Today's dodos ?

(figures 2a and b The only examples we have of the existence of the dodo)

Species diversity refers to the variety of living organisms on earth and has been variously estimated to be between 5 and 50 million or more, though only about 1.4 - 1.8 million have actually been described. Biologists classify life on earth into a widely accepted system that reflects evolutionary relationships among organisms. In ascending order, the main categories or taxa, of living things are:

• Species,
• Genus,
• Family,
• Order,
• Class,
• Phylum,
• Kingdom

Humans, for example, are classified as follows:

(Kingdom), Animalia
(Phylum), Chordata
(Class), Mammalia
(Order), Primates
(Family), Hominidae
(Genus), Homo
(Species). sapiens

These last two descriptions are referred to as the Latin binomial, are used to identify an organism, and distinguish it from any other.

Most biologists recognise five kingdoms of organisms:

• Prokaryotae(bacteria),
• Protoctista(includes algae and protozoans),
• Fungi(mushrooms, molds, and lichens),
• Animalia(animals), and
• Plantae(plants).

Relative Number of Species by Major Taxa

The following table indicates the relative number of species by major taxa according to species type and number of described species. The sample is not thought to be particularly representative of true species distribution. This is due to a number of factors; Vertebrates are more widely studied than invertebrates, most biologists have tended to come from North America and Europe, and most studies are centred on land species ignoring the fact that 70% of the earth is covered by oceans.