Reproduction is the key to natural selection in organisms. Although the norm for organisms is sexual reproduction, that is not necessary for evolution to take place. Bdelloid rotifers (metazoans), for example, have no males, no hermaphrodites and no meiosis. As far as we can tell they have been this way for millions of years. In the absence of “standard” reproductive techniques, they divide unfertilized eggs. DNA repair mechanisms exist as in other organisms. According to Flot JF, et al. Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. Nature. 2013, the key may be that they can dry up when there is no water and rehydrate when water is available, potentially incorporating fragments of foreign DNA from ingested food.
This harkens back to the idea of endosymbiosis as the originating mechanism responsible for the establishment of mitochondria and chloroplasts in genetic reproduction. These have played a critical role for the evolution of eukaryotes. The idea is that plastids in an ancient asexual cell may have come from an ingested cyanobacterial ancestor, and then some red and green algal ancestors were incorporated into other phagotrophic eukaryotes via secondary endosymbiosis and retained as secondary plastids. Almost all of the plastids in secondary and tertiary algae are thought to ultimately have originated from a secondary endosymbiosis of a red algal ancestor. Two exceptions include the Euglenophyta and Chlorarachniophyta which possess secondary plastids of green algal origin. It is probable this was parallel independent secondary endosymbioses. Our own genetic material is derived from these ancient processes.
Just to add to the complexity, we can add observations about prions. Prions are non-living pieces of infectious protein devoid of DNA or RNA that can cause fatal neurodegenerative disease. They are capable of Darwinian evolution. The study elucidating this was actually a medical study examining the development of drug resistance in prions. (Jiali Li*, Shawn Browning*, Sukhvir P. Mahal, Anja M. Oelschlegel, Charles Weissmann, Science 12 Feb 2010:Vol. 327, Issue 5967, pp. 869-872.)
The study suggests that “Prions can develop large numbers of mutations at the protein level and, through natural selection, these mutations can eventually bring about such evolutionary adaptations as drug resistance, a phenomenon previously known to occur only in bacteria and viruses.” However, these mutations may be more correctly described as “mistakes” in the way the prion protein is folded. This is all reminiscent of the the idea of a “quasi-species.” An example is an RNA-only virus population, which originally had only one sequence, but was constantly creating mutations and eliminating the unfavorable ones. In both semantic and conceptual terminology we might include the idea of a “quasi-population” where it begins with a single particle, but it becomes very heterogeneous as it grows into a larger population.
This has an almost science fiction feel to it, but the concept of two different origins of sexual reproduction is intriguing, especially when a true metazoan appears to have abandoned sexual reproduction but still maintains a successful evolutionary history. And to top it all off, Darwinian evolution may not require living organisms, although it does require reproduction.
As we move to explore distant planets, the nature of “life” and how it develops, may not be much like what we see here on our planet. The results also make it clear that life does not have to begin before Darwinian evolution can take place. All that is required is for organic molecules to link together to become compounds and be able to reproduce.