Vertebrates, Invertebrates and Plants

Although the data from this paper does show very strong trends towards positive directional selection on overall size, there are some potential biases. First of all, there were many samples of organisms such as birds, plants and insects and there was a lack of mammals. This would cause problems in the results if certain taxa did follow Copes rule while others did not. One example of this is that some mammals which live on islands tend to evolve towards smaller body sizes (Clegg and Owens, 2002). Secondly, bias could also be present due to the fact that the authors
From the Devonian until the late Cretaceous extinction shelled cephalopods were top marine carnivores. They could grow to several meters in diameter. Today they are much smaller.
only used a single component of fitness to estimate selection (Kingsolver and Pfennig, 2004). If multiple components of fitness had been used, more comprehensive results may have been obtained. Lastly, the results could be skewed since only previously published data were used. Other important data may not have been published if it was only marginally significant. This could potentially cause the results of this paper to be exaggerated beyond what they should actually be. An additional bias from this paper was brought about by Hone and Benton (2005). They pointed out that positive selection may also be observed due to the smallest individuals being unhealthy and dying more easily as opposed to there being active selection for large individuals (Hone and Benton, 2005).