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Hybridization, Hennig, and the Species-as-Individuals Hypothesis

Ian Smith

In a recent work (Book X), I argued that currently existing sexual species can flourish, and that their flourishing is the basis of their intrinsic value. In that work, I appealed to the Hennigian Species Concept (HSC) along with the Species-as-Individuals (S-A-I) hypothesis in understanding what species are, for understanding what species are will help us to understand how they could flourish. Under the HSC, species are reproductively isolated populations or metapopulations. When they speciate, they create two new daughter taxa, and this process thereby renders the parent species dissolved. So, species persist until they dissolve or go extinct.

In that work, I mentioned the problem of hybridization, which is a great example of how lineages can supposedly converge on one another. I dismissed the problem of hybridization by arguing in the following way: Suppose that two sexual “species” hybridize such that the organisms of such “species” sexually produce a new sexual species (whose organisms are fertile) that happens to be reproductively isolated from the two parental “species.” This would presumably be speciation not based on a split in lineages, but based on a convergence among lineages to form a new species. In other words, what would appear to occur here is reticulation in the phylogenetic species tree rather than divergence, thus appearing to violate the logic of the phylogenetic species tree. Those who write on this kind of speciation tend to assume that the two “species” are truly of different species. But if we do not view the two parental populations as of different species, but instead understand them to be of the same species, then there is no reticulating problem. The correct tree, on the HSC, should have these two populations in the same species lineage. Why is this the case? The two populations, in hybridizing and producing organisms that are fertile, are not reproductively isolated. They cannot logically be reproductively isolated if they have been able to hybridize and produce organisms that are fertile.

I now realize that the conjunction of the HSC and the S-A-I hypothesis in light of the hybridization problem leads to potential inconsistency. The populations mentioned above that “hybridize” cannot easily be seen as part of the same individual, prior to the “hybridization.” As David Hull argued, metapopulations that are individuals are considered as such (partly) because they occasionally exchange individual organisms. But populations that do not occasionally exchange individual organisms cannot be seen as individuals. He uses an analogy with two drops of mercury to illustrate: as long the two drops of mercury do not come together, they are not one drop. That they could be viewed as potentially of the same drop of mercury does not of course mean that they are at the time of being separate. Accordingly, it does not appear that the two populations in the above passage can be considered as part of the same species prior to the “hybridization,” assuming we accept the S-A-I hypothesis, since they are not yet exchanging organisms.

The hybridization problem is not just a problem with the conjunction of the S-A-I hypothesis and the HSC, however. The problem is deeper, for it affects the view that there is a phylogenetic tree that correctly represents the evolutionary history of speciation (or even whether there is more than one tree that could represent the evolutionary histories of speciation). Phylogenetic systematists could just deny that hybridization occurs by explaining it away, as I have attempted to do. But they may not be able to do so very easily.

This project will thus consider if it is indeed the case that true hybridization is inconsistent with phylogenetic systematics. It will also consider the narrower question for my view, whether the conjunction of the HSC and the S-A-I hypothesis is tenable, considering the hybridization problem.

Last Updated: 5/5/22