Not only do individuals of the same species differ from each other (at the individual or the population level), but different lineages of organisms belonging to different species obviously differ from each other. Coyotes differ from wolves, members of the dog family (Canidae) differ from members of the cat family (Felidae), members of the order Carnivora differ from members of the order Ungulata (hooved mammals), mammals differ from fish, etc. Some of the differences between lineages of organisms is due to their differing ancestries. For example the differences between members of the Canidae and Felidae can be traced back long before the species alive today ever existed. On the other hand, closely-related species can differ from each other as well, in ways that can often be attributed to the immediate effects of natural selection and other evolutionary factors. In this section we illustrate differences between species by two interesting phenomena: convergent and divergent evolution. In convergent evolution, distantly related species have evolved to appear similar (at least superficially). In divergent evolution, closely-related species have evolved differences.
Distantly related organisms that live in similar environments often come to resemble one another. This is because organisms that have experienced similar environmental problems are subject to similar selective pressures, and their inherent genetic variation allows the evolution of similar phenotypes (appearances) in spite of different genotypes (genetic composition).
What features do these two unrelated plants have in common?
Divergent evolution happens when different selection pressures cause two species to follow different evolutionary courses. Considerable phenotypic variation can result among species that genetically are closely related. One of the better examples of evolution within a genus of plants is the diversity of growth form in the genus Tillandsia of the pineapple family. Diversity in Tillandsia is the result of adaptations to obtain water in habitats ranging from tropical rain forests to the driest deserts. Most species of Tillandsia are epiphytes that live attached to branches trunks of trees. In some species such as "spanish moss" (neither spanish nor moss, by the way), the ability to form roots has been lost completely. Rather than depending upon root hairs for water uptake, these epiphytes rely on special leaf scales that trap water. Green, leafy species of Tillandsia inhabit wet areas and have their leaves organized in such a way as to catch water in the center of the plant, forming a kind of reservoir, where scales at the bases of the leaves take up the water. Species in drier habitats have leaves completely covered with scales, giving a silvery appearance. Such leaves do not hold a reservoir of water, but the scales are capable of taking up sufficient moisture from dew or fog. Also, the scales are highly reflective and protect the leaves from excessive heat. Some of these silvery species of Tillandsia thrive in the Atacama desert of western South America, one of the drier regions in the world.
Examples of divergent evolution: