Wilbur H M, Collins J P
Science. 1973 Dec 28;182(4119):1305-14. doi: 10.1126/science.182.4119.1305.
A synthetic theory of the ecology of amphibian metamorphosis is founded on the observation that the large variation in length of larval period and body size at metamorphosis typical of a particular species of amphibian cannot be directly explained by differences in dates of hatching or egg sizes. It is proposed that as development proceeds, variation in exponential growth coefficients causes a trend from a normal distribution to a skewed distribution of body sizes. The degree of skewing increases and the median of the distribution decreases with increasing initial densities of populations. The relative advantages of the largest members of a cohort may arise from a variety of mechanisms including the production of growth inhibitors, interference competition, and size-selective feeding behavior. These mechanisms result in a nonnormal distribution of competitive ability, a possible source of the density-dependent competition coefficient found in systems with many species (1). In our model the ranges of body sizes and dates of metamorphosis are determined by a minimum body size that must be obtained and a maximum body size that will not be exceeded at metamorphosis. Between these two size thresholds the endocrinological initiation of metamorphosis is expected to be related to the recent growth history of the individual larva. Species that exploit uncertain environments will have a wide range of possible sizes at metamorphosis. Species exploiting relatively certain environments will have a narrower range. The evolution of neoteny and direct development logically follow from the application of these ideas to the ecological context of the evolution of amphibian life histories. Species that live in constant aquatic habitats surrounded by hostile environments (desert ponds, caves, high-altitude lakes) may evolve permanent larvae genetically incapable of metamorphosis. Other populations may evolve a facultative metamorphosis such that populations are a mixture of neotenes and terrestrial adults. Direct development results from selection to escape the competition, predation, and environmental uncertainty characteristic of some aquatic habitats and is usually accompanied by parental care. The relation between our ecological model and the physiological mechanisms that initiate metamorphosis can only be suggested and it remains an open problem for developmental biologists.
特定两栖动物物种典型的幼体期长度和变态时体型大小的巨大差异，不能直接用孵化日期或卵大小的差异来解释。有人提出，随着发育的进行，指数生长系数的变化导致体型分布从正态分布向偏态分布转变。偏斜程度增加，分布的中位数随着种群初始密度的增加而降低。一个群体中最大个体的相对优势可能源于多种机制，包括生长抑制剂的产生、干扰竞争和大小选择性摄食行为。这些机制导致竞争能力的非正态分布，这可能是在多物种系统中发现的密度依赖性竞争系数的一个来源（1）。在我们的模型中，体型范围和变态日期由变态时必须达到的最小体型和不会超过的最大体型决定。在这两个大小阈值之间，变态的内分泌启动预计与单个幼体最近的生长历史有关。利用不确定环境的物种在变态时会有广泛的可能体型。利用相对确定环境的物种体型范围会更窄。幼态延续和直接发育的进化逻辑上源于将这些观点应用于两栖动物生活史进化的生态背景。生活在被恶劣环境包围的恒定水生栖息地（沙漠池塘、洞穴、高海拔湖泊）中的物种可能进化出遗传上无法变态的永久幼体。其他种群可能进化出兼性变态，使得种群是幼态延续个体和陆生成体的混合体。直接发育是为了逃避某些水生栖息地特有的竞争、捕食和环境不确定性而进行选择的结果，通常伴随着亲代抚育。我们的生态模型与启动变态的生理机制之间的关系只能进行推测，这仍然是发育生物学家面临的一个未解决问题。
