THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

15. How We Name Living Things

15.6. The Kingdoms of Life

Classification systems have gone through their own evolution of sorts, as illustrated in figure 15.9. The earliest classification systems recognized only two kingdoms of living things: animals, shown in blue in figure 15.9a, and plants, shown in green. But as biologists discovered microorganisms (the yellow-colored boxes in figure 15.9h) and learned more about other organisms like the protists (in teal) and the fungi (in light brown), they added kingdoms in recognition of fundamental differences. Most biologists now use a six-kingdom system (indicated by the six different-colored boxes in figure 15.9c) first proposed by Carl Woese of the University of Illinois.

Figure 15.9. Different approaches to classifying living organisms.

(a) Linnaeus popularized a two-kingdom approach, in which the fungi and the photosynthetic protists were classified as plants and the nonphotosynthetic protists as animals; when prokaryotes were described, they too were considered plants. (b) Whittaker in 1969 proposed a five-kingdom system that soon became widely accepted. (c) Woese has championed splitting the prokaryotes into two kingdoms for a total of six kingdoms or even assigning them separate domains, with a third domain containing the four eukaryotic kingdoms (d).

In this system, four kingdoms consist of eukaryotic organisms. The two most familiar kingdoms, Animalia and Plantae, contain only organisms that are multicellular during most of their life cycle. These groups of animals and plants are no doubt familiar to you. The kingdom Fungi contains multicellular forms, such as mushrooms and molds, and single-celled yeasts, which are thought to have multicellular ancestors. Fundamental differences divide these three kingdoms. Plants are mainly stationary, but some have motile sperm; fungi have no motile cells; animals are mainly motile. Animals ingest their food, plants manufacture it, and fungi digest it by means of secreted extracellular enzymes. Each of these kingdoms probably evolved from a different single-celled ancestor.

The large number of unicellular eukaryotes are arbitrarily grouped into a single kingdom called Protista (see chapter 17). They include the algae and many kinds of microscopic aquatic organisms. This kingdom is an artificial group in that many of these organisms are only distantly related, and the classification of the protists is in flux.

The remaining two kingdoms, Archaea and Bacteria, consist of prokaryotic organisms, which are vastly different from all other living things (see chapter 16). The prokaryotes with which you are most familiar, those that cause disease or are used in industry, are members of the kingdom Bacteria. Archaea are a diverse group including the methanogens and extreme thermophiles, and they differ greatly from bacteria in many ways. The characteristics of these six kingdoms are presented in table 15.1.

TABLE 15.1. CHARACTERISTICS OF THE SIX KINGDOMS

Domains

As biologists have learned more about the archaea, it has become increasingly clear that this ancient group is very different from all other organisms. When the full genomic DNA sequences of an archaean and a bacterium were first compared in 1996, the differences proved striking. Archaea are as different from bacteria as bacteria are from eukaryotes. Recognizing this, biologists have in recent years adopted a taxonomic level higher than kingdom that recognizes three domains (figure 15.9d). Bacteria (yellow-colored box) are in one domain, archaea (red-colored box) are in a second, and eukaryotes (the four purple boxes representing the four eukaryotic kingdoms) in the third. While the domain Eukarya contains four kingdoms of organisms, the domains Bacteria and Archaea contain only one kingdom in each. Because of this, the kingdom level of classification for Bacteria and Archaea is now often omitted, biologists using just their domain and phyla names.

Key Learning Outcome 15.6. Living organisms are grouped into three categories called domains. One of the domains, Eukarya, is divided into four kingdoms: Protista, Fungi, Plantae, and Animalia.