Hybrids

    Hybridization seems to be a tricky concept in evolutionary biology. It's an inarguable fact of nature but frameworks like the biological species concept force us to deny it in order to create artificial, human-serving categories. But why is it important? Even though it makes things messy for us, there should be some sort of sense to why it occurs in the first place, right? A beneficial mechanism of communities or perhaps just a happy coincidence of probability and biochemistry?

    While we maybe can't say whether it "exists for a reason" or not, we can definitely see that it has an impact on biological processes. It also presents some challenges in the conservation world (i.e., if you don't know what things constitute a species, how do you protect that species?). In terms of population genetics, hybridization can fix beneficial alleles and it can also dispose of detrimental ones (Quilodrán et al, 2020). So evolutionary, if another "species" has a good line of genetic coding that could benefit another closely related, geographically overlapping species, then when two individuals (or more) decide to make some babies then you instantly have children with a new beneficial allele (or dozens!), assuming they're not reproductively isolated by sterility. If these genes are beneficial enough, then they could quickly spread into the target "species" population via these interbreeding events and reproduction of the alleles by hybrids rearing more and more diluted-hybrid offspring. In simple terms I feel that this could act as a sort of evolutionary springing board: a slew of new genetic material is thrown out into the pool to be acted upon by selection, and if it's good enough, it explodes in the population. A quote from the same paper: "hybrids have the potential to exploit a unique ecology that is intermediate between those of the parental species." While this is in reference to a speciation of yeast, I think that if you extrapolate the information from that quote you can find the gold nuggets of interspecies breeding. Sometimes, it could actually be ecologically beneficial to lose the extremities of two parental species and instead act within an intermediate habitat. If this habitat is novel and made accessible by hybridization, then there's an instead potential for colonization and utilization of that resource the diversifies an ecological community. 

    Interestingly enough, this article also stresses how hybridization can be detrimental to biodiversity. More interesting, it seems to be when it's specifically "forced" anthropogenically, rather than coming about through natural overlap of allopatry. It seems that demographic decline (presumably from habitat loss) and interspecies breeding interact in a negative way to expedite extinction. Radically different allopatric species can bring maladaptive alleles into populations that are on an unwanted genetic extreme, if you will, as opposed to before when geographically related populations could offer genes that were beneficial because of their relatedness. Something known as "speciation reversal" can occur when these maladaptive traits effectively reverse local adaptations and eliminate ecological opportunity.

All in all, it appears that hybridization, like most things, is not good or bad in and of itself. Rather, depending on the context of the hybridization events, biodiversity can either be benefited or damaged according to the background of the interspecies breeding. 

Resources:

Harmonizing hybridization dissonance in conservation; Quilodrán et al. 2020

https://www.nature.com/articles/s42003-020-1116-9