Data from: Ecological specialization in fossil mammals explains Cope's rule

Cope’s rule is the trend toward increasing body size in a lineage over geological time. The rule has been explained either as passive diffusion away from a small initial body size or as an active trend upheld by the ecological and evolutionary advantages that large body size confers. An explicit and phylogenetically informed analysis of body size evolution in Cenozoic mammals shows that body size increases significantly in most inclusive clades. This increase occurs through temporal substitution of incumbent species by larger-sized close relatives within the clades. These late-appearing species have smaller spatial and temporal ranges and are rarer than the incumbents they replace, traits that are typical of ecological specialists. Cope’s rule, accordingly, appears to derive mainly from increasing ecological specialization and clade-level niche expansion rather than from active selection for larger size. However, overlain on a net trend toward average size increase, significant pulses in origination of large-sized species are concentrated in periods of global cooling. These pulses plausibly record direct selection for larger body size according to Bergmann’s rule, which thus appears to be independent of but concomitant with Cope’s.科普法则（Cope’s rule）指的是在地质时间尺度上，演化支系内生物体型逐渐增大的演化趋势。针对该法则的成因，学界先后提出两种解释：一种认为体型演化是从初始小型体型被动扩散的结果；另一种则主张其为主动演化趋势，由大型体型所带来的生态与演化优势所支撑。一项针对新生代哺乳动物体型演化的严谨且基于系统发育信息的分析显示，绝大多数最大包容性演化支的体型均呈现显著增大趋势。此类体型增大通过演化支内原有存续物种被体型更大的近缘物种在时间维度上的更替实现。这些后期出现的物种相较于其所取代的原有物种，拥有更小的空间与时间分布范围，且种群更为稀有，这类特征正是生态特化物种的典型表征。由此可见，科普法则的成因主要源于生态特化程度的提升与演化支层面的生态位扩张，而非针对大型体型的主动选择。然而，在体型平均增大的整体趋势之上，大型物种的显著爆发式起源事件集中发生于全球变冷时期。此类爆发事件可合理地归因于依据伯格曼法则（Bergmann’s rule）对大型体型的直接选择，因此伯格曼法则看似独立于科普法则，但二者又相伴相随。