ella[191] and protein sorting machinery[192] evolved by the recruitment of several pre-existing proteins that previously had different functions.[144] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.[193][194]

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.[195] This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features.[196] These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.[197] It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles.[198] It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.[199]

Co-evolution


Common Garter Snake (Thamnophis sirtalis sirtalis) which has evolved resistance to tetrodotoxin in its amphibian prey.
Further information: Co-evolution
Interactions between organisms can produce both conflict and co-operation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called co-evolution.[200] An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.[201]

Co-operation
Further information: Co-operation (evolution)
Not all co-evolved interactions between species involve conflict.[202] Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil.[203] This is a reciprocal relationship as the plants provide