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Species diversity: the beetle collection at the Natural History Museum

Organismal Diversity

Organismal (or taxic) diversity refers to the diversity within and between taxa.

• How are organisms classified in an hierachical way?

The species is the level of biological organisation which is the most easy to conceptualise and thus the most commonly used when discussing organismal diversity.

• What is a species?

However, although it is still the most convenient way of evaluating biodiversity,species richness should not simply be regarded as a synonym for biodiversity.

Organismal diversity can be studied at taxonomic levels higher than that of the species.

• How does diversity vary across taxonomic levels?

Levels of Organismal Variation

The elements of organismal diversity (taxa) can be ordered in a hierarchical way, in a variety of different classification schemes:

• domain
• kingdom
• phylum / division
• class
• order
• family
• genus
• species.

Five kingdoms and three domains classifications

Classification Schemes

The five kingdoms classification scheme [Whittaker & Margulis 1978] is based on three main criteria: cell type, level of organisation, and nutritional type.

Whittaker and Margulis hoped that their five kingdoms scheme would stimulate the study of the protists.

The three domains classification scheme [Woese et al. 1990] is considered to be a more natural scheme, based on comparative ribosomal RNA sequences.

Molecular systematics has revolutionised the study of the evolutionary relationships of microorganisms [Olsen et al. 1994] .

What is a Species?

The biological species concept is the most widely used definition.

This states:

• species are groups of interbreeding or potentially interbreeding natural populations that are reproductively isolated from other such groups [Mayr 1942] .

However, this is far from the only definition that is available.

List the pros and cons of the following species concepts:

• Biological Species Concept, [Mayr 1942] , [Dobzhansky 1950] ;
• Ecological Species Concept;
• Morphospecies Concept; [Mayr 1992]
• Phyogenetic Species Concept [Nixon & Wheeler 1990] ;
• Evolutionary Species Concept [Wiley 1981] .

Use [Quicke 1993] , [Ridley 1993] (chapter 15) or [Groombridge 1992] to help you.

Marine vs. terrestrial realms

How do marine and terrestrial realms compare in their organismal diversity?

Marine realms have many more animal phyla, but far fewer species, than terrestrial realms.

This clearly shows the point that any comparisons of diversity depend on what taxonomic level you examine.

The species-scape

Diversity Across Taxa

• Are your views of organismal diversity influenced by the everyday animals and plants you see?

If so, look at the species-scape of [Wheeler 1990] , which is a way of representing the size of organisms in proportion to the number of known species in each group.

This shows the importance of insect and non-insect arthropods, fungi and nematodes in terms ofspecies numbers.

• Aremicroorganisms of importance in biodiversity?