To anyone who accepts
Occasionally scientists do find a parthogenetic clone competing with its ancestral sexual species. Usually the clone is found to be steadily outcompeting the ancestral species. So why have sexual species survived and prospered?
To give a specific example, consider a lake populated by a million fish of one species that lives for only one year, and in which fathers do not help raise offspring. Every year, half the fish (the females) lay eggs. For the population to be stable, on average two eggs from each female must be fertilized and successfully reach adulthood.
Now suppose one egg is a mutant. It survives and becomes an adult parthogenetic female. There is no reason to believe that the survival rate of this female’s eggs will be different, so next year there will be two parthogenetic females, genetically identical to their mother, or in other words the beginning of a clone.
Just to keep the arithmetic simple, let us make the unlikely assumption that the lake expands just quickly enough to keep the number of sexually reproducing fish constant at one million. Under these circumstances, the number of parthogenetic fish will double every year. After twenty years, there will be one million parthogenetic fish, the same as the number of sexual fish.
To continue our arithmetically simplifying assumptions, let the lake shrink at just the rate required to increase competition to the point that on average only one egg from each female fish survives. Now the number of parthogenetic fish will stay constant, but the number of sexual fish will be cut in half every year, until after another twenty years there is only one sexual fish and the lake has returned to its original size. At this point, the sexual species is doomed.
If the size of the lake remained constant, or changed randomly, the proportion of parthogenetic fish would be the same, and so the same result would be achieved. For a larger population, more generations would be needed to eliminate sex, but this analysis would seem to indicate that all sexual species are doomed within a few hundred generations of a viable parthogenetic mutant appearing. Yet sexual reproduction has continued to thrive for at least two billion years. Some other factor must be present.
In our fishy example, sexual reproduction would seem to disappear after forty years. Whatever factor causes sex to survive must destroy clones with great certainty, and within forty years. What factor can be so powerful?
The only factor that I can imagine eliminating clones with such efficiency is disease. Every successful species acquires its retinue of deadly parasites, from worms to bacteria. Let us focus on bacteria, for example. Bacteria divide (parthogenetically) two or three times an hour (viruses reproduce even more rapidly), so during the course of a year (one fish generation) there will be something like 25,000 bacterial generations. Each time a bacterium divides there is a significant chance of mutation, because bacteria copy their DNA less carefully than higher organisms.
While the bacteria are infecting a single fish, most mutations will be disadvantageous (i.e. the mutated bacterium will either fail to reproduce, or will reproduce more slowly) but there will be an occasional beneficial mutation that will actually speed up reproduction in the specific environment of that individual fish. If it takes a month for an infection to kill a fish, that is approximately 2,000 bacterial generations. If there is a beneficial mutation every 200 generations, then by the time the fish dies, the most rapidly reproducing bacteria will have accumulated ten mutations helping them prosper in that particular fish.
To outlast the individual fish, the bacteria must have the capability of leaving that individual and infecting another. In a sexual species, the bacterium with ten mutations will find itself in a genetically different fish, in which the very mutations that helped it in its prior home may now be an impediment to survival and reproduction. Before the bacteria manage to shed these mutations and adjust themselves to this new environment, the fish’s immune system may destroy them. If both fish are members of the same clone, on the other hand, the bacterium entering the new fish will find itself in exactly the same environment as it previously prospered in, and should have no difficulty in multiplying rapidly and ultimately killing this fish.
The more numerous the clone becomes, the higher the concentration of bacteria specifically bred to infect and destroy them. It seems almost inevitable that the clone will be completely eliminated well within the 40 years (one million bacterial generations) that it would otherwise take for the clone to destroy the ancestral sexual species.
Problem solved. Sex is here to stay. Perhaps we should be grateful to bacteria, and to pathogens generally!