Based on Pough et. al, 1989. Vertebrate Zoology - 3rd Ed. MacMillan
Hickman et al., 1996. Integrated Principles of Zoology - 10th Ed. William C. Brown Publishers
These two subjects are closely tied together. Before we can understand classification, it is necessary to understand the basics of the vertebrate body, since much of our classification system is based to some extent on external characteristics (which we presume to reflect evolutionary relationships).
There are various levels of organization in biology:
We will be dealing with the organismal level for the most part. As it turns out, the major factors affecting the construction of vertebrate bodies arise as a result of the physiological constraints imposed by the size of the vertebrate body:
Effects of Scale
Vertebrates, as it turns out, all are large and complex enough to require circulatory and respiratory systems. Some vertebrates, such as birds and mammals, are also homeotherms and homeothermy increases O2 and energy demands by a factor of 10. This places additional requirements on the body's systems, particularly the gas exchange and circulatory system.
By the way, the material on page 184 that talks about Cope's law of phyletic increase is plain wrong. Cope's law postulates that there is a natural tendency to increased size over evolutionary time. This is not the case; there is no directional evolutionary trend. Many groups simply start out small; they can't get much smaller, so the only route of increase is for larger size. As many species (if not most) actually remain small or get as small as practical. See Full House by Stephen J. Gould (Harmony Books, 1996).
The vertebrate body is composed of cells and the various materials laid down by cells, as well as a lot of water. The water and the material laid down by cells can be grouped into the category of extracellular components:
The cells of the vertebrate body are grouped into tissues; each tissue has a specialized function. You should know the types of tissues pictures in Figures 10-4 through 10-7 in your text.
By only considering vertebrates, we omit many of the body plans in the animal kingdom. The vertebrate body is fairly simple in its organization. Vertebrates have bilateral symmetry, which means they can be divided into "equal" right and left halves along only one plane. They have a jointed endoskeleton, which means that the hard supporting part of the body, the skeleton, is contained under the various other tissues and that it has flexible joints. Vertebrates have a specialized dorsal (along the back) nervous system, which is unique in the animal kingdom. The body cavity, or coelom, of vertebrates is known technically as a modified schizocoel. This body cavity arises when the mesoderm splits (schisms) open to form the cavity. Vertebrates are cephalized, which means that they have the nervous system concentrated in the head.
To know your way around the vertebrate body, you need to know the various planes and directions used to describe it. These are listed below; details are in Figure 10-11 on page 194.
planes and directions
Anterior refers to the end of the organism bearing the head; the tail end is posterior. Cranial and caudal are also used to refer to the head and tail regions, respectively. The transverse plane divides the anterior from the posterior. Dorsal refers to the back of the animal; ventral refers to the belly. These terms refer to the normal orientation of most vertebrates, not the unusual attitude of humans; in other words, the top of our head is anterior, not dorsal. The dorsal and ventral parts of the animal are divided by the frontal plane. Medial refers to along the midline of the body (the spine is medial); lateral means to the side of the midline. The sagittal plane separates the two lateral halves of an organism. Proximal describes parts of appendages closest to the body; distal parts are further away. The hand is distal to the elbow, the elbow is proximal to the hand, but distal to the shoulder. A structure on the body connecting more distal parts of an appendage to the body is called basal. In vertebrates, which have only two sets of paired appendages, pectoral refers to the more anterior set of appendages (our arms, for instance), while pelvic refers to the appendages joined posteriorly.
In addition, you should know which tissues develop from which of the three germ layers of the embryo (see Figure 7-25 on page 121):
ectoderm: gives rise to
outer epithelium of body, and its derivatives:
lining of mouth
enamel of teeth
lens of eye
neural tube and its derivatives:
neural crest and its derivatives:
dentine of teeth
mesoderm gives rise to:
lining of thoracic and abdominal cavities
circulatory system, including:
endothelium of blood vessels
somites (body sections), including:
cartilage of skeleton (except skull)
organs of the urogenital system, including:
endoderm gives rise to the primitive gut, which in turn produces:
I already introduced some of this material in the first section on evolution, so if some of this seems like review, great; if not, maybe you better go back and do some more reading.
