Brent Cornell

    
Historically, classification was based primarily on morphological differences (i.e. structural characteristics)

• Closely related species were expected to show similar structural features, indicating common ancestry

However, there are two key limitations to using morphological differences as a basis for classification:

• Closely related organisms can exhibit very different structural features due to adaptive radiation (e.g. pentadactyl limb)
• Distantly related organisms can display very similar structural features due to convergent evolution

Convergent evolution is the independent evolution of similar features in species with distinct lineages

• It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures
• The shared conditions cause common adaptations to be selected in different species, resulting in structural similarity
  • An example of convergent evolution is the development of wings in birds, bats and insects

Convergent Evolution

    
Structural traits are not commonly used to determine clades as such features may not necessarily indicate shared heritage

• Traits that are similar because they are derived from common ancestry are termed homologous structures 
• Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures

Using molecular evidence, scientists have discovered that many species thought to be closely related based on shared structural characteristics actually demonstrate distinct evolutionary origins

• Crocodiles have been shown to be more closely related to birds than lizards, despite closely resembling lizards in structure
• Many species of plants previously classified as figworts have been reclassified based on molecular evidence

Homologous Structures versus Analogous Structures