Parchman Thomas L, Benkman Craig W
Department of Biology, MSC 3AF, New Mexico State University, Las Cruces, New Mexico 88003, USA.
Evolution. 2008 Feb;62(2):348-60. doi: 10.1111/j.1558-5646.2007.00295.x. Epub 2007 Nov 12.
Recent research demonstrates how the occurrence of a preemptive competitor (Tamiasciurus) gives rise to a geographic mosaic of coevolution for crossbills (Loxia) and conifers. We extend these studies by examining ponderosa pine (Pinus ponderosa), which produces more variable annual seed crops than the conifers in previous studies and often cooccurs with tree squirrels in the genus Sciurus that are less specialized than Tamiasciurus on conifer seed. We found no evidence of seed defenses evolving in response to selection exerted by S. aberti, which was apparently overwhelmed by selection resulting from inner bark feeding that caused many developing cones to be destroyed. In the absence of S. aberti, defenses directed at crossbills increased, favoring larger-billed crossbills and causing stronger reciprocal selection between crossbills and ponderosa pine. However, crossbill nomadism in response to cone crop fluctuations prevents localized reciprocal adaptation by crossbills. In contrast, evolution in response to S. griseus has incidentally defended cones against crossbills, limiting the geographic range of the interaction between crossbills and ponderosa pine. Our results suggest that annual resource variation does not prevent competitors from shaping selection mosaics, although such fluctuations likely prevent fine-scale geographic differentiation in predators that are nomadic in response to resource variability.
最近的研究表明，先发制竞争对手（美洲花鼠属）的出现如何导致交嘴雀（交嘴雀属）和针叶树之间形成共进化的地理镶嵌体。我们通过研究黄松（西黄松）来扩展这些研究，黄松每年产生的种子作物比之前研究中的针叶树更具变异性，并且经常与松鼠属的树松鼠共生，这些树松鼠对针叶树种子的专一性不如美洲花鼠属。我们没有发现种子防御因阿氏松鼠施加的选择而进化的证据，阿氏松鼠显然被内树皮取食导致许多发育中的球果被破坏所产生的选择所压倒。在没有阿氏松鼠的情况下，针对交嘴雀的防御增加，有利于嘴更大的交嘴雀，并导致交嘴雀和黄松之间更强的相互选择。然而，交嘴雀因球果产量波动而进行的游牧行为阻碍了交嘴雀的局部相互适应。相比之下，对灰松鼠的进化反应偶然地保护了球果免受交嘴雀侵害，限制了交嘴雀和黄松之间相互作用的地理范围。我们的结果表明，年度资源变化并不能阻止竞争对手塑造选择镶嵌体，尽管这种波动可能会阻止因资源变化而游牧的捕食者进行精细尺度的地理分化。
