, some plants
- fossils
- plants – some form fertile hybrids with other species
A new definition is needed and will hopefully be developed based on similarities of DNA sequences
Linnaeus – binomial system of classification (nomenclature)

Levels of classification

Diagnostic features are those characteristics that distinguish one group from others.

Features Used To Classify Organisms

Similarities of structure
- Convergent evolution – adaptations of different organisms to a similar environment – producing similar structures rather than having a common ancestor
Differences of structure
- Divergent evolution – adaptations of similar organisms (with a common ancestor) to different environments, hence different structures
Internal as well as external structures are important
Homologous structures – common ancestor, not necessarily similar structures or functions
Analogous structures – similar appearance, similar function, but no evidence of being related
Analysis of blood and other proteins
- Provides clues to similarities in DNA & RNA
Embryology, early development, life cycles, parental involvement in “rearing” young

How many kingdoms?

(Learn Table 3.2 in Text p35)