Coltman D W, Smith J A, Bancroft D R, Pilkington J, MacColl A D, Clutton-Brock T H, Pemberton J M
Institute of Cell, Animal, and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, United Kingdom.
Am Nat. 1999 Dec;154(6):730-46. doi: 10.1086/303274.
Variation in male lifetime breeding success (LBS) is central to understanding selection, yet it has rarely been measured in natural populations of large mammals. Here, we first describe variation in the opportunity for selection in cohorts of Soay rams (Ovis aries) on the archipelago of St. Kilda, Scotland, that were born during years of varying population density. Variation in LBS is closely coupled with demography, as rams born in years of low density following population crashes enjoy greater LBS than do those born in high-density years. Paradoxically, the opportunity for selection was greatest in the largest cohorts, those born in years of high population density, owing to low juvenile breeding success and overwinter survival. Variation in longevity and the contribution of nonbreeders were the most important components of the total variance in LBS in cohorts born in years of high density, while variation in fecundity was more important in cohorts born in low-density years. The opportunity for sexual selection is thus stronger in cohorts born in low-density years, as many rams in these cohorts survive to compete for mates as adults in subsequent ruts. Variation in population density in the year of birth also influenced the intensity of selection. Individuals born in years of high population density underwent strong natural selection in favor of longer hindlimbs over their first winter. In contrast, in cohorts born in low-density years, there was no natural selection on hindlimb in the first year of life. Longer hindlimbs were associated with increased fecundity over the entire lifetime of individuals born in low-density years. Natural and sexual selection thus act on the same trait in the same direction at different life-history stages in Soay rams, depending on the population density experienced in the year of birth.
雄性一生繁殖成功率（LBS）的差异是理解选择的核心，但在大型哺乳动物的自然种群中却很少被测量。在这里，我们首先描述了苏格兰圣基尔达群岛上索艾羊（Ovis aries）群体中选择机会的差异，这些羊出生于种群密度不同的年份。LBS的差异与种群统计学密切相关，因为在种群数量崩溃后的低密度年份出生的公羊比在高密度年份出生的公羊享有更高的LBS。矛盾的是，选择机会在最大的群体中最大，即那些在高种群密度年份出生的群体，这是由于幼年繁殖成功率低和越冬存活率低。在高密度年份出生的群体中，寿命差异和非繁殖者的贡献是LBS总方差中最重要的组成部分，而在低密度年份出生的群体中，繁殖力差异更为重要。因此，在低密度年份出生的群体中，性选择的机会更强，因为这些群体中的许多公羊成年后能存活下来，在随后的发情期争夺配偶。出生年份的种群密度差异也影响了选择强度。在高种群密度年份出生的个体在第一个冬天经历了强烈的自然选择，有利于更长的后肢。相比之下，在低密度年份出生的群体中，出生后第一年对后肢没有自然选择。在低密度年份出生的个体一生中，更长的后肢与更高的繁殖力相关。因此，在索艾羊中，自然选择和性选择在不同的生命史阶段对同一性状起着相同方向的作用，这取决于出生年份所经历的种群密度。
