Jackson Sue, Diamond Jared
Physiology Department, UCLA Medical School, Los Angeles, California, 90095-1751.
Evolution. 1996 Aug;50(4):1638-1650. doi: 10.1111/j.1558-5646.1996.tb03936.x.
Compared to ancestral wild jungle fowl, domestic broiler chickens have been consciously selected for large body size, relatively large pectoral muscles, rapid growth, and high feed efficiency. Hence intraspecific comparisons of these two strains could help identify consequences of unconscious artificial selection, trade-offs in energy allocation, and factors limiting energy budgets. We therefore compared our measurements of many corresponding parameters in both strains: growth rate, energy intake, digestive efficiency, metabolic rate and its components, organ masses, and intestinal brush-border nutrient transporter and hydrolase activities and capacities, as functions of age and body mass in zero- to nine-week-old chicks. Both strains prove to have the same digestive efficiency. Compared to equal-sized jungle fowl, broilers have higher daily energy intake and activity costs. Broilers have relatively longer and wider, hence heavier, small intestines, and their other gut compartments are also relatively larger. Offsetting these increases, broilers have relatively smaller brains and leg bones, these being much less important to a captive bird than to a wild bird exposed to predators. Broilers have generally lower intestinal transporter activities, but relatively higher transporter capacities because of their larger guts. Among domestic chicken strains, comparison of broilers with layers, the former having been consciously selected for much higher growth rates, yields generally similar conclusions. Thus, as recognized in broad outline by Darwin, domestication provides clear examples of conscious selection, of unconscious selection for traits prerequisite to the consciously selected traits, and of unconscious selection against traits rendered less important or competing for space or energy.
与野生原鸡相比，家肉鸡在体型大小、胸肌相对较大、生长速度快以及饲料效率高方面经过了有意识的选育。因此，对这两个品系进行种内比较有助于确定无意识人工选择的后果、能量分配的权衡以及限制能量预算的因素。我们因此比较了两个品系中许多相应参数的测量值：生长速率、能量摄入、消化效率、代谢率及其组成部分、器官质量、肠道刷状缘营养物质转运蛋白和水解酶的活性及能力，这些参数是0至9周龄雏鸡年龄和体重的函数。两个品系的消化效率相同。与体型相当的原鸡相比，肉鸡的每日能量摄入量和活动成本更高。肉鸡的小肠相对更长、更宽，因此更重，其其他肠道部分也相对更大。作为这些增加的抵消因素，肉鸡的大脑和腿骨相对较小，对于圈养的鸡来说，这些器官的重要性远低于野生环境中易受捕食者攻击的鸡。肉鸡的肠道转运蛋白活性普遍较低，但由于其肠道较大，转运蛋白能力相对较高。在家鸡品系中，将肉鸡与蛋鸡进行比较，前者经过有意识的选育，生长速度要高得多，得出的总体结论大致相似。因此，正如达尔文大致认识到的那样，驯化提供了有意识选择、对有意识选择的性状的先决性状进行无意识选择以及对变得不那么重要或争夺空间或能量的性状进行无意识选择的明显例子。
