This blog post explores how parasitic and non-parasitic species coexist in equilibrium, based on Richard Dawkins’ The Selfish Gene and using ESS theory.
In his book The Selfish Gene, Richard Dawkins rejects the species-based concept predominantly used in the scientific community and educational systems when discussing biological evolution. Instead, he views the living system centered on genes. In this book, he mentions cuckoos, which lay their eggs in other birds’ nests, stating this is an act that increases the chances of gene survival. Parasitic birds exploit the fact that their hosts exert significant effort to raise genetically similar offspring, enabling them to propagate their own offspring and preserve their genes with minimal effort.
According to Dawkins’ theory, the selfishness of genes drives individuals to spread their genes as widely as possible. However, this raises a question. From an objective standpoint, the behavior of brood parasites is clearly far more advantageous for individual survival than that of non-parasitic birds. Yet, ironically, not all birds engage in brood parasitism. According to Dawkins’ theory, the selfishness of genes should compel all birds to adopt the survival-advantageous strategy of ‘brood parasitism’. However, this phenomenon is only observed in some brood parasite species; brood parasitism does not spread as a uniform behavior across the entire avian population. This can be seen as a phenomenon that contradicts his theory when viewed from the perspective of the selfishness of genes. To explain this logically, let’s examine the ESS (evolutionarily stable strategy) theory mentioned in the book and understand its relevance.
ESS theory describes a strategy that cannot be surpassed by other strategies when adopted by the majority of members within a population. Since following the strategy adopted by the majority within the population offers the greatest survival advantage, individuals conform to the existing strategy rather than adopting an alternative one. Let’s examine ESS theory in more detail. Suppose a population contains two distinct groups with different traits: hawks, who are fierce and assertive, and doves, who are gentle and pragmatic.
If the entire population consists solely of hawks, all individuals will fight until they are severely injured, resulting in significant average losses for each individual. However, if a dove appears, adopting the dove’s strategy becomes more advantageous and increases the survival probability compared to the hawks who fight until they are injured and suffer losses. This disrupts the genetic equilibrium. Conversely, if all individuals in a population adopt the dove strategy, injuries from conflicts between individuals would not occur. However, if an individual with the hawk strategy appears in this population, it would gain an advantage over the doves and reap significant benefits. This would naturally increase the number of individuals adopting the hawk strategy, again disrupting the equilibrium. Therefore, for the overall benefit and stable survival of the entire population, individuals do not simply act on their genetic self-interest by leaning toward one strategy. Instead, they calculate the optimal environment and balance the number of individuals following each strategy. The aforementioned hawk and dove strategies are most advantageous for the survival of the entire group when they exist in a ratio of approximately 5:7, so most populations tend to follow this ratio. Examining this process reveals that the ESS strategy emerged and stabilized through the genetic selfishness of each individual.
While the book explains ESS solely as an intraspecific phenomenon, extending this concept to interspecific interactions could provide significant clues for understanding cuckooing. Therefore, by applying ESS to interspecific relationships, we aim to explain not only the selection and behavior of cuckooing birds but also those of non-cuckooing birds.
First, let’s divide birds into cuckooing birds and non-cuckooing birds. If all birds were non-parasitic, each individual would reproduce independently, with little direct impact on egg hatching and offspring survival. However, if parasitic birds appear and engage in parasitism, the benefits they gain are substantial, and their survival ability improves, leading to an increase in the number of parasitic birds. Conversely, if all birds were brood parasites, non-parasitic birds would suffer because brood parasites do not build nests or raise eggs directly, preventing them from laying eggs and reproducing. In this scenario, if non-parasitic birds appear, they can build nests and successfully reproduce their own offspring, giving them a survival advantage over brood parasites. Therefore, the non-parasitic birds, being more advantageous for survival and reproduction, would increase in number.
The equilibrium between brood parasitic and non-brood parasitic birds is only stable when both strategies coexist. This can be seen as the result of behavior patterns, stemming from the selfishness of genes, diversifying into various strategies under the pressure of natural selection. For example, while brood parasitic birds benefit from laying eggs in other birds’ nests, if all birds became brood parasitic, the ecosystem would descend into chaos. Conversely, non-brood parasitic birds raise their own offspring, maintaining ecosystem balance by tolerating the presence of brood parasites.
Therefore, expressing an individual’s genetic selfishness does not necessarily mean unconditionally engaging in brood parasitism; rather, it involves achieving an appropriate balance between brood parasites and non-brood parasites based on the ESS. The initial problem raised regarding interspecific selfishness observed in brood parasitism was that if brood parasitism is genetically advantageous, the behavior of non-parasitic birds cannot be interpreted from the perspective of genetic selfishness. However, applying ESS theory to interspecies interactions revealed that non-parasitism also reflects the characteristics of selfish genes, as it favors the survival of the species.
Furthermore, the interaction between brood parasitic and non-parasitic birds demonstrates how the selfishness of genes operates not only within a population but also between populations. This serves as a crucial example illustrating how the selfishness of genes, as argued by Dawkins, is not merely a survival strategy at the individual level, but how it drives evolution and adaptation within a broader ecological context.
From this perspective, Dawkins’ ‘The Selfish Gene’ theory clearly explains how genes maximize their own survival and reproduction through individual behavior. Simultaneously, combined with ESS theory, it provides crucial insights into understanding the diversity of genetic strategies and the resulting ecological equilibrium. This is an essential element for deeper comprehension of life’s complex evolutionary processes, offering a useful framework for exploring various aspects of natural selection.
In summary, Dawkins’ theory makes a significant contribution to explaining the behavior and evolution of living organisms from the perspective of genes. Its explanatory power is further enhanced through its integration with ESS theory. This provides important implications for biological research and education, demonstrating that it is an essential concept for understanding the complexity of life.
