A speciation event produces orthologs of a gene in the two daughter species. Subsequent hybridisation of those species generates a hybrid genome with a homoeolog copy of each gene from both species. (en)
A whole genome duplication event produces a genome with two ohnolog copies of each gene. (en)
A speciation event produces orthologs of a gene in the two daughter species. A horizontal gene transfer event from one species to another adds a xenolog of the gene to its genome. (en)