Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 21

Taxonomy is the classification of life-forms into groups, or taxa. Taxonomic names reflect what we know about an organism's evolutionary relationships. The more closely related two species, the more taxa they share.
Taxonomic designations are based on similarities between species, including visible characteristics, behavior, microscopic features, and sequences of genes and proteins. Computers assist taxonomists by rapidly supplying information. Field work and museums are still important parts of taxonomy.
Taxonomy has changed as technology enabled biologists to discern more intricate details of cellular and molecular structure, anatomy and physiology, and behavior. The initial two-kingdom system that classified all organisms as plants or animals expanded to a three-kingdom plan recognizing fungi and then to various four-kingdom schemes, some acknowledging the great difference between prokaryotes and eukaryotes. The five-kingdom system, classifying life-forms as plants, animals, fungi, protista, and monera, takes both of these changes into account. Archaea are usually considered to be a part of Monera or a sixth kingdom.
A system proposing three domains divides life into Archaea, Bacteria, and Eukarya. The domain system contradicts the five-kingdom system because it includes a separate grouping for the archaea.
Taxa more restrictive than kingdom include division or phylum, class, order, family, genus, and species.
Taxonomy is subjective because biologists perceive different degrees of difference between types of organisms and may consider different classification criteria with different weights. Classification is confounded when an organism looks very different at different stages of its life cycle, when the sexes look very different, or if it takes part in a symbiotic relationship.
Taxonomists' attempts to number all species on earth are biased by observing only certain species. Biologists estimate the number of species by extrapolating from the percentage of new species in small samples.