Comparative anatomy provides powerful evidence for evolution through two fundamental patterns: homology and analogy. Homologous structures share the same evolutionary origin and same basic structural pattern (derived from the same ancestral structure), but may be adapted to perform very different functions in different organisms, reflecting divergent evolution from a common ancestor. Analogous structures perform similar functions in different organisms but have entirely different evolutionary origins and structural organisation, reflecting convergent evolution — independent evolution of similar solutions to similar ecological challenges in different lineages. Distinguishing between these two patterns is crucial for reconstructing evolutionary relationships: homologies reflect true evolutionary ancestry (shared inheritance from a common ancestor), while analogies reflect similar selective pressures but not shared ancestry (convergent evolution). Modern phylogenetics based on molecular data (DNA sequences) has largely confirmed the distinction between homologous and analogous characters, though it has also revealed many "hidden homologies" at the molecular genetic level between structures that appear superficially analogous at the morphological level.

The pentadactyl (five-fingered/five-toed) limb of tetrapod vertebrates represents one of the most famous and compelling examples of homologous structure in comparative anatomy, providing strong anatomical evidence for the common ancestry of all four-limbed vertebrates from a fish-like ancestor. The basic vertebrate forelimb skeletal plan — one proximal bone (humerus in forelimb), two more distal bones (radius and ulna), a series of small wrist/ankle bones (carpals/tarsals), five hand/foot bones (metacarpals/metatarsals), and up to five sets of toe/finger bones (phalanges) — is shared among all tetrapod vertebrates despite remarkable diversity in external appearance, size, proportions, and function. In humans and many primates: elongated digits for grasping, manipulation, and fine motor control. In bats: dramatically elongated digits 2-5 supporting a flight membrane (patagium), with only digit 1 (thumb) free. In whales: digits buried within the flipper, compacted into a paddle-like structure for aquatic propulsion. In horses: only digit 3 (the "middle finger") remains, forming the single hoof, with vestigial splint bones representing remnants of digits 2 and 4. In frogs: modified for jumping with elongated limbs and reduced digit numbers. The consistent presence of this same underlying skeletal organisation across such diverse vertebrate groups, despite adaptation for radically different functions, is most parsimoniously explained by descent from a common ancestor that possessed this five-element limb plan, which then became modified differently in different lineages — precisely the prediction of evolutionary theory and the most powerful evidence of vertebrate common ancestry.

Divergent evolution describes the process by which organisms sharing a common ancestor evolve increasingly different characteristics over time as they adapt to different environments, ecological niches, or selective pressures, potentially resulting in very different-looking organisms that nonetheless share homologous structures reflecting their common ancestry. The adaptive radiation of Darwin's finches in the Galapagos Islands, where a single ancestral finch colonised the islands and diversified into approximately 14 species with dramatically different beak shapes and feeding habits (seed crackers, cactus feeders, insect hunters, blood-feeding — the remarkable Vampire Finch), represents one of the most celebrated examples of divergent evolution in a defined geographical and ecological context. Convergent evolution describes the independent evolution of similar characteristics in unrelated organisms in response to similar environmental pressures, producing analogous structures that resemble each other functionally but share no structural homology or common ancestry in the strict sense. Classic examples: ichthyosaurs (Mesozoic marine reptiles), dolphins (mammals), and sharks (cartilaginous fish) all evolved streamlined, fish-like body forms with dorsal fins and fusiform body shapes for efficient aquatic locomotion — but these morphological similarities reflect similar hydrodynamic constraints on large aquatic predators rather than shared ancestry. Wings in birds, bats, and insects serve the same aerodynamic function but are derived from entirely different ancestral structures and developed through completely independent evolutionary pathways.

The concept of homology extends far beyond morphological structures to encompass molecular biology, where homologous genes (genes derived from a common ancestral gene through evolutionary descent) provide the most fundamental level of evidence for evolutionary relationships. Orthologous genes are homologous genes in different species that diverged from a common ancestral gene through speciation — they typically perform the same or similar function in both species (e.g., the haemoglobin genes of humans and chimpanzees are orthologous, both encoding oxygen-transport proteins, with extremely similar sequences reflecting recent common ancestry). Paralogous genes are homologous genes within the same species that diverged through gene duplication — they may perform related but distinct functions (e.g., the alpha and beta haemoglobin chains in humans are paralogous, both encoding haemoglobin subunits but with different properties, having originated by duplication of an ancestral haemoglobin gene in vertebrate evolutionary history). Molecular sequence comparisons have revealed unexpected deep homologies between distantly related organisms — for example, the Hox genes (homeotic selector genes controlling body axis and segment identity during development) show striking sequence conservation and positional collinearity across virtually all bilaterally symmetrical animals from simple worms through insects to vertebrates, suggesting these fundamental developmental control genes were present in the last common ancestor of all bilaterian animals over 600 million years ago.