Final answer:
The pattern of limb structures in tetrapods is due to homologies, indicating a common evolutionary origin. Complex structures like these are unlikely to have evolved independently multiple times, thus they represent evidence of an ancestral relationship rather than convergent evolution.
Step-by-step explanation:
The presence of a common structural pattern in the limbs of most tetrapods, such as one-bone (humerus), two-bones (radius and ulna), and lots of bones (carpals, metacarpals, phalanges), is the result of homologies. This means that these structures are homologous and share a common ancestral origin. The complexity of these structures being similar across different species indicates that they evolved from a common ancestor rather than being an example of convergent evolution, which is when similar features evolve independently in species of different lineages.
Anatomical and embryological evidence supports this idea. Homologous structures, like the limb bones of humans, dogs, birds, and whales, have the same basic organization due to their evolution from the appendages of a common ancestor. Over time, while the shapes and sizes of these bones have adapted to the needs of different species, the overall layout has been conserved, which demonstrates descent from that common ancestor.