The kingdom protista consists of a vast assortment of primarily aquatic eukaryotic organisms whose diverse body forms, types of reproduction, modes of nutrition and lifestyles make them difficult to characterize. Basically, this kingdom is defined by exclusion i.e., all members have characteristics that exclude them from the other four kingdoms.
All protists are eukaryotic and have evolved from prokaryotes. Another reason for creating a separate kingdom arises from the difficulty in placing certain eukaryotic organisms in the appropriate kingdom. This difficulty is a consequence of the fact that the other eukaryotic kingdoms have their evolutionary origin in kingdom protista. The other eukaryotic kingdoms Plantae, Fungi, and Animalia arose from protists in various ways.
The protists are unicellular, colonial or simple multi cellular organisms that possess a eukaryotic cell organization. Eukaryotic cells, the unifying feature of protists, are common to complex multi-cellular organisms belonging to the three eukaryotic kingdoms (Fungi, Plantae and Animalia) but clearly differentiate protists from members of the prokaryotic kingdom (Monera). Unlike plants and animals. However, protists do not develop from a blastula or an embryo.
The kingdom protista contains four major groups of eukaryotic organisms which are: single celled protozans, unicellular algae, multicellular algae, slime molds and comycotes.
In 1861, Jhon Hogg proposed the kingdom protoctista for microscopic organisms. In 1866, Ernst Haeckel suggested creating the Kingdom Protista to include bacteria and other microorganisms (such as Euglena) that did not fit into plant or animal kingdom. He, however, separated blue green algae and bacteria (prokaryotes) from nucleated protists and placed them in a separate group he called Monera, within the kingdom Protista.
In 1938, Herbert Copeland elevated the prokaryotes to kingdom status, thus separating them from protista. In five kingdom system of Robert Whittaker (1969) only unicellular eukarayotes were placed in kingdom Protista. Currently this kingdom also includes colonial and simple multicellular eukaryotes as well. Margulis and Schwartz (1988) modified the five kingdom system. Protista or Protoctista is one of the five kingdoms.
Diversity Among Protista
During the course of evolutionary history, organisms in the kingdom protista have evolved diversity in their (a) size and structure, (b) means of locomotion, (c) ways of obtaining nutrients, (d) interactions with other organisms, (e) habitat and (f) modes of reproduction. Diversity is exhibited by all of the major protest groups (fig. 7.1).
Based on the diversity, most biologists regard the protest kingdom as a polyphyletic group of organisms; that is, the protists probably do not share a single common ancestor. Margulis and Schwartz have listed 27 phyla to accommodate this diverse assemblage of organism.