text: is there evidence to support the evolution of sexual reproduction from a viral or parasitic origin? why would selfish, replicating genes choose to engage in a system where the offspring only have half of those present in an adult organism?  why not just fission?  choosing the chance to not be passed on to the next generation to better enable the offspring to handle environmental changes seems an entirely implausible answer given that genes don't have future thoughts and why would genes choose a 50% chance of not replicating now, versus an y% chance of not surviving some 'time' later (post environment shift). sexual reproduction is just better for generating lots of species ... i.e., it increases the adaptation of new organisms to changes in environment, and creates new organisms that can move into new environments.  whether it is more or less *likely* to originate than asexual reproduction is irrelevant.  (most of the history of life was without it.)  what matters is that, once it does occur, it is *very* effective at generating *lots* of new organisms that have it ... and it can way outpace asexual reproduction in populating the world with new species that have it.  this is not to say that asexual reproduction can't work quite well for many organisms that continue to thrive and produce new species through occasional mutation.  but the type of widespread variation and *inventiveness* we see in much of the plant kingdom and all of the animal kingdom, is a direct result of sexual reproduction.  (and i use "inventiveness" metaphorically, as i would use the word "beautiful" ... i of course don't mean "deliberate invention", but just that sexual reproduction can produce 20 different solutions to a problem of survival, 2 or 3 of which survive long enough to branch out new species.)  in other words, (and this is the key) we see way more sexual than asexual reproduction in plants and animals only because it is *so* effective at generating new species.  oh, and to address your original question, evidence ... the evidence that sexual reproduction arose from asexual organisms includes (1) the fact that the two have exactly the same genetic machinery (same dna, same orientation of double-helix, same codons, a, c, g, t, same amino acids, same proteins); (2) the fact that the genes themselves have a huge amount of commonality (% of shared genes); and (3) the fact that the amount of this commonality is always consistent with a common-descent model (the % of genes in common between humans, apes, salamanders, sponges, and amoebas don't all have to have a sensible proportions that point to genetic "distance" from each other ... but they do).