Protists Characteristics – Introduction. Protists (Proto = Greek : first) are unicellular eukaryotes (eu = G : true; karyo = G : nuclei) that blur the distinction between animals and plants. Indeed, all algal, plant and animal life that is, all multicellular life must have evolved from protist ancestors. As with mitochondria and the other membrane bound organelles that characterize eukaryotes, chloroplasts have been acquired by many unicellular organisms through ancient symbiotic relationships. Thus, while some protists ingest solid food particles, others photosynthesize. Still other live amidst decaying plant and animal matter or live as parasites and feed by taking up dissolved organic material and other nutrients across their surface. Some species are capable of 2 or even all 3 nutritional modes either simultaneously or at different times. For this reason, about 140 years ago the great German scientist Ernst Haeckel suggest placing all of the troublesome organisms that weren’t clear plants or animals within a new kingkom, the Protista (protist = G : the very first). Eventually, the Protista came to include everything except plants, animals, fungi and bacteria. But the diversity of protist ultrastructure, life cycles, lifestyles and evolutionary trajectories proved to extreme for a single kingdom to support. Indeed, protists are now distributed among all kingkom. In consequence, in the classification scheme adopted for this edition there is no longer a formal taxonomic category called the protista, although “protist” still serve as a useful general term for referring to this remarkable collection of unicellular eukaryotes.
Invertebrates Classification – Why Determine Evolutionary Trees ? One goal of classification schemes is simply to facilitate discussions about different groups of animals, and ideally to arrange those groups in the correct evolutionary context. But knowing with certainty the precise pattern of evolutionary change that give rise to the present diversity of animal form would give us far more than a convenient and stable classification system. Finding one species of coral, for example, that produces a particular defensive compound of great biomedical potential, we might know which other species were most likely to synthesize related compounds. We would also be better able to understand the sequence of genetic changes involved in body plan evolution and would be able to tell with certainty how many times certain traits had evolved independently within any particular group of animals.
Invertebrates Classification – Classification by Evolutionary Relationship. Probably the most familiar classification scheme is the taxonomic framework established about 250 years ago (1758) by Carolus Linnaeus. The system is hierarchical, that is one category contain less inclusive groups, which in turn contain still less inclusive groups and so on :
Kingdom – Phylum – Class – Order – Family – Genus – Species
Many subgroups are superimposed upon this basic framework. One encounters among arthropods, for example, subclasses within classes, suborders within orders, infraorders within suborders, and even sections within infraorders, and families are grouped together within superfamilies. Any named group of organisms that is sufficiently distict to be assigned to such a category is called a taxon. The member of any given taxon show a high degree of similarity morphological, developmental, biochemical, genetic and sometimes behavioral and are presumed to be more closely related to each other than to the members of any other taxon at the same taxonomic level. The members of a particular order of snails, for example, are all presumed to have evoleved from a single ancestor that is not an ancestor of snail in the orders. Similarity, all the members of any particular phylum are presumed to have evolved from a single ancestral form. Such group, at every taxonomic level, are said to be monophyletic. Most modern workers ar now agree that all monophyletic groups must also include all descendants of originating ancestor. A group that does not do so is said to be paraphyletic. By this definition, the invertebrates form a paraphyletic group, since their vertebrate descendants are excluded.