This blog post explores how the selfishness of genes and the ESS strategy can explain the phenomenon of brood parasitism. Can these two theories truly be connected?
In The Selfish Gene, Dawkins argues that genes are the core unit of evolution, referring to their inherent nature to replicate and spread widely as “selfishness.” He explains that this selfishness of genes has driven the evolution and proliferation of life. According to this theory of the selfish gene, all living organisms are merely machines for preserving and propagating genes.
Dawkins also contends that when organisms compete for resources, they follow Evolutionarily Stable Strategies (ESS). When conflicts arise within or between species for gain, the strategy chosen can be disadvantageous or advantageous relative to the opponent’s strategy. Within a species, the ESS that maximizes the average benefit per individual is implemented. In interspecies competition, strategies following an ESS are also highly likely to be selected. If an individual emerges within an existing ESS system possessing a strategy that offers an advantage in survival and reproduction over others, the proportion of ESS within that population may change. Dawkins states this phenomenon can be explained at the genetic level.
Reading this book raised questions: Could ESS strategies be linked to explain cuckoo parasitism? Could ESS strategies be extended to interspecies problems? While the book’s mention of cuckoo parasitism is brief, it presented a topic of considerable discussion value for our group. Indeed, from the perspective of the host, explaining cuckoo parasitism through the lens of gene selfishness proved difficult. The text explains that cuckoos exploit a blind spot in the maternal instinct to parasitize nests. This made me wonder why genes carrying such a flaw weren’t eliminated, and why special individuals didn’t emerge through mutation. If genetic selfishness were fully realized, one might expect all birds to engage in parasitism, yet reality doesn’t reflect this.
The ESS strategy, or Evolutionarily Stable Strategy, is treated in the book as a concept applicable only within a species. However, given that it is a strategy stabilized by the selfishness of individuals, I wondered if it could be extended to an interspecies concept. Although my biological knowledge is limited, I also questioned why non-parasitic birds do not engage in brood parasitism and instead incubate other birds’ eggs. Furthermore, the author uses numerical values to assess the effectiveness of strategies in various examples, such as the Prisoner’s Dilemma, ESS in mating, and others. However, the criteria for determining these values were unclear, which was disappointing. While the attempt to explain phenomena within the biological world using existing theories was meaningful, I felt there were clear limitations.
Therefore, I aim to focus on these aspects and expand ESS theory to explain the interspecific phenomenon of brood parasitism. The phenomenon of brood parasitism, characteristic of some birds, appears to align with Dawkins’ theory of the selfish gene. Brood parasites lay eggs in other birds’ nests, allowing them to focus more on laying eggs and thus leave behind a larger number of offspring. This seems consistent with the nature of genes seeking to increase their own survival probability. However, if brood parasitism benefits the parasites, other birds should also engage in it according to the selfishness of genes. The fact that they do not suggests that the selfish gene theory alone cannot fully explain brood parasitism.
One solution to this problem is the ESS strategy, which this book explains applies only within species. However, since ESS represents a strategy stabilized by individual selfishness, it seems applicable between species as well. Therefore, I formulated the hypothesis that “introducing ESS can explain the phenomenon of brood parasitism” and sought evidence. Because ESS aims for strategies stabilized by individual selfish tendencies, it is highly likely that strategies stabilized by selfish tendencies can also form between species. Cuckoo parasitism is a result of the cuckoo’s selfish tendencies, so it can be explained by ESS. While Dawkins’ explanation alone struggled to account for the existence of non-parasitic birds, if ESS can be applied interspecifically, it could explain why non-parasitic birds exist in nature.
In ‘The Selfish Gene’, it is explained that genes determine whether to fight in competition through strategic cost-benefit calculations, avoiding unnecessary competition by waiting for opportunities or avoiding fights. Maynard Smith posits that different strategies coexist within populations until an evolutionarily stable strategy (ESS) emerges. According to this theory, stable strategies emerging from population conflicts are preserved by natural selection, while individuals exhibiting deviant behavior are eliminated.
As a concrete example, consider a population with two groups: ‘hawks’ and ‘doves’. Hawks are combative and unwilling to yield to others, while doves are peaceful. Assuming that in combat, the winner receives 50 points, the loser 0 points, the seriously injured -100 points, and time wasted -10 points, and that these points are proportional to gene survival, a population entirely composed of doves would have no fights. Due to time wasted, the winner gains +40 points and the loser loses -10 points, resulting in an average score of +15 points.
Next, if a hawk appears in this population, the hawk defeats all doves to gain +50 points, while the doves avoid fighting and gain 0 points. In this case, the population’s average score becomes +25 points. If all individuals are hawks, the winner gains +50 points, but the loser suffers serious injury and loses -100 points, resulting in an average score of -25 points. Ultimately, the most stable strategy is achieved when hawks and doves maintain a 5:7 ratio.
Now, let’s extend this strategy to an interspecific situation. Birds can adopt two strategies for laying eggs: ‘parasite’ and ‘non-parasite’. The brood parasite lays eggs in other species’ nests, while the non-brood parasite incubates both its own eggs and those of the brood parasite. Scoring is set as follows: if the brood parasite’s egg hatches, +100 points; if the non-brood parasite’s egg hatches, +30 points; if neither egg hatches, -20 points.
In an ecosystem where all members are non-brood parasitizers, each gains 30 points, resulting in an average score of +30 points. When a brood parasitizer appears, it gains 100 points while non-brood parasitizers gain 30 points, raising the average score to +65 points. Conversely, in an ecosystem where all are non-brood parasites, repeated egg-laying results in an average score of -20 points. Therefore, an optimal balance is achieved when brood parasites and non-brood parasites maintain a certain ratio.
From this, we can conclude that optimal results occur when brood parasites and non-brood parasites exist in a fixed ratio. The reason non-brood parasitic birds incubate other birds’ eggs can also be seen as a form of ‘selfish behavior’ driven by genetic selfishness. Alternatively, it could be the result of brood parasitic and non-brood parasitic birds diverging and evolving within the same species, stemming from a common ancestor. Thus, the logic of ESS can be seen as applying even between species, leading to speciation.
