Fig. 13

Evolutionary relatedness and physical similarity are not necessarily linked. The rates at which physical features change can differ among lineages (Fig. 8), and superficially similar morphologies can evolve independently in more than one lineage. As a result, close relatives may look different from one another or distant relatives may look misleadingly similar. This tree presents evolutionary relationships among “reptiles,” with frogs as the outgroup. Although they look very different, birds and crocodiles are actually more closely related to each other than either is to any other group of reptiles. This particular phylogenetic hypothesis shows birds, crocodiles, lizards, and snakes to all be equally related to turtles (a detail that remains a subject of debate) and birds and all “reptiles” to be equally related to frogs (which is well accepted). The similarities between birds and mammals (e.g., four-chambered hearts, homeothermy) evolved independently in the two lineages well after their split from a distant reptilian ancestor. As with “fishes,” the category of “reptiles” is phylogenetically inconsistent