Natural History of Vertebrates
Lecture Notes
Chapter 1 - The Diversity, Classification, and Evolution of Vertebrates
These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course.
We will not spend much time in Chapter 1. Several of these concepts you have probably covered in other courses (GB 140 or ZO 214).
List of Terms
Be sure to study carefully figures 1-1 and 1-3. We will cover the numbers of species of each group as we go through the semester, but an overview will help greatly from the beginning.
Be sure to study carefully the concepts in the sections entitled "Classification of Vertebrates" (1-2) and "Earth history and vertebrate evolution" (1-4). These concepts will permeate much of what we talk about in the course.
Cladistics (pages 7 - 12).
One of the reason that I chose this book is that the story of the vertebrates is told from the
standpoint of their evolution. The way that phylogenies are proposed is through a process called
cladistics. Cladistics attempts to produce an hypothesis about the evolutionary sequence of
events that led to a group of organisms through the use of shared derived characters. These
hypothesized phylogenies are put forth as cladograms. It is important to realize that a cladogram
is an hypothesis about a group of organisms. Cladograms, like any hypothesis in science, are
subject to change as more data are accumulated or data are reevaluated. Cladograms are NOT
truth. It might be (and we certainly hope) that a cladogram portrays the correct phylogeny, but
we will never know if it does. For example, many studies have addressed the evolutionary
relationships among the great apes and humans. There have been may cladograms produced.
The majority of the cladograms show humans as the sister taxon to the chimpanzees, however
other studies (with their cladograms) show the gorilla to be the sister taxon to the chimpanzees.
We will never know which taxon diverged first, but we have more data (the preponderance of
evidence) supports the cladogram that shows humans as the sister taxon to the chimpanzees.
Thus the cladogram is a hypothesis that, like any hypothesis or theory in science, is subject to change as more data are accumulated.
When you read the textbook you will see many cladograms. You must not only learn the
relationships among the taxa but you must learn and understand the data that supports these
cladograms. Ask yourself, "What other data could I collect to address the hypothesized
relationships shown in the cladogram?" Be sure to learn the following terms.
- Shared derived character (synapomorphy) is a character that is derived from the ancestral
conditions and is shared among several taxa that all descended from a common ancestor that first
exhibited the derived character. These characters carry information regarding the phylogeny of
the organisms under study.
- Shared primitive character (sympleisomorphy) is a character that is the same as the ancestral
condition and is shared among several taxa. These characters carry no information regarding
phylogeny of the organism under study.
- Uniquely derived character (autapomorphy) is a character that is derived from the ancestral
condition but is present in only a single taxon and thus not shared. These characters carry no
information regarding the phylogeny of the organisms under study.
- The groupings of organism in a cladogram are referred to as clades. The goal of cladistics
(phylogenetic systematics) is the identification of monophyletic groups. They are groups which
contain the common ancestor plus all of the descendants. For example, Mammalia is a
monophyletic group as it contains all living and extinct mammals plus the ancestor of all
mammals.
- Reptiles, as commonly defined, is not a monophyletic group. It is a paraphyletic group in that
some descendant groups are not included in the reptiles, for example birds or mammals. The
group called reptiles really has no validity in a cladistic analysis and as such would not be given
a name.
- Polyphyletic groups are groups that do not contain the common ancestor for the group. For
example, to try to name a group that was composed of Aves and Mammalia (perhaps on the basis
of endothermy) would not include the common ancestor of these two groups and thus the named
group would not be monophyletic.
- A sister taxon is the most closely related monophyletic clade to the clade in question. For
example, in figure 1-3, the Sauropsida in the sister taxon to the Mammalia; the Crocodilia is the
sister taxon to the Aves; the Amphibia is the sister taxon to the Amniota.
Be sure to study carefully figure 1-3. One of the major goals of this course is to understand the evolution of the extant vertebrates and this cladogram is the current hypothesis about that evolution. This cladogram will be a recurring theme throughout this course. You will need to not only know this cladogram but the evidence that supports the phylogenetic hypotheses
that the cladogram represents.
Last updated on 15 January 2003
Provide comments to Dwight Moore at mooredwi@emporia.edu
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