Invertebrates Classification 4 – Evolutionary Relationship

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.

Invertebrates Classification – Phylum

Invertebrates Classification – Phylum is gererally the highest taxonomic level that will concern us in this text. Invertebrate animals are presently distributed among at least 23 phyla, each representing a unique body plan, and unicellular invertebrates (protest) are distributed among still more phyla. Based on existing fossil evidence, no new phylum-level body plans have arisen in the past 600 million years, despite substantial radiation following each of the 5 major and about 10 smaller extinctions that took place during that time. In the most devastating extinction event to date, 251 million years ago at the Permian – Triassic boundary, nearly 95% of extinction species – level animal diversity was lost. In the subsequent 250 million years many new species evolved, often representing new orders and classes, but no new phylum – level body plans seem to have appeared. It is possible, of course, that some groups with no fossil record are of more recent origin.

The category of species has particular biological significance, although a single, precise, functional definition has not been found. Theoretically, the members of one species are reproductively isolated from members of all other species. The species, therefore, forms a pool of genetic material that only members of that species have access to and that isolated from the gene pool of all other species. The scientific name of a species is binomial (has two parts) : the generic name and the specific name. The generic and specific names are usually italicized in print and underlined in writing. The generic name begis with a capital letter, but the specific name does not. For example, the proper scientific name for one of the common shallow–water marine snails fond off Cape Cod, Massachusetts, is Crepidula fornicata. Related species are Crepidula plana and Crepidula convexa. Once the generic name is spelled out, it may be abbreviated when used subsequently, as long as no confusion result (for example, Crepidula and Conus, neither genus name can be abbreviated as “C.”) Thus, Crepidula fornicate, C. plana and C. convexa are common shallow – water marine gastropods found near Woods Hole, Massachusetts. They all belong to the phylum Mollusca and are contained within the class Gastropoda, family Calyptraeidae. The family Calyptraeidae contains other genera besides Crepidula; the class Gastropoda contains other families besides the Calyptraeidae; and the phylum Mollusca contains other classes besides the Gastropoda. The taxonomic classification system is indeed hierarchical.

The name of the person who first described the organism often follows the species name. It is capitalized, but not italicized. A barnacle common along the coast of the southeastern United States, for example, is Balanus Amphitrite Darwin, first described by Charles Darwin. Linnaeus’s name is often abbreviated as L., since he is associated with the descriptions of so many species. If the organism was originally described as being in a different genus than the one in which it is currently placed, the describer’s name is enclosed within parentheses. Thus, the snail Ilyanassa obsolete (Say) was described by a man name Say, who originally assigned the species to another genus (the genus Nassa). This snail was later determined to be sufficiently dissimilar from other members of the genus Nassa to warrant its assignment to a different genus. Occasionally, a person’s name is followed by a date, identifying the year in which the species was first described.

Invertebrates Classification – Inferring Evolutionary Relationships

Invertebrates Classification – An ideal taxonomic classification scheme reflects degrees of phylogenetic relatedness; that is all members of given taxonomic group should have descended from a single ancestral species and thus be more closely related to each other than to the members of any other group. Biologists have long made logical, reasoned guesses about the origin of various animal groups, based upon detaile studies of developmental patterns, studies of morphological and biochemical characteristics, and careful examination of animals preserved in the fossil record. Comparative molecular analyses of protein structure and of DNA and ribosomal RNA (rRNA) sequences among species have altered some of these views substantially. The difficulty concerns the relative importance of phenotypic similarities among taxa, phenotypic differences among taxa and the degree to which one is willing to admit (and deal with the fact) that phenotype may be a very misleading indicator of underlying genetic similarities and differences. Though the process of convergence, distantly related animals may come to resemble each other rather closely. Features that resemble each other though convergence are refferto as analogous, as opposed to homologous. For example, the eye of an octopus (a cephalopod mollusc) is remarkably like that of a human, but these visual organs are believed to be analogues, not homologues and not indicate any close evolutionary relationship between vertebrates and molluscs.

