I've mentioned before how, as two diverging nascent species each evolve specialisation, hybrids will often be at a disadvantage being good an neither of their parent's specialities. As isolated and diverged populations of finches came back together, at the end of the last Ice Age for example, one might have evolved a long thin beak for eating small seeds and the other a short stout beak for eating large hard seeds. A hybrid with a long stout beak or a short thin one would find feeding difficult so it would be in the interests of both sets of genes to set up barriers to interbreeding.
The very fact that they are diverging accelerates the process of speciation, not as an end in itself but to protect the advantageous genes from being wasted on hybrids which have a lower probability of passing them on (see Why Species?, Creationist's Macro-Evolution Lie and More Mimetic Evolution).
We now have a rather surprising, even shocking example of one such barrier in closely related nematode worms.
Conflict between the sexes over reproductive interests can drive rapid evolution of reproductive traits and promote speciation. Here we show that inter-species mating between Caenorhabditis nematodes sterilizes maternal individuals. The principal effectors of male-induced harm are sperm cells, which induce sterility and shorten lifespan by displacing conspecific sperm, invading the ovary, and sometimes breaching the gonad to infiltrate other tissues. This sperm-mediated harm is pervasive across species, but idiosyncrasies in its magnitude implicate both independent histories of sexually antagonistic coevolution within species and differences in reproductive mode (self-fertilizing hermaphrodites versus females) in determining its severity. Consistent with this conclusion, in androdioecious species the hermaphrodites are more vulnerable, the males more benign, or both. Patterns of assortative mating and a low incidence of invasive sperm occurring with conspecific mating are indicative of ongoing intra-specific sexual conflict that results in inter-species reproductive incompatibility.
The sexes have divergent reproductive interests, and conflict arising from this disparity can drive the rapid evolution of reproductive traits and promote speciation. Here we describe a unique reproductive barrier in Caenorhabditis nematodes that is induced by sperm. We found that mating between species can sterilize maternal worms and even cause premature death, and we were able to attribute this phenomenon directly to the sperm themselves. Sperm from other species can displace sperm from the same species and, in some cases, can invade inappropriate parts of the maternal reproductive system and even their non-reproductive tissues. We find that mating to males of another species harms females far more than does within-species mating. Overall, our observations are consistent with ongoing sexual conflict between the sexes within species, arising as a byproduct of sperm competition among the gametes of different males. Finally, patterns of assortative mating indicate that mating behaviours that reduce the likelihood of costly inter-species mating have evolved in this group of animals. These findings support an important role of sexual selection and gametic interactions contributing to reproductive boundaries between species, as predicted by evolutionary theory.
Janice J. Ting, Gavin C. Woodruff, Gemma Leung, Na-Ra Shin, Asher D. Cutter, Eric S. Haag;
Intense Sperm-Mediated Sexual Conflict Promotes Reproductive Isolation in Caenorhabditis Nematodes;
PLOS Biology, 29 July, 2014. DOI: 10.1371/journal.pbio.1001915
Interestingly, this gruesome barrier to hybridisation seems to have evolved out of a more readily understandable conflict between the sperms of different males of the same species, so that females had to evolved robust reproductive organs to reduce injury, and possibly driving evolution towards hermaphroditism. When some species became hermaphrodite the need for this layer of protection was removed and lost, so rendering hermaphrodites liable to damage if they mate with males from related species.
As someone once said, and I can't find out who, "Remember, Natural Selection is a lot cleverer than you!". This would seem to be especially true of creationists.
Intelligent design solution to the killer sperm phenomenon, anyone?
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