Gómez José M
Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain.
Evolution. 2004 Jan;58(1):71-80. doi: 10.1111/j.0014-3820.2004.tb01574.x.
Most theoretical treatments of the evolutionary ecology of offspring size assume a simple and direct effect of investment per offspring on offspring fitness. In this paper I experimentally determine the relationship between seed mass and several main fitness components of the oak Quercus ilex, to estimate phenotypic selection acting on seed mass during the early life cycle and to discover any potential selective conflicts occurring between different stages from dispersal to establishment. I found a positive effect of acorn size on most fitness components related to seedling establishment. Large size increased germination rate and seedling survival, accelerated germination timing, and enhanced seedling growth. Nevertheless, there was also a direct negative effect of acorn size on survival to predation, because large acorns were highly preferred by the main postdispersal seed predators at the study site, wild boars and wood mice. Because of the low probability of escape from predation, the fitness of large acorns estimated on this component was significantly lower than the fitness of smaller acorns. Therefore, seed size affected fitness in two different ways, yielding opposing and conflicting selective forces. These findings suggest that the general assumption that offspring fitness is a fixed positive function of seed size needs to be reconsidered for some systems. The existence of conflicting selection might explain the occurrence of an optimal seed size in some plant species without invoking a seed number-size trade-off.
大多数关于后代大小的进化生态学的理论探讨都假定每个后代的投入对后代适合度有简单而直接的影响。在本文中，我通过实验确定了栓皮栎种子质量与几个主要适合度组成部分之间的关系，以估计在生命周期早期作用于种子质量的表型选择，并发现从扩散到定居的不同阶段之间可能存在的任何潜在选择冲突。我发现橡子大小对与幼苗建立相关的大多数适合度组成部分有积极影响。较大的尺寸提高了发芽率和幼苗存活率，加快了发芽时间，并促进了幼苗生长。然而，橡子大小对被捕食后的存活率也有直接的负面影响，因为在研究地点，野猪和林鼠等主要的扩散后种子捕食者非常喜欢大橡子。由于逃脱捕食的概率很低，基于这一组成部分估计的大橡子的适合度明显低于小橡子的适合度。因此，种子大小以两种不同的方式影响适合度，产生了相反和冲突的选择压力。这些发现表明，对于某些系统而言，后代适合度是种子大小的固定正函数这一普遍假设需要重新考虑。冲突选择的存在可能解释了在一些植物物种中出现最优种子大小的现象，而无需引入种子数量 - 大小权衡的概念。
