Complex sentence: However, reductions in genome size specially in parasites and symbionts can also be caused by direct selection to get rid of genes that have become unnecessary so that a smaller genome is not a sure indication of the action of Muller's Ratchet.
Split into simpler sentences: However, reductions in genome size specially in parasites and symbionts can also be caused by direct selection to get rid of genes that have become unnecessary. Therefore a smaller genome is not a sure indication of the action of Muller's Ratchet.