Biologists now routinely test for homology between genes in different species. if genes are determined to be homologous, it means that they are related

The answer is ‘by descent from a common ancestor’. Sequence homology (nucleotide or amino acid sequence similarity) is used  to compare two segments of DNA for shared ancestry through either of the following three phenomena: either a speciation event (orthologs), or a duplication event (paralogs), or else a horizontal (or lateral) gene transfer event (xenologs).

answer; projective;