Section 4: Classification

In the 1730s, Carolus Linnaeus, a Swedish botanist, developed a two-word naming system called binomial nomenclature.  The first part of the name is the genus, and the second part of the name refers to the species.  Classification is a way of separating a large group of closely related organisms into smaller subgroups.  Biologists use a dichotomous key to help them identify unknown organisms.  The word dichotomous comes from the word dichotomy, meaning “two opposite parts or categories.”  A dichotomous key consists of paired statements that are opposite of one another about the same trait.  The user picks the best description that fits the organism and is either led to another set of paired statements or the organism’s identity.

Linnaeus also developed a classification system to organize living things into groups.  This helps in our understanding of their differences and similarities.  The science of naming and identifying organisms is called taxonomy.  Seven groups make up the system for scientific classification.  At the end is species, which refers to a group of organisms that breed and produce offspring.  Next, the genus is where similar species are grouped.  Next, closely related genera are grouped into a larger category called the family, then placed into larger groups called orders.  Then orders are grouped into classes; classes are grouped into phyla, and phyla are grouped into kingdoms.  Kingdoms are the most prominent groups and include plants and animals.

Sometimes classification keys are used to identify organisms by visible traits; however, scientists today look at how closely members of a group are related.  Phylogeny is the study of how living and extinct organisms are related to one another.  This group’s species into categories that show lines of evolutionary descent, not just similarities and differences.  Classifying them this way places them into clades, which is a group of species that includes a single common ancestry and all the descendants of that ancestor.  A cladogram is a diagram that links groups of organisms by showing how evolutionary lines, or lineages, branched off from common ancestors.  These focus on studying derived character, a trait that arose in a particular lineage’s most recent common ancestor and was passed on to its descendants.


  1. What is binomial nomenclature?
  2. Which group is the largest and most inclusive taxonomic category?
  3. What is a cladogram?

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