Parchman Thomas L, Benkman Craig W
Department of Biology, New Mexico State University, Las Cruces 88003-8001, USA.
Evolution. 2002 Aug;56(8):1663-72. doi: 10.1111/j.0014-3820.2002.tb01478.x.
Coevolution is increasingly recognized as an important process structuring geographic variation in the form of selection for many populations. Here we consider the importance of a geographic mosaic of coevolution to patterns of crossbill (Loxia) diversity in the northern boreal forests of North America. We examine the relationships between geographic variation in cone morphology, bill morphology, and feeding performance to test the hypothesis that, in the absence of red squirrels (Tamiasciurus hudsonicus), black spruce (Picea mariana) has lost seed defenses directed at Tamiasciurus and that red crossbills (L curvirostra) and black spruce have coevolved in an evolutionary arms race. Comparisons of cone morphology and several indirect lines of evidence suggest that black spruce has evolved defenses in response to Tamiasciurus on mainland North America but has lost these defenses on Newfoundland. Cone traits that deter crossbills, including thicker scales that require larger forces to separate, are elevated in black spruce on Newfoundland, and larger billed crossbills have higher feeding performances than smaller billed crossbills on black spruce cones from Newfoundland. These results imply that the large bill of the Newfoundland crossbill (L. c. percna) evolved as an adaptation to the elevated cone defenses on Newfoundland and that crossbills and black spruce coevolved in an evolutionary arms race on Newfoundland during the last 9000 years since glaciers retreated. On the mainland where black spruce is not as well defended against crossbills, the small-billed white-winged crossbill (L leucoptera leucoptera) is more efficient and specializes on seeds in the partially closed cones. Finally, reciprocal adaptations between crossbills and conifers are replicated in black spruce and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia), with coevolution most pronounced in isolated populations where Tamiasciurus are absent as a competitor. This study further supports the role of Tamiasciurus in determining the selection mosaic for crossbills and suggests that a geographic mosaic of coevolution has been a prominent factor underlying the diversification of North American crossbills.
协同进化越来越被认为是一个重要过程，它以对许多种群的选择形式构建地理变异。在此，我们探讨协同进化的地理镶嵌体对北美北方针叶林交嘴雀（Loxia）多样性模式的重要性。我们研究球果形态、喙形态和取食性能的地理变异之间的关系，以检验以下假设：在没有红松鼠（Tamiasciurus hudsonicus）的情况下，黑云杉（Picea mariana）已经失去了针对红松鼠的种子防御机制，并且红交嘴雀（L curvirostra）和黑云杉在进化的军备竞赛中协同进化。球果形态的比较以及几条间接证据表明，北美大陆的黑云杉针对红松鼠进化出了防御机制，但在纽芬兰却失去了这些防御。在纽芬兰，黑云杉中那些能阻止交嘴雀的球果特征，包括更厚的鳞片（需要更大的力量才能分开）有所增加，而且喙较大的交嘴雀在纽芬兰黑云杉球果上的取食性能比喙较小的交嘴雀更高。这些结果意味着，纽芬兰交嘴雀（L. c. percna）的大嘴是作为对纽芬兰球果防御增强的一种适应而进化出来的，并且在冰川消退后的过去9000年里，交嘴雀和黑云杉在纽芬兰的进化军备竞赛中协同进化。在大陆上，黑云杉对交嘴雀的防御没有那么强，小嘴的白翅交嘴雀（L leucoptera leucoptera）效率更高，专门取食部分闭合球果中的种子。最后，交嘴雀和针叶树之间的相互适应在黑云杉和落基山黑松（Pinus contorta ssp. latifolia）中也有体现，在没有红松鼠作为竞争者的孤立种群中，协同进化最为明显。这项研究进一步支持了红松鼠在决定交嘴雀选择镶嵌体中的作用，并表明协同进化的地理镶嵌体一直是北美交嘴雀多样化的一个突出因素。
