Final answer:
Classification based on overall similarity, not considering evolutionary history, is known as phenetic classification and is exemplified by the Linnaean system. Cladistics, on the other hand, categorizes organisms based on evolutionary relationships, using cladograms and clades. Today's scientists use morphological and genetic evidence to identify homologous structures and trace evolutionary pathways.
Step-by-step explanation:
The classification based on overall similarity regardless of evolutionary history is known as phenetic classification. This system groups organisms by observable physical traits, rather than by phylogeny or evolutionary relationships. The Linnaean system, developed by Carl Linnaeus, is an example of such a classification system. It organizes organisms into a hierarchy of taxa, ranging from kingdom to species, based on shared physical characteristics.
In contrast, cladistics is a method that classifies organisms based on their phylogenetic relationships, using clades that include an ancestor and all its descendants.
This approach emphasizes evolutionary ancestry and is represented by cladograms. However, before the acceptance of Darwin's theory of evolution, scientists grouped organisms primarily on the principle of phenetics without considering evolutionary pathways. Even after Darwin, the challenge was identifying features that genuinely reflected the evolutionary past, as much of it is untraceable due to an incomplete fossil record.
Today, scientists often rely on both morphological evidence (form and function) and genetic evidence to uncover the evolutionary connections between organisms. Homologous structures are one of the key indicators of shared ancestry. They demonstrate morphological and genetic similarities due to a common evolutionary path, as seen in the homologous bones in the limbs of bats, birds, and horses.