PHYLOGENY

the history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms. Fundamental to phylogeny is the proposition, universally accepted in the scientific community, that plants or animals of different species descended from common ancestors. The evidence for such relationships, however, is nearly always incomplete, for the vast majority of species that have ever lived have become extinct, and relatively few of their remains have been preserved. Most judgments of phylogenicity, then, are based on indirect evidence and cautious speculation. Even when biologists use the same evidence, they often hypothesize different phylogenies, though they do agree that life is the result of organic descent from earlier ancestors and that true phylogenies are discoverable, at least in principle. Taxonomy, the science of classifying organisms, is based on phylogeny. Early taxonomic systems had no theoretical basis; organisms were grouped according to apparent similarity. Since the publication of Charles Darwin's Origin of Species in 1859, however, taxonomy has been based on the accepted propositions of evolutionary descent and relationship. Biologists who postulate a phylogeny derive their most useful evidence from the fields of paleontology, comparative anatomy, comparative embryology, and biochemistry. Studies of the fine structure of cells and geographic distribution of flora and fauna are also useful. The fossil record is often used to determine the phylogeny of groups containing hard body parts; soft parts are generally not preserved. Most of the data used in making phylogenetic judgments have come from comparative anatomy and from embryology. In comparing features common to different species, anatomists try to distinguish between homologies, or similarities inherited from a common ancestor, and analogies, or similarities that arise in response to similar habits and living conditions. Biochemical investigations carried out in the latter half of the 20th century have contributed valuable data to phylogenetic studies. By counting differences in the sequence of units that make up protein and deoxyribonucleic acid (DNA) molecules, researchers have devised a tool for measuring the degree to which different species have diverged since evolving from a common ancestor. The earliest organisms were probably the result of a long chemical evolution, in which random reactions in the primeval seas and atmosphere produced amino acids and then proteins. It is supposed that droplets containing proteins then formed membranes by binding molecules to their surface, and these membrane-bound proteins are said to have become organisms when they developed the capacity to reproduce. It is not certain whether these earliest self-reproducing organisms were proteins, nucleic acidprotein associations, or viruses. There is general agreement that they were heterotrophic organismsi.e., those that required nourishment in the form of organic matter from early seas. Later, autotrophic forms appeared, having the ability to make their own food from inorganic matter. These organisms were the earliest bacteria; they could store energy as food and release energy as needed through respiration. Figure 1: A summary of probable lines of plant evolution Cyanobacteria (sometimes called the blue-green algae) are thought to have been the next evolutionary step (Figure 1) in that they were able to use photosynthetic pigments to manufacture their own supply of food and therefore were not totally dependent on their environment for nutrients. After the cyanobacteria there appeared an extensive array of algae, molds, protozoans, plants, and animals. Three groups of algae can be dismissed with passing mention, as they arose from uncertain ancestors and have given rise to no further groups. These groups are the chrysophytes (yellow-green and golden-brown algae, chiefly diatoms); the pyrrophytes (cryptomonads and dinoflagellates); and the rhodophytes, or red algae. Three more groups have greater phylogenetic importance: the chlorophytes (green algae), which almost certainly gave rise to the land plants, i.e., the bryophytes (mosses and liverworts) and the tracheophytes, or vascular plants (including all of the higher plants); the euglenoids (unicellular, flagellate organisms), which suggest a broad connection between plants and animals at this primitive level; and the phaeophytes (brown algae), which some biologists have considered to be a probable source of the animal kingdom. Finally, the protozoans (unicellular prokaryotic microorganisms) were derived from unknown, more primitive ancestors, and one or more groups of protozoans have given rise to metazoansi.e., multicellular animals. Land plants contain two major groups, bryophytes and tracheophytes, which differ in many ways but which share distinctive characteristics for adaptation to dry land. These include the housing of the plant embryo in maternal tissue. Bryophytes are descended from green algae and include mosses, liverworts, and hornworts. Only small quantities of water are needed for their reproduction, so that the sperm may travel to the eggs. The fertilized egg matures within the maternal tissue. The plant is protected from dessication by a waxy cuticle. Bryophytes have apparently not advanced far beyond their algal predecessors and do not seem to be the evolutionary source of other groups. All the dominant plants on Earth are included in the tracheophytes. The tracheophytes' development of large plant bodies has been made possible by having vascular parts that carry water and food inside these plants, and by a dominant sporophyte stage with a microscopic-sized gametophyte. Tracheophytes' tissues have differentiated into leaves, stems, and roots, and in the highest plants seeds and flowers are featured. In explaining the evolution of tracheophytes, it has been suggested that a mutant form of green algae developed a primitive rootlike function with which to supply itself with water and minerals. The progeny of this organism eventually developed bundles of vascular tissues, a stem and leaves, and a cuticle for protection. The early vascular plants are called psilophytes. The development of seeds arose from the retention of the embryo inside maternal tissue. Early seed ferns gave rise to the gymnosperm group, including pines, spruces, and firs. Flowering plants, the angiosperms, probably came from the gymnosperm phase and have two subgroups: the dicotyledons and the monocotyledons. The problem of the origin of multicellular animals (metazoans) was long dominated by the German embryologist Ernst Haeckel's theory that the original metazoan ancestor was a spherical protozoan that was structurally similar to the coelenterates (e.g., jellyfishes, corals). Today there are two alternative explanations. The first traces metazoans back to flagellates, the presumed ancestors of flattened, ciliated animals (planulas) that eventually led to coelenterates and flatworms. Another theory hypothesizes that multinucleated protozoans, dividing into subcells, were the original metazoans, which developed into simple flatworms. No decisive information, however, yet exists to sustain either contention. Lower metazoan forms developed the first symmetrical arrangement of body parts about a main axis, thus establishing the bilateral symmetry that characterizes most animals; major exceptions are the echinoderms (e.g., starfishes, sea cucumbers). The development of tissues into an outer ectoderm, which provides protection and carries sense apparatus, and an inner endoderm, serving digestion and reproduction needs, was an important phase. Another important trend was cephalization (head formation). The anterior end of the body generally holds the central nervous system, sense organs, and mouth. Figure 2: Hypothetical relationships in the animal kingdom Two current theories postulate the lineage of the higher metazoans. The monophyletic sequence suggests that four groups evolved from lower forms to higher: Ameria (unsegmented animals), which includes flatworms, coelenterates, and mollusks; Polymeria (segmented animals), which includes annelids and arthropods; Oligomeria (reduced segmentation), which includes insects and echinoderms; and Chordonia (chordates). The (alternative) diphyletic theory has been proposed by many zoologists. It contends that the higher metazoans had two lines of descent, one of which led to annelids, arthropods, and mollusks and the other of which led to echinoderms and chordates, as in Figure 2. Both groups emanated from an ancient flatworm. Humans are included in the chordates. Three basic structures are shared by all chordates: a dorsal nerve tube (brain and spinal cord in vertebrates); a notochord (supporting rod under the nerve tube); and a pharynx perforated by gill slits, at least during the embryonic stage. The history of evolution is full of examples of primitive groups giving rise to more advanced groups, but it should be noted that it is the more primitive and less specialized members of a groupnot the advanced membersthat produce new groups. For example, birds and mammals arose not from advanced reptiles but from primitive, unspecialized reptiles. The data and conclusions of phylogeny show clearly that the world of life is the product of a historical process of evolution and that degrees of resemblance within and between groups correspond to degrees of relationship by descent from common ancestors. A fully developed phylogeny is essential for the devising of a taxonomy that reflects the natural relationships within the world of living things.