Data from: Can natural selection favour altruism between species?

Darwin suggested that the discovery of altruism between species would annihilate his theory of natural selection. However, it has not been formally shown whether between-species altruism can evolve by natural selection, or why this could never happen. Here, we develop a spatial population genetic model of two interacting species, showing that indiscriminate between species helping can be favoured by natural selection. We then ask if this helping behaviour constitutes altruism between species, using a linear-regression analysis to separate the total action of natural selection into its direct and indirect (kin selected) components. We show that our model can be interpreted in two ways, as either altruism within species, or altruism between species. This ambiguity arises depending on whether or not we treat genes in the other species as predictors of an individual's fitness, which is equivalent to treating these individuals as agents (actors or recipients). Our formal analysis, which focuses upon evolutionary dynamics rather than agents and their agendas, cannot resolve which is the better approach. Nonetheless, because a within-species altruism interpretation is always possible, our analysis supports Darwin's suggestion that natural selection does not favour traits that provide benefits exclusively to individuals of other species.

达尔文曾提出，物种间利他主义（altruism）的发现将推翻其自然选择（natural selection）理论。然而，学界尚未正式证明物种间利他主义能否通过自然选择演化，亦未阐明为何此类行为不可能出现。为此，我们构建了两个互作物种的空间种群遗传模型（spatial population genetic model），证实不加区分的跨物种互助行为可受到自然选择的青睐。随后，我们借助线性回归分析（linear-regression analysis）将自然选择的总作用拆解为直接作用与间接（亲缘选择（kin-selected））分量，以此探讨此类互助行为是否构成物种间利他主义。我们的模型可从两种视角解读：既可视为物种内利他主义，亦可视为物种间利他主义。这种歧义源于我们是否将其他物种的基因视为个体适合度（fitness）的预测因子，这等价于将这些个体视作行动者（施动者或受动者）。我们的形式化分析聚焦于演化动力学而非行动者及其行动意图，无法判定哪种解读方式更优。尽管如此，由于物种内利他主义的解读路径始终可行，我们的分析支持了达尔文的观点：自然选择不会青睐仅为其他物种个体提供益处的性状。