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Why can counting all shared traits give a misleading picture of evolutionary relationships?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

Counting all shared traits can provide a misleading picture of evolutionary relationships because it fails to distinguish between traits inherited from a common ancestor and those that evolved independently.12

In biology, this distinction is fundamental:

Homology: Refers to features shared due to common descent, such as the skeletal structure of human, whale, and bat forelimbs. These are reliable indicators of evolutionary history.13
Analogy (or Homoplasy): Refers to similarities in form or function that arise independently, often due to convergent evolution or parallel evolution in similar environments. For example, the wings of birds and flies are analogous; they serve the same function but do not share a common ancestral origin.143

The methodology known as phenetics—which classifies organisms by counting as many shared characteristics as possible regardless of ancestry—is often criticized because it treats these analogous traits as equal to homologous ones. By failing to weight characters based on their evolutionary significance, this approach can incorrectly group unrelated species together simply because they share superficial similarities driven by environmental pressures. Modern cladistics avoids this by focusing specifically on shared derived characters, or synapomorphies, which provide a more accurate reflection of common ancestry.25