Breeding populations as “biological individuals”

In order to define species as individuals, Sober takes the first step to argue that breeding populations are “biological individuals”. By this he means, breeding populations, “a set of local demes linked to each other by reproductive ties but not so linked to demes outside the set” (156), can be defined as individuals.

Sober writes that the key of individuality is “causality” (153), instead of similarity. In order to define an individual, we have to consider the existence of synchronic and diachronic interrelations. Synchronic interrelation means that the units in the individual group should have causal interrelations during the same period of time, such as when the cells in your body cooperate together. Diachronic interrelation means that during different periods of time, different units should be linked together by the “casual processes of ontogeny” (154). The units should have a kind of “ancestor/decent relationship” (156), such as when all the cells in your body grow from the very first embryo cell. Moreover, to define an individual, we also have to distinguish when the individual ceases to exist, which means the individual should have a precise “life-death

According to Sober, breeding populations can be considered “individuals”, since the “earmarks” of individuality can be applied to breeding populations (157). Breeding populations have reproductive links within the group, which is a synchronic interrelation. And they have ancestor/decent relationship because of the reproductive links. This is the diachronic interrelation. Breeding populations emerge when one breeding population buds off from a parent population, cease to exist when all member in the population “die or fall to reproduce.” This meets the requirement of life-death cycle.

I agree with Sober’s claim that breeding populations are individuals. Sober’s definition of “biological individuals” is fairly solid, and breeding populations do indeed fall into his definition of “biological individuals.” However, there will be some disagreement on this claim. I will discuss about these critics in following passages. When defending Sober’s claim, I will pay attention to avoid the discussion of “species” here, and focus on Sober’s definition of individuality.

If a breeding population separates from the parent population at certain time, according to Sober’s definition, we should say a new individual population emerges. But can we rashly make this conclusion? For instance, if these two populations reintegrate later, can we still be so confident about our previous judgment? The answer is yes. The occurrence of reintegration does not affect the fact that two breeding populations had existed as two individuals. The German reunification in 1990 did not deny the fact that, when West Germany and East Germany separated, they had been existed as two independent countries. The property of individuality is based on the time period we observe the breeding population.

Other possible critics may argue that the extent of individuality is rather arbitrary. To what extent can a group of breeding population be counted as an individual? For example, we assume that all gorillas in one continent have the possibility to interbreed with each other. If we define all gorillas on this continent as a single individual, how can we define the smaller troops of gorilla? In fact Sober did answer this question. He said that “whether a given collection counts as one individual or as many will depend on the magnifying power of lens we use” (158). When saying breeding populations are individuals, we do not deny the relation between each population. As long as breeding populations meet Sober’s definition of “biological individuals”, they can be grouped as bigger “individuals”. 

There are some controversies when we divide populations into individual according to the criteria of “interbreeding”. Consider this example: we assume that a group of gorilla is genetically identical and breed from same group of ancestor, but they have reproduction isolation. For instance, a portion of them would like to interbreed one month later than others. In Sober’s view, breeding isolation leads to the separation of two individuals. However, if these gorillas live together, forage together, and even form into a same troop under the same adult male leader, then they do have interrelation with each other, compared to the other groups that have a geographic isolation. Why cannot we define them as a single “biological individual”? In my opinion, these gorillas can be regarded as one single group, but the breeding barrier violates Sober’s biological consideration. Sober’s “biological individual” is based on the interrelations, especially breeding relations, among the organisms. The gene flow is the key to distinguish a single biological individual. In our gorilla example, even though they still live in the group, the breeding barrier will finally lead to the isolation in these gorillas’ social structure. The physical isolation will appear when the male leader gorillas no longer regard those female gorillas. On the longer time scale, these two groups may even develop physical breeding isolation. In this sense, breeding isolation is critical when we consider a group of organisms as “biological individuals.”