Department of Biological Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
J Theor Biol. 2009 Aug 21;259(4):744-50. doi: 10.1016/j.jtbi.2009.04.023. Epub 2009 May 4.
Mutualism is ubiquitous in nature and is thought to have played a key role in the history of life. However, how mutualism could evolve despite being prone to unilateral exploitation is a puzzling question in evolutionary ecology. Some theoretical studies have shown that spatial structure of habitat can facilitate the emergence and maintenance of mutualism. However, they are based on the simple assumption that the trait in question is discrete: each individual is either a mutualist or a non-mutualist. In this article I develop a simple simulation model of coevolution of facultative symbiosis using a one-shot continuous Prisoner's Dilemma game to investigate the evolutionary dynamics of mutualism between two species. In this model I assume continuous traits for both species from -1 (fully deceptive) to 1 (fully cooperative). The habitat has a dual-lattice structure, each layer is inhabited by one species. Interspecific interaction is restricted between two corresponding sites of the two layers. Without limitation on the magnitude of a single mutation, I find that mutualism can arise and persist when the intrinsic reproduction rate is low (but is above a threshold) and the benefit/cost ratio of the cooperative strategy is large, which is consistent with Yamamura et al. [2004. Evolution of mutualism through spatial effects. J. Theor. Biol. 226, 421-428]. In these cases, extreme antagonism often evolves starting from a neutral population that seems nearly stable, but once mutualism arises, the cooperative individuals quickly increase and both the populations eventually become mutualistic on average, although they are polymorphic. However, when the effect of a single mutation was limited to be small, extreme antagonism is much likely to dominate unless the intrinsic reproduction rate is low. When only one species is allowed to evolve, mutualism arises when the initial strategy of the other species is cooperative. Otherwise, excessive deception evolves in the former, and the latter often becomes driven to extinction.
互利共生在自然界中无处不在，被认为在生命历史中发挥了关键作用。然而，尽管互利共生容易受到单方面的剥削，但它是如何进化的，这在进化生态学中是一个令人困惑的问题。一些理论研究表明，栖息地的空间结构可以促进互利共生的出现和维持。然而，它们基于一个简单的假设，即所讨论的特征是离散的：每个个体要么是互利共生者，要么是非互利共生者。在本文中，我使用一次性连续囚徒困境博弈来开发一个简单的共生关系的协同进化模拟模型，以研究两种物种之间互利共生的进化动态。在这个模型中，我假设两个物种的特征都是连续的，从-1（完全欺骗）到 1（完全合作）。栖息地具有双重晶格结构，每个层都由一个物种居住。种间相互作用仅限于两层的两个相应位置之间。在没有单个突变幅度限制的情况下，我发现当固有繁殖率较低（但高于阈值）且合作策略的收益/成本比较大时，互利共生可以出现并持续存在，这与 Yamamura 等人的结果一致。[2004. 空间效应导致互利共生的进化。J. 理论生物学 226, 421-428]。在这些情况下，极端的对抗性往往从看似几乎稳定的中性种群中进化而来，但一旦互利共生出现，合作个体就会迅速增加，最终两个种群平均都成为互利共生的，尽管它们是多态的。然而，当单个突变的影响限于小幅度时，除非固有繁殖率较低，否则极端的对抗性很可能占主导地位。当只有一个物种可以进化时，另一个物种的初始策略是合作时，互利共生就会出现。否则，前者会进化出过度的欺骗行为，后者往往会被推向灭绝。
