Animals and their parasites coevolve in an endless game of cat and mouse. As one escapes, the other finds new ways of attack. Of what long-term evolutionary consequence is all of this dodging and weaving? New research published in The American Naturalist by Camillo Berenos and colleagues at the University of Zurich in Switzerland provides evidence that it might cause the formation of new species.
How new species form remains an evolutionary puzzle. The classical view holds that new species emerge passively following geographic separation. Adaptation can play a similar role in speciation. Here, speciation occurs when some part of a population evolves along one adaptive path while another takes a different route. But what causes these adaptive splits to begin with? Berenos and colleagues tested the idea that parasites send hosts scattering down different paths of resistance. As a consequence, because these separate paths are mutually incompatible, the host populations become less able to interbreed.
Using an experimental approach where evolution is followed in real time, the researchers forced the flour beetle, Tribolium castaneum, to coevolve with its parasite, the microsporidian Nosema whitei, for 17 generations in the laboratory. Although only a brief evolutionary period, it was sufficient to drive extensive changes in the beetle host. And crucially, these changes were specific to the presence of the parasite. In each of the six coevolved lineages, the rate of parasite-induced mortality dropped by nearly twofold, while resistance in parasite-free populations remained unchanged.
Because these parasites are killers, it is unsurprising that beetles evolved increased resistance. More interesting is how these changes influenced mating success. When the researchers crossed coevolved resistant beetle lineages they found that they were happy enough to mate; there were no barriers to reproduction. However, fecundity in crosses between beetles from different coevolved populations declined by 16% compared with fecundity in crosses of beetles from the same population. This suggested that different modes of resistance had become fixed in the six coevolved lineages and that these separate solutions were genetically incompatible. Supporting this, the researchers found a significant positive relationship between the reduction in fecundity in between-population crosses and their level of evolved resistance. That is, lines that had evolved greater parasite resistance were more reproductively isolated from the others.
Populations separated by time or space will slowly drift apart genetically, and this can eventually lead to reproductive isolation – a precondition for speciation. What this study elegantly shows is that biotic factors can serve as a proxy for time and space. Like an arsonist's accelerant, parasites stoke the flames of genetic change, rapidly pushing hosts to different slopes of the genetic landscape. Pushed far enough, hosts become less able to interbreed. How often this scenario unfolds in nature is unknown. However, although mountains emerge to split populations only rarely, parasites are ubiquitous. This study shows that parasites not only kill and maim, but, through their actions, they may also be the wind in the sails of speciation.
