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.
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