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BIO.7
The student will investigate and understand bases for modern classification systems. Key concepts include
a) structural similarities among organisms;
b) fossil record interpretation;
c) comparison of developmental stages in different organisms;
d) examination of biochemical similarities and differences among organisms; and
e) systems of classification that are adaptable to new scientific discoveries
Carl Linnaeus is considered the father of modern taxonomy. He developed the system of binomial
nomenclature that is in use today. It is flexible enough that it has been able to incorporate new methods of
comparison such as gene sequences and allows rearrangement of taxa to accommodate new information.
The basic unit of classification is the species. The biological species concept is defined as a group of
populations capable of interbreeding and that are reproductively isolated from other such groups. The clear
drawback to this definition is that it cannot be applied to asexually reproducing organisms.
Species are grouped into a higher taxon called a genus. In the early days of taxonomy, groupings
into higher, more inclusive taxa were based on similarities in anatomy. The problem is that convergent
evolution can produce similar structures as a response to similar selection pressures. For example, the
forelimbs of whales and penguins are very similar; both are adapted to swimming, yet the first is in the
class mammalia and the second in the class aves. Further examination of their internal anatomy, body
covering, methods of rearing young, and the development of their young would indicate that they are in
very different taxa, in particular, they are in different classes. A review of BIO.5 shows the data that are
used to classify organisms into different kingdoms, and in the case of the Chordates, into different classes.
Genera are classified into families, and families into orders. Orders are grouped into classes and
classes into phyla. Phyla are grouped into kingdoms. The order from the most inclusive taxon to the least
is kingdom, phylum, class, order, family, genus, species. A good mnemonic device is “King Phillip, come
out for God’s sake.” Every organism has a unique name, given by its genus and species. Genus is
capitalized and its species is lower case. In print, the genus and species are italicized; in writing, they are
underlined. For example, humans are in the genus Homo and the species sapiens. So our biological
name is Homo sapiens.
family
genus 1
today
genus 2
genus 3
fossil
ancestor?
fossil ancestor?
time
Ideally, classification should reveal evolutionary relationships or phylogeny as in the theoretical
phylogenetic tree in the illustration above. Each small branch would be a biological species. Those
grouped into a common genus would presumably have a more recent common ancestor than other such
genera. Groups of larger branches would represent higher taxa. To make such a tree, taxonomists have
to identify ancestral characters. For example, all those species in genus three would possess an ancestral
character that would be found in fossil lineages as well. This is a trait that unites them in the same genus.
Derived characters are those that divide the genus into separate species. Each taxonomic level would
have unifying ancestral characters and the next less inclusive taxon would have differentiating derived
characters. A simplistic example is to compare modern reptiles with mammals. An ancestral character
might be the presence of four limbs (either vestigial or functional). The position of those limbs is derived. In
reptiles the body is suspended between the limbs. In mammals the limbs are positioned under the body,
an adaptation for sustained running that endothermy allows.
Modern classification is based not just on structure or anatomy, but on development of the
organism as well. Behavior can play a part in reproductive isolation. In many similar birds, closely related
species are isolated by songs. In fiddler crabs, the method used by males to signal females (drumming
and waving of the large claw) can separate species. Behavior is not readily apparent in fossils. Today
similarities in amino acid sequences in common proteins or point mutations in DNA can be used to infer
taxonomic relationships and can make possible the construction of phylogenetic trees that accurately
reflect the time of divergence of species. For example, among the primates, examination of a section of the
hemoglobin molecule shows no differences between man and chimpanzee and only one difference
between man and the gorilla. Thus we share a more recent common ancestor and are more closely related
to the chimpanzee than to the gorilla.
http://jrscience.wcp.muohio.edu/lab/TaxonomyLab.html
This page has a lab in which students develop a classification scheme for various pieces of
hardware. Various types of pasta work just as well
http://mclibrary.nhmccd.edu/taxonomy/taxonomy.html
A clearing house of biological taxonomy web sites
After reviewing material in your textbook, go to the file labeled BIO.7 Review Response and open it in
Word. Type your answers below each question and make them a distinctive readable color or font. E-mail
this file as an attachment