Can Genetic Loss Increase Diversity?

The only example of reproductive isolation that is not dependent, fundamentally, upon differences between alleles present in divergent subpopulations of a single species, is the case of wolbachia infection in arthropods.Wolbachia is a bacterium that infects arthropods such as insects, spiders, isopods, etc. The infection has a variety of interesting effects on its carriers (including such things as biasing the sex ratio of offspring, transforming individuals into hermaphrodites, and other strange things varying from species to species) but as far as speciation is concerned there are two common effects of wolbachia infection that are of interest.First, an infected male is unable to fertilize an uninfected female. Second, if both male and female are infected, they must be infected with the same strain of the bacterium if fertilization is to be successful. Imagine a single species of insect, with two neighboring population A and B. If A gets infected and B does not, the result will be severely diminished gene flow between the two populations, because while males of population B can mate with members of either population, males of population A can only mate with members of their own population. The exact amount of gene flow reduction will depend upon patterns of migration between the two populations. Now if population B gets infected with a different strain of wolbachia, reproductive isolation will be complete, and in the long term speciation could follow.The exact mechanism whereby wolbachia effects these changes to mating success are currently unknown. We know that the bacterium does not cause direct changes to the DNA of its host because the effects can be removed by treatment with antibiotics. An overview of some possible mechanisms can be found at this articleThe bacterium has been demonstrated to be the sole source of reproductive isolation between two species of parasitic wasp, Nasonia giraulti and Nasonia longicornis (see this article). So it does really happen in nature.As far as I know, cytoplasmic incompatibility caused by wolbachia is the only known source of reproductive isolation that does not depend on mutations to the species' genome itself. Whether it qualifies as "speciation without mutation" is a different matter. Obviously the phenomenon requires different strains of wolbachia to exist if it is to generate complete reproductive isolation, and these different strains could only arise through mutation of wolbachia. So you might get speciation in an insect, driven by mutation of its wolbachia parasite rather than by mutation of the insect itself. Mutation is still at the bottom of it all.Hope this is of interestMick

That sounds like a testable hypothesis to me!Here is some data on the level of microsatellite heterozygosity in 20 dog breeds. First comes the name of the dog breed, followed by the heterozygosity. The data is from an analysis of 100 microsatellites in this article. Now, according to your theory, these breeds have undergone strong selection and multiple severe bottlenecks, therefore should exhibit considerably lower heterozygosity than wild canids such as wolves or foxes which have never been domesticated and have never been subject to human breeding programs in order to generate novel breeds.Studies of wild canids do NOT find higher levels of heterozygosity. Here is a summary of a few different articles (each species has a number of measured heterozygosity values, each one from a different location in the world):Gray Wolf: 0.421, 0.536, 0.605, 0.532, 0.593, 0.533, 0.547
Coyote: 0.540, 0.554, 0.653, 0.649, 0.596, 0.502
Red Wolf (captive): 0.507
Golden jackal: 0.412
Red fox: 0.7, 0.68, 0.72, 0.63, 0.56 reference 1reference 2So, your prediction fails. Your understanding of the population genetic consequences of bottlenecks and selective breeding is incorrect.Mickin edit: In order to preempt the complaint that these are microsatellite heterozygosity measures rather than allele counts, I would like to point out that allele diversity is correlated with heterozygosity, populations with high allele diversity tending to be more heterozygous.Edited by mick, : No reason given.