Final answer:
An ortholog is two genes in different species that have the same function and were inherited from a common ancestor, supporting the concept of conserved genetic functions and evolutionary relationships derived from a shared lineage.
Step-by-step explanation:
An ortholog refers to option 4) two genes in two different species that serve the same function and were inherited from a common ancestor. These genes are a result of speciation, where a single species diverges into two separate lineages, and the equivalent gene of each lineage retains the original function that was present in the common ancestor. Orthologs are important for understanding evolutionary relationships between species because they provide evidence of conserved genetic functions across different organisms that have a shared lineage.
Homologous structures, such as the similar bone patterns found in the limbs of various mammals, support the concept of orthologs as they demonstrate the inheritance of traits from a common ancestor. Despite superficial differences in the present-day function of limbs among different species, their underlying structural similarity is due to their evolutionary history. Moreover, these structures show that organisms can possess similar physical and genetic features that denote a more intimate evolutionary relationship.