Fleming Ian A, Gross Mart R
Department of Zoology, University of Toronto, Toronto, Ontario, M5S 1A1, Canada.
Evolution. 1994 Jun;48(3):637-657. doi: 10.1111/j.1558-5646.1994.tb01350.x.
In the breeding system of Pacific salmon, females compete for oviposition territories, and males compete to fertilize eggs. The natural selection in females and sexual selection in males likely has been responsible for their elaborate breeding morphologies and the dimorphism between the sexes. We quantified direct-selection intensities during breeding on mature coho salmon (Oncorhynchus kisutch), measured for seven phenotypic characters, including three secondary sexual characters. Wild and sea-ranched hatchery coho were used to enhance the range of phenotypes over which selection could be examined. The fish were allowed to breed in experimental arenas where we could quantify components of breeding success as well as estimate overall breeding success. We found that without competition, natural selection acts only on female body size for increased egg production; there is no detectable selection on males for the phenotypic distribution we used. Under competition, the opportunity for selection increased sixfold among females. Natural selection favored female body size and caudal-peduncle (tail) depth. Increased body size meant increased egg production and access to nesting territories. The caudal peduncle, used in burst swimming and nest digging, influenced both successful egg deposition and nest survival. Increasing density increased competition among females, though it did not significantly intensify natural selection on their characters. In males, competition increased the opportunity for selection 52-fold, which was nine times greater than for females. Sexual selection favored male body size and hooked snout length, both characters directly influencing male access to spawning opportunities. Selection on male body size was also affected significantly by breeding density. The ability of large males to control access to spawning females decreased at higher densities reflecting an increase in the operational sex ratio. Further, the relative success of small males, which could sneak access to spawning females, appeared to increase as that of intermediate-sized males decreased. Such disruptive selection may be responsible for the evolution of alternative reproductive tactics in salmon.
在太平洋鲑鱼的繁殖系统中，雌鱼会争夺产卵领地，雄鱼则竞争使卵子受精。雌鱼的自然选择和雄鱼的性选择可能造就了它们复杂的繁殖形态以及两性异形。我们对成熟银大麻哈鱼（Oncorhynchus kisutch）繁殖期间的直接选择强度进行了量化，测量了七个表型特征，包括三个第二性征。野生和海上网箱养殖的银大麻哈鱼被用于扩大可研究选择的表型范围。这些鱼被允许在实验场地进行繁殖，在那里我们可以量化繁殖成功的各个组成部分，并估算总体繁殖成功率。我们发现，在没有竞争的情况下，自然选择仅作用于雌鱼的体型，以增加产卵量；对于我们所使用的表型分布，未检测到对雄鱼的选择。在有竞争的情况下，雌鱼的选择机会增加了六倍。自然选择青睐雌鱼的体型和尾柄深度。体型增大意味着产卵量增加以及获得筑巢领地的机会增加。用于爆发式游泳和挖巢的尾柄，对成功产卵和巢穴存活都有影响。密度增加会加剧雌鱼之间的竞争，不过这并没有显著增强对其特征的自然选择。在雄鱼中，竞争使选择机会增加了52倍，这比雌鱼的选择机会大九倍。性选择青睐雄鱼的体型和钩状吻长，这两个特征都直接影响雄鱼获得产卵机会。雄鱼体型的选择也受到繁殖密度的显著影响。在较高密度下，大型雄鱼控制接近产卵雌鱼的能力下降，这反映了两性 operational sex ratio 的增加。此外，小型雄鱼（能够偷偷接近产卵雌鱼）的相对成功率似乎随着中型雄鱼成功率的下降而增加。这种分裂选择可能导致了鲑鱼中替代繁殖策略的进化。
