Final answer:
Orthologous genes are genes in different species that originated from a common ancestral gene and usually retain the same function over time.
Step-by-step explanation:
When we say two genes are orthologous, we refer to the fact that they originated from a common ancestral gene and have diverged following a speciation event, meaning that they are found in different species now. Orthologous genes typically retain the same function over the course of evolution, despite being in different organisms. This concept is different from paralogous genes, which diverge within the same species due to gene duplication. An example of orthologous genes can be found in the gene for hemoglobin in humans and the gene for a similar oxygen-transport protein in other mammals; they originate from a common ancestor but are found in different species.