Download Classification of Biological Diversity

Classification of Biological Diversity
Slide 2
Classification systems serve a number of important roles. Firstly, they
allow us to recognize and interpret similarities and differences among organisms and
arrange them into groups according to specific traits. Secondly, having similar organisms
classified into a group also allows predictions of a specific trait that is consistent with the
group. For example, a secondary metabolite isolated from a particular plant species may
have pharmaceutical properties. The same molecule may be found in higher
concentrations in other plant species in the same group. Thirdly, when organisms are
classified into groups they are given a unique name according to their genera and species.
The language used in naming organisms is universal and people throughout the world are
able to recognize a specific organism from its scientific name.
Slide 3
Carl Linnaeus, a Swedish professor, physician and naturalist devised the
binomial system for classifying organisms into the hierarchical system that we use today.
Evolution was not an accepted component of biology in 1758; hence there is no
consideration of evolutionary relationships between organisms classified by this system.
It is useful however, in that an organism classified by this system can be identified
anywhere in the world and by anyone. Much of the classification was done on
morphological characteristics and many organismal relationships have been confirmed
after analyzing molecular traits.
Slide 4
Classification of organisms into their highest taxonomic rank is an
ongoing process. Linnaeus (1758) divided all known forms of life between two
kingdoms, the plant and animal kingdoms. Since then, the five kingdom system was
adopted in 1969 and has been the standard for the past twenty years or so. Recent
evidence has divided the kingdom Monera (prokaryotes) into two separate kingdoms, the
Bacteria and the Archaea resulting in the three-domain system. The organisms in the
Archaea domain are prokaryotes that are adapted to live in extreme environments. The
remaining four kingdoms in the five kingdom system are included in the Eukarya
domain. Currently, there is much controversy over the classification of the Protista
kingdom which includes an assortment of organisms. Sometime in the not too distant
future the classification system will undergo another change as this kingdom is
subdivided into more closely related groups.
Slide 5
The binomial nomenclature, devised by Linnaeus, names organisms
according to genus and species. In addition, he grouped similar species into hierarchical
categories. Organisms are ranked by kingdom, phylum, class, order, family, genus and
species. The kingdom classification is the most general category in which to place an
organism; the species is more specific sometimes being further reduced into subspecies.
Each of the subsequent groups (or taxa) becomes more specific, the organisms in each
grouping having more characteristics in common. In this example, the two organisms
classified are Dendroica fusca, the blackburnian warbler and Rosa gallica the moss rose.
The genus name is always capitalized and both genus and species names should be
italicized.
Slide 6
Ancestral traits that show anatomical similarities between species are
considered as homologous structures. For example, the forelimbs of humans, cats, bats,
whales and birds all have similar skeletal structures even though these appendages have
evolved different functions. Sometimes the trait being examined is not always obvious
in the adult form of an organism therefore, early stages of development are often
observed for homologies.
On the other hand, a derived trait is different from its ancestral form in that similarities
within a group are distinguished. Derived traits are often more difficult to classify since
they change over the course of evolution. Also, identification of derived traits is often
complicated by the occurrence of homoplastic traits i.e. traits that are similar in different
individuals but they are not descended from a common ancestor. For example,
convergent evolution produces organisms that evolve independently but have similar
features. Some plants that evolved in desert habitats have features similar to cactus, i.e.
succulent and prickly, but they do not belong to the cactus family. These organisms
occupy similar habitats and are influenced by similar selective pressures producing
similar phenotypes.
Slide 7
Fossil records of extinct organisms are useful in distinguishing ancestral
and derived traits. These records also provide an evolutionary time line of events on
which to base the history of life. In extant or living organisms, structural traits such as
morphology and development are an important resource in classifying organisms.
Organisms possessing similar characteristics are arranged in the same group.
The fossil record does not provide all the answers to the history of life, hence molecular
structure has become a powerful tool in determining the relationship between organisms.
Proteins such as cytochrome c and nucleic acids are particularly useful in identifying
relationships. Mitochondrial DNA is often used to investigate the evolutionary
relationships in animals whereas chloroplast DNA is often chosen to investigate the
relationships between plants.
Many different heritable traits are analyzed in order to classify organisms in relation to
one another and in the reconstruction of phylogenetic trees.
Slide 8
Systematists analyze evolutionary changes within a group of organisms
and reconstruct phylogenetic trees showing the relationship of organisms to one another
and their common ancestor. Phylogenetic trees show the sequential order of evolutionary
change; in this example the oldest split is to the left of the tree, the youngest to the right.
Each taxonomic unit is a monophyletic group (also known as a clade) containing all the
descendants of a particular ancestor. Phylogenetic trees represent monophyletic groups
nested within monophyletic groups. In this example mosses, ferns and flowering plants
are each monophyletic groups but they all belong to the monophyletic group Plantae
since they are all descended from a common ancestor believed to be a green algae.
We will be investigating the classification of biological diversity in more detail in the
next course BIO 116, “Organisms and Environments”.