Horizontal gene transfer is invoked to explain how there is often extreme similarity between small portions of the genomes of two organisms that are otherwise very distantly related. Horizontal gene transfer seems to be common among many microbes. Also, eukaryotic cells seem to have experienced a transfer of some genetic material from their chloroplast and mitochondrial genomes to their nuclear chromosomes.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What does horizontal gene transfer explain about nuclear chromosomes?
unanswerable