A major challenge when trying to determine a plausible relationship between organisms based on anatomy has been to distinguish homologous from convergent (analogous) traits. Homologous traits, known as synapomorphies, are the basis of placing organisms together within a common group. In contrast, convergent traits are independent solutions to a common problem, and so are irrelevant when it comes to defining evolutionary relationships. It is, however, also true that evolution can lead to the loss of traits; this can confuse or complicate the positioning of an organism in a classification scheme. It is worth noting that very often developing a particular trait, whether it is an enzyme or an eye, requires energy. If the trait does not contribution of an organism’s reproductive success it will not be selected for; on the other hand, it is expensive to build, but has not useful function, its loss may be selected for. As organisms adapt to a specific environment and lifestyle, traits once useful can become irrelevant and may be lost (such as the ability to synthesize ascorbic acid). A classic example is the reduction of hind limbs during the evolution of whales [→]. Another is the common loss of eyes often seen as populations adapt to environments in which light is absent. The most dramatic case of loss involves organisms that become obligate parasites of other organisms. In many cases, these parasitic organisms are completely dependent on their hosts for many essential functions, this allows them to become quite simplified even though they are in fact highly evolved. For example, they lose many genes as they become dependent upon the host. The loss of traits can itself be an adaptation if it provides an advantage to organisms living in a particular environment. This fact can make it difficult to determine whether an organism is primitive (that is, retains ancestral features) or highly evolved.