The drive of selection
Conceptually, small populations with low genetic diversity are thought of as having increased risk of extirpation due to the emergence of deleterious or maladaptive alleles. Even medium sized populations with low diversity in genetic material are often modeled as small populations. There is a paradigm that small populations require increased genetic diversity to sustain through demographic pressures over time.
Evolutionary rescue
The idea of evolutionary rescue (ER) seems to offer an alternative or more robust / complex view of the stability and viability of populations. Simplified, ER is the concept that there can be an event where a population or species reaches near extinction but is brought up in an heroic last minute change in viability direction and is saved via a rare set of alleles - or just rare allele(s)? Note: please excuse a bad or ignorant definition…
The Scope of Risk
However, ER still nests smoothly in the conventional paradigm sense no new conceptualization is needed. I would like to present the idea of the scope of risk as a function of genetic material and ecosystem parameters in which the selecting factors reside. In other words, the risks associated with selection from e.g. disease spillover events, anthropogenic factors, etc. are constrained within whatever genetic parameters, from evolutionary processes, and ecosystem parameters, from ecological time, offer. Although the results of these processes seem limitless - with nature’s ability to turn simple genetic materials into the complex structures of humans, language, incredibly colored birds, and so on - there are actual a finite number of possibilities for the next steps in evolution. The myriad of potential is still finite, nonetheless!
I like to use this example to understand the incredible result of natural processes (and perhaps anthropogenic as well) on the Channel Island foxes. A paper by Robinson et al. 2016 demonstrated unprecedented low genetic diversity for any eukaryote as well as demographically stable low population sizes in the subspecies of these foxes. The paradigm of small population size needing increased genetic diversity is at least called into question. It appears to me that a move complex behavior is occurring in the natural world. Although I do not provide a solution to this apparent gap in knowledge at this time, I am proposing a slight deviation in the way we conceptualize aspects of the natural world in genetic popoulation ecology and evolutionary models.
Where does selection act?
Selection acts within instantaneous snapshots of the genetic material (evolutionary result) and ecosystem material (ecological result) in the lineage of the species / population in question. Further, selection is a process that can only modify and shape the genetic material for the next generation based on the available material in the current generation. A binary operator, such as some abstract genetic material in a population of fox, can not be influenced via selection to obtain a third state or value (note: please excuse influence of programming logic). The next generation may only be able to be influenced to have a higher or lower probability of 0 or 1. Thus, there is only and always a relative risk based on the evolutionary and ecosystem scope. The scope of the binary operator is 0 or 1 and the ability to influence the release of those values into the next generation.
The scope of risk is a function of the genetic and ecosystem material, which are both products of evolutionary time within a finite sandbox of life’s building blocks.