Moreover, in the evolutionary process, structures sometimes become less complex rather than more complex. Suppose, for example, you discover a new species of wing – less insect. How can you tell wether this species evolved before insect wings evolved or wether it instead descended from a winged ancestor and lost the wings over time ? It is often very difficult to determine which of 2 character states is the original (primitive, or plesiomorphic) condition and which is the advanced (derived, or apomorphic) condition. Until very resently, evolutionary relationships have been deduced entirely through anatomical and ultrastructural studies, whith phenotypes serving as reflections of the underlying genotypes. During the past 20 years or so, however, biochemical and molecular studies have allowed us to examine genotypic diversity directly. Particularly remarkable are recent interspecific comparison of nucleotide sequences of genes coding for rRNA, comparisons made feasible through development of the polymerase chain reaction (PCR) in the mid – 1980s. The PCR permits biologists to very quickly and inexpensively generate many copies of specific DNA sequences : a billion copies of a single DNA molecule can be obtained in a few hours producing sufficient material for analysis.

Invertebrates Classification – The Molecular Studies

Invertebrates Classification – Molecular studies often produce some remarkable and surprising results, results that differ considerably from those of earlier, organismal studies. These results are frequently controversial; in some cases, there is considerable disagreement among workers about the procedures used to prepare and analyze the data, and about how the results of molecular studies should be interpreted. But even before molecular Biologists joined the fray, proposed phylogenetic relationships were controversial. A variety of phylogenetic trees have been proposed over the years. None of the proposed schemes represents idle speculation; all reflect hard work and detailed and careful reasoning. The oldest scheme assumes that all multicellular animals descended from some form of single – celled protest, most likely a colonial flagellate and present sponges (phylum porifera) as the earliest experiments in multicellularity with no close relationship to any other existing phyla.

Classifications also change when biologists discover organisms having characteristics not shared with any existing groups. For example, 2 arthropod classes (the Remipedia and Tantulocarida) and 3 small but remarkably distinct phyla of recently discovered marine animal called loriciferans, micrognathozoans and cycliophorans have been established in the last 25 years or so. Cycliophorans were first described in 1995 and micrognathozoans in 2000.

Sometimes classifications change when biologists reexamine previously studied material, or acquire new material. A small but fascinating group of gutless worms, for example the pogonophorans were originally characterized as unquestionable deuterostomes, based on adult morphology. Years later, speciments with a small additional body part were obtained the posterior part of the animal had detached unnoticed from previous specimens and the animals were quickly reclassified as a phylum of protostomes. Indeed, largely on the basis of features of that small terminal portion, pogonophorans have recently been incorporated into the pylum Annelida, a group that contains earthworms and leeches. Such placement has now been supported by molecular data.

Finally, molecular studies comparing selected gene sequences and more recently by analysis of entire genomes among representative of different groups are quickly altering our understanding of many invertebrate relationships. While molecular data often support previous conclusions based on morphology and developmental pattern, such as the monophyly of living animals and the distinction between protostomes and deuterostomes, the frequently suggest relationships quite different from those based on the other criteria. Where molecular data produce phylogenies very different from those based on morphology, decisions will have to be made about which evidence is more likely to be correct. And it is worth nothing that molecular studies, as powerful as they are, will never resolved all phylogenetic issues, no matter how sophisticated these studies become. For one thing, when species diversified too rapidly, molecular studies are unable to resolve the order of divergence. Moreover, molecular studies will never be able to tell us the precise sequence of steps that took place as one form gave rise to another or what selective pressures brought about these morphological changes. And molecular studies can never tell us what ancestral, unfossilized animal looked like. Perhaps molecular, paleontological, ecological and morphological evidence can be used in concert to deduce relationships, but we will still need to decide how much weight to give each line of evidence when the different approach imply different evolutionary scenarios.

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