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.柯普法则（Cope’s Rule）指演化支在地质时间尺度上体型逐渐增大的演化趋势。针对该法则的成因，学界主要存在两种解释：一种观点认为体型从初始较小的状态被动扩散增长，另一种则提出大体型所带来的生态与演化优势，推动形成了主动的体型增大趋势。一项针对新生代哺乳动物体型演化的系统性、基于系统发育的分析显示，绝大多数包容性较强的演化支均存在显著的体型增大现象。这类体型增大是通过演化支内部，体型更大的近缘类群在时间维度上取代原有常驻物种的过程实现的。相较于被其取代的原有物种，这些后期出现的物种种群的空间与时间分布范围更小，种群密度也更低，这类特征正是生态特化物种的典型表现。由此可见，柯普法则主要源自生态特化程度的提升以及演化支层面的生态位扩张，而非对大体型的主动选择。然而，在整体平均体型增大的趋势之上，大型物种的显著爆发式起源事件集中发生在全球变冷时期。这类爆发事件恰好符合伯格曼法则（Bergmann’s Rule）中关于大体型受直接选择的推论，因此伯格曼法则看似独立于柯普法则，但二者又相伴而生。