This is not the promised post about vertebrates who don’t use haemoglobin (yet). Instead, “red queening” came up as an expression, and it inspired me to want to write about the Red Queen hypothesis in science. The hypothesis gets its name from the Red Queen of Alice Through the Looking Glass, by Lewis Caroll: in her world, she says, “it takes all the running you can do, to keep in the same place”. The Red Queen hypothesis is an evolutionary theory, suggesting that because all species are constantly evolving, all species are also constantly evolving relative to each other. For example, a predator species gains an advantage by evolving traits allowing it to run faster, but the prey species evolves to negate it by evolving traits allowing it to run even faster. The net result is the maintenance of the status quo.
(Or perhaps more likely, though less simplistically, the prey species evolves traits allowing it to hide from the predator better, or to fight back. Regardless, the point is the net result.)
This hypothesis has actually been used to build other hypotheses further down the line: one of particular interest to me is the hypothesis by Hartung and Bell, suggesting why such a disadvantageous trait as sexual reproduction has evolved and persists. First, it’s important to understand why sexual reproduction is disadvantageous. In the end, the best outcome for every gene in an organism is to survive in that organism’s offspring.
Sexual reproduction, however, splits up traits, jumbles them around, and creates offspring significantly different to both parents. It can even split up traits that that benefit each other — for example, if a particular kind of skin pigment provides excellent camouflage, but tends to lead to runaway cancer in organisms carrying the gene, it can be mitigated by the presence of a tumour suppressor gene. But if mated with another organism of the same species, it’s entirely possible for the offspring to receive the gene for the pigment and not the tumour suppressor gene — leading to a dead end for those genes as the offspring dies young without reproducing.
It also takes effort. You have to find a mate, and in many species you have to court that mate. Then you have to ensure the survival of the offspring, or invest resources into creating many many gametes. By contrast, asexual reproduction is incredibly efficient. The new organism contains all the genes of the old. From a gene’s eye view, that’s great.
So what makes sex a good idea for a species? Really, it’s the same thing that makes it not a good idea from a gene’s eye view: it introduces variation. Hartung and Bell specifically suggest that sex has evolved to combat parasite infection. An asexual organism’s offspring is identical to the parent — so a parasite perfectly adapted to the parent organism is perfectly adapted to the offspring. Each generation formed by sexual reproduction, though, has a slightly different mixture of genes, preventing the parasite from perfectly adapting to the host species and then just infecting it over and over.
There is some proof of this hypothesis in action, which is always pleasing: genes controlling the immune system are the quickest to evolve, and we know in humans that immune systems get a lot of variation in each generation through the different combinations of the parents’ genes. An actual controlled experiment by Morran et al using C. elegans and a parasitic bacterium used the fact that C. elegans can participate in both sexual and asexual reproduction — populations that were only allowed asexual reproduction were maintained alongside populations that were only allowed sexual reproduction. The sexual reproducers stayed on top of the bacterial infection, while asexual reproducers were driven extinct — rapidly.
Main source: Morran, L., Schmidt, O., Gelarden, I., Parrish, R. and Lively, C. (2011). Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex. Science, 333(6039), pp.216-218.