Classification arises out of several needs that are peculiar to humans. We have language, which is an abstract way of communicating. Language requires names, and therefore an important part of classification involves simply putting names on things. Humans have a further need to abstract and simplify things so that we can think about them. Classification seeks to simplify the natural world by taking the multitude of basic units - the species - and grouping them together in ways that can help us see overall patterns. It is far simpler to say "fish have gills" than it is to say "Pomoxis annularis Rafinesque, Pomoxis nigromaculatus (Lesueur), Ambloplites rupestris rupestris (Rafinesque), Micropterus dolomieui dolomieui Lacepede, Micropterus punctulatus punctulatus (Rafinesque), Micropterus salmoides salmoides (Lacepede), Lepomis cyanellus Rafinesque, Lepomis macrochirus macrochirus Rafinesque, Lepomis megalotis megalotis (Rafinesque), ... and so on for 28,000 or so species ... have gills." Finally, our classification system takes on an additional challenge - to relate the evolutionary relationship between the different species. To serve this end, it is necessary for the classification system to group together those species that are most closely related. This final requirement was not a part of original classification schemes, including the binomial system of nomenclature devised by Linnaeus in the 1700's - a century before Darwin and evolution gave us cause to suspect that the species were related.
You should know the relationship between 3 terms. Classification means to name organisms and place them in a hierarchical system. Taxonomy is the science of doing classification. Since Darwin, taxonomy has become part of a larger science of systematics (just as entomology is part of the larger science of biology). Systematics seeks to understand the evolutionary relationships between organisms.
A taxon (plural taxa) is any hierarchical grouping in a classification system. Thus the Kingdom Protista or the genus Plethodon, or the family Canidae, are all examples of taxa. The levels in the hierarchy - kingdom, phylum, class, order, family, genus, and species - are taxonomic ranks.
How do taxonomists place organisms into taxa? Primarily, they look for characters. A character is any distinctive feature of an organism, perhaps a bone structure, maybe a unique gene or protein, or even a behavior, that helps separate the organism from others. If the character is shared between several organisms, that provides a clue that they should be placed in the same taxon, but caution must be used here. Because one goal of taxonomy is to produce a classification that depicts evolutionary relationships, it is important to be sure that characteristics are both shared and inherited from a common ancestor before they are used to unite organisms into a taxon. Both flies and birds have wings, but that doesn't mean you want to group them closely together in your taxonomy. There are several ways of insuring (to some extent) that characteristics are derived from a common ancestor; the most accepted of these today is cladistics. Cladistics was formalized by an entomologist working on one of the most complicated, speciose families in the world - the Chironomidae. Cladistics can be very complicated; for us it will suffice to say that it is a system for rigorously evaluating characters that results in a classification system in which all the members of a taxon share a common ancestor, and all descendants of that ancestor are included within the group. This creates what is known as a monophyletic group, see Figure 11-4 and realize that the figure is wrong (the monophyletic group should only include groups E, F, G, and H). Other possibilities for grouping taxa include paraphyletic grouping (all members share an immediate common ancestor, but not all of the descendants are included) and polyphyletic grouping (which does not include the most recent common ancestor of all of the descendants). Cladistics only allows monophyletic groups. A strict cladistic interpretation of modern species will probably mean that the current Linnaean hierarchy will have to be scrapped. In fact, a cladistic system argues against using fixed ranks.
The cladistic method is probably a better (though still imperfect) way of getting at evolutionary relationships without bias. Another method, evolutionary taxonomy, uses both common descent and the amount of evolutionary change to group organisms. This is the system that produced many of the groups we still recognize today. Reptiles hold together as a group here because they can be considered to have about the same level of evolutionary change, and because they share many characters inherited from common ancestors. Cladists, however, would argue that turtles split off before the common ancestor of lizards and snakes arose, and that to include all the descendants of the reptiles would mean including birds and mammals as well (and that would be just fine with the cladists, who lump many fish, all amphibians, reptiles, birds and mammals into the "bony fish" Osteichthyes).
How cladistics works:
Example: Turtle, Fish, Bird
To determine a synapomorphy, one must first determine homologous structures, and to do this, one must trace modifications (transformation) within the fossil lineage or closely scrutinize the structure involved in comparison with others known to belong to the taxon as well as those which do not belong (these latter taxa are known as outgroups). One must be careful not to be mislead by convergence (i.e. similar, but not homologous) structures. For instance, birds and bats have wings, but if one looks to the ancestor of a bird one finds a lizard-like dinosaur. On the other hand, it is apparent that bats evolved from furry mammals (but not rats or mice). Neither the dinosaur nor the mammal had wings, so the wings of birds and of bats must have evolved separately. They cannot be synapomorphies.
Read through all of Chapters 10 and 11. Most of the material is directly related to vertebrates. You do not need to know all of the patterns presented in Figure 10-9, only the vertebrate pattern.
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